LED7706
6-rows 30 mA LEDs driver with boost regulator for LCD panels backlight
Features
Boost section 4.5 V to 36 V input voltage range Internal power MOSFET Internal +5 V LDO for device supply Up to 36 V output voltage Constant frequency peak current-mode control 250 kHz to 1 MHz adjustable switching frequency External synchronization for multi-device application Pulse-skip power saving mode at light load Programmable soft-start Programmable OVP protection Stable with ceramic output capacitors Thermal shutdown Backlight driver section Six rows with 30 mA maximum current capability (adjustable) Parallelable rows for higher current Rows disable option Less than 500 ns minimum dimming time (1 % minimum dimming duty-cycle at 20 kHz) 2 % current matching between rows LED failure (open and short-circuit) detection
VFQFPN-24 4x4
Description
The LED7706 consists of a high efficiency monolithic boost converter and six controlled current generators (rows) specifically designed to supply LED arrays used in the backlighting of LCD panels. The device can manage an output voltage up to 36 V (i.e. 10 white LEDs per row). The generators can be externally programmed to sink up to 30 mA and can be dimmed via a PWM signal (1 % dimming duty-cycle at 20 kHz can be managed). The device allows to detect and manage the open and shorted LED faults and to let unused rows floating. Basic protections (Output Over-Voltage, internal MOSFET Over-Current and Thermal Shutdown) are provided.
Applications
LCD monitors and TV panels PDA panel backlight GPS panel backlight Device summary
Order code LED7706 VFQFPN-24 4x4 (exposed pad) LED7706TR Tape and reel Rev 1 1/31
www.st.com 31
Table 1.
Package
Packaging Tube
February 2008
Contents
LED7706
Contents
1 2 Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 2.2 Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 3.2 3.3 Maximum rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Recommended operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 5 6
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Operation description - boost section . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1 6.2 6.3 6.4 6.5 6.6 Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Soft-star t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Over voltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Switching frequency selection and synchronization . . . . . . . . . . . . . . . . . 15 System stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.6.1 6.6.2 Loop compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Slope compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.7 6.8
Boost current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Thermal protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2/31
LED7706
Contents
7
Backlight driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1 7.2 Current generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 PWM dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8
Fault management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1 8.2 8.3 8.4 FAULT pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 MODE pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Open LED fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Shor ted LED fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
9 10
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3/31
1
23
8
19
AVC C SLOPE 9 17
SYN C
6 OVSEL
VI N
C av c c 18
F AU LT C ldo5 7 LD O5 F AU LT EN DIM MOD E C OMP F SW R I LI M BI LI M SGN D SS TH PD 25 PGN D 22 21 20 5 1 24
R f ilt
EN
Typical application circuit
PM 77 0 LED6606
16 15 14 13 12 11
DIM
AVC C
R OW 6 R OW 5 R OW 4 R OW 3 R OW 2 R OW 1
SW 3
MOD E
4
2
3
R c om p AVC C SW 2 F SW R rilim R bilim
Css
C c om p
Rf sw
VI N -
10
LX
4/31
Figure 1.
L VBOOST
D
VI N + C in
Typical application circuit
Application circuit
R1
C 13
R s lope
C out
AVC C
R2
C 10
LED7706
LED7706
Pin settings
2
2.1
Pin settings
Connections
Figure 2. Pin connection (through top view)
FAULT
SYNC
DIM
EN
SS
LX
19 18
24
COMP RILIM BILIM FSW MODE AVCC
1
OVSEL PGND
LED7706
ROW6 ROW5 ROW4
6 7 12
13
ROW3
LDO5
VIN
SLOPE
SGND
ROW1
ROW2
5/31
Pin settings
LED7706
2.2
Pin description
Table 2.
N 1 2 3
Pin functions
Pin COMP RILIM BILIM Function Error amplifier output. A simple RC series between this pin and ground is needed to compensate the loop of the boost regulator. Output generators current limit setting. The output current of the rows can be programmed connecting a resistor to SGND. Boost converter current limit setting. The internal MOSFET current limit can be programmed connecting a resistor to SGND. Switching frequency selection and external sync input. A resistor to SGND is used to set the desired switching frequency. The pin can also be used as external synchronization input. See Section 6.5 on page 16 for details. Current generators fault management selector. It allows to detect and manage LEDs failures. See Section 8.2 on page 25 for details. +5 V analog supply. Connect to LDO5 through a simple RC filter. Internal + 5V LDO output and power section supply. Bypass to SGND with a 1 F ceramic capacitor. Input voltage. Connect to the main supply rail. Slope compensation setting. A resistor between the output of the boost converter and this pin is needed to avoid sub-harmonic instability. Refer to Section 6.6 on page 17 for details. Signal ground. Supply return for the analog circuitry and the current generators. Row driver output #1. Row driver output #2. Row driver output #3. Row driver output #4. Row driver output #5. Row driver output #6. Power ground. Source of the internal Power MOSFET. Over-voltage selection. Used to set the desired OV threshold by an external divider. See Section 6.4 on page 15 for details. Switching node. Drain of the internal Power MOSFET. Dimming input. Used to externally set the brightness by using a PWM signal. Enable input. When low, the device is turned off. If tied high or left open, the device is turned on and a soft-start sequence takes place. Fault signal output. Open drain output. The pin goes low when a fault condition is detected (see Section 8.1 on page 25 for details). SYNC SS Synchronization output. Used as external synchronization output. Soft-star t. Connect a capacitor to SGND to set the desired soft-start duration.
4
FSW
5 6 7 8 9
MODE AVCC LDO5 VIN SLOPE
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
SGND ROW1 ROW2 ROW3 ROW4 ROW5 ROW6 PGND OVSEL LX DIM EN
6/31
LED7706
Electrical data
3
3.1
Electrical data
Maximum rating
Table 3.
Symbol VAVCC VLDO5 AVCC to SGND LDO5 to SGND PGND to SGND VIN VLX VIN to PGND LX to SGND LX to PGND RILIM, BILIM, SYNC, OVSEL, SS to SGND EN, DIM, SW, MODE, FAULT to SGND rowx to PGND/ SGND SLOPE to VIN SLOPE to SGND Maximum LX RMS current PTOT Power dissipation @ TA = 25 C Maximum withstanding voltage range test condition: CDF-AEC-Q100-002- "Human Body Model" acceptance criteria: "Normal Performance"
Absolute maximum ratings (1)
Parameter Value -0.3 to 6 -0.3 to 6 -0.3 to 0.3 -0.3 to 40 -0.3 to 40 -0.3 to 40 -0.3 to VAVCC + 0.3 -0.3 to 6 -0.3 to 40 VIN - 0.3 to VIN + 6 -0.3 to 40 2.0 2.3 2000 A W V V Unit
1. Stresses beyond those listed under "absolute maximum ratings" may cause permanent damage to the device. Exposure to absolute maximum rated conditions for extended periods may affect device reliability.
3.2
Thermal data
Table 4.
Symbol RthJA TSTG TJ
Thermal data
Parameter Thermal resistance junction to ambient Storage temperature range Junction operating temperature range Value 42 -20 to 125 -50 to 150 Unit C/W C C
7/31
Electrical data
LED7706
3.3
Recommended operating conditions
Table 5.
Symbol VIN VBST Input voltage range Boost section output voltage FSW sync input Duty-Cycle fSW Irowx Switching frequency rows output current 250 5
Recommended operating conditions
Value Parameter Min 4.5 Max 36 36 40 1000 30 % kHz mA V Unit
8/31
LED7706
Electrical characteristics
4
Table 6.
Symbol
Electrical characteristics
Electrical characteristics (VIN = 12 V; TA = 0 C to 85 C and LDO5 connected to AVCC if not otherwise specified)
Parameter Test condition Min Typ Max Unit
Supply section VLDO5 VAVCC IIN,Q IIN,SHDN VUVLO,ON VUVLO,OFF LDO output and IC supply voltage EN High ILDO5 = 0 mA RRILIM = 51 k, RBILIM = 220 k, RSLOPE = 680 k DIM tied to SGND. EN low 4.4 5 5.5 V
Operating quiescent current Operating current in shutdown LDO5 Under Voltage Lock Out upper threshold LDO5 Under Voltage Lock Out lower threshold
1 20 3.8 3.3 3.6 30 4.0
mA A
V
LDO linear regulator Line regulation LDO dropout voltage LDO maximum output current Boost section tON,min fSW Minimum switching on-time Default switching frequency Minimum FSW sync frequency FSW sync input threshold FSW sync input hysteresis FSW sync min ON time SYNC output Duty-Cycle SYNC output high level SYNC output low level Power switch KB RDSon LX current coefficient
Internal MOSFET on-resistance
6 V VIN 28 V, ILDO5 = 30 mA VIN = 4.3 V, ILDO5 = 10 mA VLDO5 > VUVLO,ON VLDO5 < VUVLO,OFF 25 80 40 20
25 120 60 30
mV
mA
200 FSW connected to AVCC 570 660 210 240 300 20 270 FSW connected to AVCC (Internal oscillator selected) ISYNC = 10 A ISYNC = -10 A VAVCC -20V 20 34 40 750
ns kHz
mV
ns %
mV
RBILIM = 600 k
5E5
6E5 280
7E5 500
V m
9/31
Electrical characteristics Table 6.
Symbol
LED7706
Electrical characteristics (continued) (VIN = 12 V; TA = 0 C to 85 C and LDO5 connected to AVCC if not otherwise specified)
Parameter Test condition Min Typ Max Unit
OC and OV protections VTH,OVP VTH,FRD VOVP,FRD Over voltage protection reference threshold (OVSEL) Floating channel detection threshold Voltage gap between OVP and FRD thresholds 1.190 1.100 1.234 1.145 90 1.280 V 1.190 mV
Soft-start and power management EN, Turn-On level threshold EN, Turn-off level threshold DIM, high level threshold DIM, low level threshold EN, pull-up current SS, charge current SS, end-of-startup threshold SS, reduced switching frequency release threshold Current generators section TDIMON,min
1.4 0.8 1.0 1.15 0.8 1.09 2.5 4 2.0 5 2.4 0.8
1.6 V
1.3
6 2.8
A
V
Minimum dimming on-time rows current accuracy (1) rows current mismatch(2) RRILIM = 51 k KR = 987 Irowx,nom = KR/RRILIM
500 2.0 +4.0 400 MODE tied to SGND MODE connected to AVCC 3.4 6.0 100 IFAULT,SINK = 4 mA 200 350
ns % mV V s mV
Irowx,1 Irowx,2 VIFB Vrowx,
FAULT
Feedback regulation voltage Shor ted LED fault detection threshold LED short circuit detection masking time FAULT pin Low-Level voltage
TMASK VFAULT,
LOW
Thermal shutdown TSHDN Thermal shutdown turn-off temperature Thermal shutdown hysteresis
1. Current accuracy calculated as Irowx,1 = (Irowx-Irowx,NOM) / Irowx,NOM 2. Current Mismatch calculated as Irowx,2 = |Irowx-Irowy| / Irowx,NOM
150 30
C
10/31
LED7706
Block diagram
5
Figure 3.
Block diagram
Functional and block diagram
VIN
SLOPE
Current Sense
LDO5
+5V LDO
Ramp Generator ++
ZCD
LX
UVLO Detector UVLO + gm _
+ _
Boost Control Logic 0.4V
COMP BILIM SS
Current Limit
PGND
Boost_EN _ FRD + + _ 1.172V
OVSEL
1.234V
OVP Soft Start Min Voltage Selector VROW6
CTRL6
Prot_EN
Current Generator 6 Current Generator 5 Current Generator 4 Current Generator 3 Current Generator 2
ROW6 ROW5 ROW4 ROW3 ROW2
SYNC
Ext Sync Detector
÷2
VROW5
CTRL5
VROW4 OSC
CTRL4
VROW3
FSW
Prot_EN
CTRL3
VROW2
CTRL2
AVCC EN MODE
CONTROL LOGIC
Boost_EN UVLO CTRL6 CTRL5 CTRL4 CTRL3 CTRL2 6V 3.4V
VTH,FLT
MODE
CTRL1 VROW1
LOGIC
FAULT DIM
Thermal Shutdown
OVP FRD
+ _ I to V
ROW1
I to V 1.2V Current Generator 1
RILIM
SGND
11/31
Operation description - boost section
LED7706
6
6.1
Operation description - boost section
Functional description
The LED7706 is a monolithic LEDs driver for the backlight of LCD panels and it consists of a boost converter and six PWM-dimmable current generators. The boost section is based on a constant switching frequency, Peak Current-Mode architecture. The boost output voltage is controlled such that the lowest row's voltage, referred to SGND, is equal to an internal reference voltage (400 mV typ.). The input voltage range is from 4.5 V up to the output voltage. In addition, the LED7706 has an internal LDO that supplies the internal circuitry of the device and is capable to deliver up to 40 mA. The input of the LDO is the VIN pin. The LDO5 pin is the LDO output and the supply for the Power MOSFET driver at the same time. The AVCC pin is the supply for the analog circuitry and should be connected to the LDO output through a simple RC filter. Figure 4. AVCC filtering
VIN
LDO5
LDO
4R7 AVCC 1u 100n
LED7706
SGND
Two loops are involved in regulating the current sunk by the generators. The main loop is related to the boost regulator and uses a constant frequency Peak CurrentMode architecture (see Table 7), while an internal current loop regulates the same current at each row according to the set value (RILIM pin). A dedicated circuit automatically selects the lowest voltage drop among all the rows and provides this voltage the main loop that, in turn, regulates the output voltage. In fact, once the reference generator has been detected, the error amplifier compares its voltage drop to the internal reference voltage and varies the COMP output. The voltage at the COMP pin determines the inductor peak current at each switching cycle. The output voltage of the boost regulator is thus determined by the total forward voltage of the LEDs strings: Equation 1
VOUT = max (
i=1
NROWS mLEDS
j=1
VF,j ) + 400mV
where the first term represents the highest total forward voltage drop over N active rows and the second is the voltage drop across the leading generator (400 mV typ.). The device continues to monitor the voltage drop across all the rows and automatically switches to the current generator having the lowest voltage drop.
12/31
LED7706
Operation description - boost section
6.2
Enable function
The LED7706 is enabled by the EN pin. This pin is active high and, when forced to SGND, the device is turned off. This pin is connected to a permanently active 2.5 A current source; when sudden device turn-on at power-up is required, this pin must be left floating or connected to a delay capacitor. When turned off, the LED7706 quickly discharges the soft-start capacitor and turns off the Power MOSFET, the current generators and the LDO. The power consumption is thus reduced to 20 A only. In applications where the dimming signal is used to turn on and off the device, the EN pin can be connected to the DIM pin as shown in Figure 5.
Figure 5. External sync waveforms
DIM BAS69 EN 220k 100n SGND
LED7706
13/31
Operation description - boost section
LED7706
6.3
Soft-start
The soft-start function is required to perform a correct start-up of the system, controlling the inrush current required to charge the output capacitor and to avoid output voltage overshoot. The soft-start duration is set connecting an external capacitor between the SS pin and ground. This capacitor is charged with a 5 A constant current, forcing the voltage on the SS pin to ramp up. When this voltage increases from zero to nearly 1.2 V, the current limit of the Power MOSFET is proportionally released to its final value. In addition, the switching frequency of the boost converter is reduced to half of the nominal value to avoid the saturation of the inductor due to current runaway; the nominal switching frequency is restored after the SS pin voltage has crossed 0.8 V.
Figure 6. Soft-start sequence waveforms in case of floating rows
OVP Floating ROWs detection
95% of OVP
Output voltage
AVCC 2.3V 4 1.2V 0.8V
SS pin voltage
Protections turn active Nominal switching frequency release
tss
100% Current limit
EN pin voltage
t
During the soft-start phase is also performed the floating rows detection. In presence of one or more floating rows, the error amplifier is unbalanced and the output voltage increases; when it reaches the Floating row Detection (FRD) threshold (95 % of the OVP threshold), the floating rows are managed according to Table 7 (see Section 8 on page 25 ). After the SS voltage reaches a 2.4 V threshold, the start-up finishes and all the protections turn active. The soft-start capacitor CSS can be calculated according equation 2.
Equation 2
C SS
ISS t SS 2 .4
Where ISS = 5 A and tSS is the desired soft-start duration.
14/31
LED7706
Operation description - boost section
6.4
Over voltage protection
An adjustable Over-Voltage Protection is available. It can be set feeding the OVSEL pin with a partition of the output voltage. The voltage of the central tap of the divider is thus compared to a fixed 1.234 V threshold. When the voltage on the OVSEL pin exceeds the OV threshold, the FAULT pin is tied low (see Section 8 on page 25) and the device is turned off; this condition is latched and the LED7706 is restarted by toggling the EN pin or by performing a Power-On Reset (the POR occurs when the LDO output falls below the lower UVLO threshold and subsequently crosses the upper UVLO threshold during the rising phase of the input voltage). Normally, the value of the high-side resistors of the divider must be chosen as high as possible (but lower than 1 M) to reduce the output capacitor discharge when the boost converter is off (during the off phase of the dimming cycle). The R2/R1 ratio is calculated to trigger the OVP circuitry as soon as the output voltage is 2 V higher than the maximum value for a given LED string (see equation 3). Two additional filtering capacitors, C10 and C13, may be required to improve noise rejection at the OVSEL pin, as shown in Figure 7. The typical value for C10 is in the 100 pF-330 pF range, while the C13 value is given by equation 4.
Equation 3
R 2 = R1
1.234 V (VOUT,OVP + 2V - 1.234 V)
Equation 4
C13 = 2 C10
R2 R1
Figure 7.
OVP threshold setting
VIN VOUT
LX
R1
COUT
OVSEL
LED7706
SGND
R2
CF
15/31
Operation description - boost section
LED7706
6.5
Switching frequency selection and synchronization
The switching frequency of the boost converter can be set in the 250 kHz-1 MHz range by connecting the FSW pin to ground through a resistor. Calculation of the setting resistor is made using equation 5 and should not exceed the 100 k-400 k range.
Equation 5
RFSW =
FSW 2.5
In addition, when the FSW pin is tied to AVCC, the LED7706 uses a default 660 kHz fixed switching frequency, allowing to save a resistor in minimum component-count applications. The FSW pin can also be used as synchronization input, allowing the LED7706 to operate both as master or slave device. If a clock signal with a 220 kHz minimum frequency is applied to this pin, the device locks synchronized (270 mV threshold). An Internal time-out allows synchronization as long as the external clock frequency is greater than 220kHz. Keeping the FSW pin voltage lower than 270 mV for more than 6 s results in device turned off. Normal operation is resumed as soon as FSW rises above the mentioned threshold and the soft-start sequence is repeated. The SYNC pin is a synchronization output and provides a 35 % (typ.) duty-cycle clock when the LED7706 is used as master or a replica of the FSW pin when used as slave. It is used to connect multiple devices in a daisy-chain configuration or to synchronize other switching converters running in the system with the LED7706 (master operation).
Figure 8. Multiple device synchronization
MASTER AVCC
SLAVE
Sync Out FSW SYNC FSW SYNC SYNC
LED7706
RFSW SGND
LED7706
SGND
When an external synchronization clock is applied to the FSW pin, the internal oscillator is overdriven: each switching cycle begins at the rising edge of clock, while the slope compensation ramp starts at the falling edge of the same signal. Thus, the external synchronization clock is required to have a 40 % maximum duty-cycle when the boost converter is working in Continuous-Conduction Mode (CCM). The minimum pulse width which allows the synchronizing pulses to be detected is 270 ns.
16/31
LED7706 Figure 9. External sync waveforms
Operation description - boost section
FSW pin voltage (ext. sync)
270 minimum 250ns 270 300mV threshold
Slave SYNC pin voltage
Slave LX pin voltage
6.6
System stability
The boost section of the LED7706 is a Fixed Frequency, Current-Mode converter. During normal operation, a minimum voltage selection circuit compares all the voltage drops across the active current generators and provides the minimum one to the error amplifier. The output voltage of the error amplifier determines the inductor peak current in order to keep its inverting input equal to the reference voltage (270 mV typ). The compensation network consists of a simple RC series (RCOMP - CCOMP) between the COMP pin and ground. The calculation of RCOMP and CCOMP is fundamental to achieve optimal loop stability and dynamic performance of the boost converter and is strictly related to the operating conditions.
6.6.1
Loop compensation
The compensation network can be quickly calculated using equations 6 through 10. Once both RCOMP and CCOMP have been determined, a fine-tuning phase may be required in order to get the optimal dynamic performance from the application. The first parameter to be fixed is the switching frequency. Normally, a high switching frequency allows reducing the size of the inductor but increases the switching losses and negatively affects the dynamic response of the converter. For most of applications, the fixed value (660 kHz) represents a good trade-off between power dissipation and dynamic response, allowing to save an external resistor at the same time. In low-profile applications, the inductor value is often kept low to reduce the number of turns; an inductor value in the 4.7 H-15 H range is a good starting choice. Even if the loop bandwidth of the boost converter should be chosen as large as possible, it should be set to 20 % of the switching frequency, taking care not to exceed the CCM-mode Right Half-Plane Zero (RHPZ).
17/31
Operation description - boost section Equation 6
LED7706
fU 0.2 FSW
Equation 7
VIN,min VOUT V 2 MR OUT IOUT = 0. 2 fU 0.2 2 L 2 L
2
Where VIN,min is the minimum input voltage and IOUT is the overall output current.
Note: The lower the inductor value (and the lower the switching frequency), the higher the bandwidth can be achieved. The output capacitor is directly involved in the loop of the boost converter and must be large enough to avoid excessive output voltage drop in case of a sudden line transition from the maximum to the minimum input voltages (VOUT should not exceed 50-100 mV): Equation 8
VOUT =
V IOUT 1 - IN _ MIN 2 fU C VIN _ MAX
Once the output capacitor has been chosen, the Equation 9
RCOMP can be calculated as:
R COMP =
Where GM=2.7 S and gEA=375 S
2 fU C GM gEA M
Equation 10 places the loop bandwidth at fU. Then, the CCOMP capacitor is determined to place the frequency of the compensation zero 5 times lower than the loop bandwidth:
Equation 10
C COMP =
Where fZ=fU/5.
1 2 fZ R COMP
The close loop gain function (GLOOP) is thus given by equation 11:
Equation 11
GLOOP = GM gEA
1 R COMP + sC COMP
L 1- s 2 MR RM 1 + sRC
A simple technique to optimize different applications is to replace RCOMP with a 20 k trimmer and adjust its value to properly damp the output transient response. Insufficient
18/31
LED7706
Operation description - boost section
damping will result in excessive ringing at the output and poor phase margin. Figure 10 a and 10 b give an example of compensation adjustment for a typical application.
Figure 10. Poor phase margin (a) and properly damped (b) load transient response
Figure 11. Load transient response measurement set-up
VIN= 6V 6.8H VBST=30÷36V
C
IN
+5V
4.7F MLCC
AVCC
LX
SLOPE
OVSEL
VIN
LDO5 BILIM RILIM SS COM P SGND DIM
FSW ROW1 ROW 2 ROW 3 ROW 4 ROW 5 ROW 6
RL =
VBST 50mA
LED7706
FAULT
MODE
SYNC
PGND
EN
Up to 10 WLEDs per row
500Hz
6.6.2
Slope compensation
The constant frequency, Peak Current-Mode topology has the advantage of very easy loop compensation with output ceramic caps (reduced cost and size of the application) and fast transient response. In addition, the intrinsic peak-current measurement simplifies the current limit protection, avoiding undesired saturation of the inductor. On the other side, this topology has a drawback: there is an inherent open loop instability when operating with a duty-ratio greater than 0.5. This phenomenon is known as "SubHarmonic Instability" and can be avoided by adding an external ramp to the one coming from the sensed current. This compensating technique, based on the additional ramp, is called "Slope Compensation". In Figure 13, where the switching duty-cycle is higher than 0.5, the small perturbation ÄIL dies away in subsequent cycles thanks to the slope compensation and the system reverts to a stable situation.
19/31
Operation description - boost section Figure 12. Main loop and current loop diagram
VI N
LED7706
LX
ROWx
PWM
Minimum voltage drop selector
SGND
RILIM
COMP
gm
0.4V
The SLOPE pin allows to properly set the amount of slope compensation connecting a simple resistor RSLOPE between the SLOPE pin and the output. The compensation ramp starts at 35 % (typ.) of each switching period and its slope is given by the following equation:
Equation 12
V - VIN - VBE SE = K SLOPE OUT R SLOPE
Where KSLOPE = 5.8 x1010S-1, VBE = 2 V (typ.) and SE is the slope ramp in [A/s]. To avoi-d sub-harmonic instability, the compensating slope should be at least half the slope of the inductor current during the off-phase for a duty-cycle greater than 50 % (i.e. at the lowest input voltage). The value of RSLOPE can be calculated according to equation 13.
Equation 13
R SLOPE
2 K SLOPE L (VOUT - VIN - VBE ) (VOUT - VIN )
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LED7706
Operation description - boost section
Figure 13. Effect of slope compensation on small inductor current perturbation (D > 0.5)
Inductor current (CCM) 0.35·TSW
Programmed inductor peak current with slope compensation (SE)
ITRIP
IL
Inductor current perturbation
TSW
t
6.7
Boost current limit
The design of the external components, especially the inductor and the flywheel diode, must be optimized in terms of size relying on the programmable peak current limit. The LED7706 improves the reliability of the final application giving the way to limit the maximum current flowing into the critical components. A simple resistor connected between the BILIM pin and ground sets the desired value. The voltage at the BILIM pin is internally fixed to 1.23 V and the current limit is proportional to the current flowing through the setting resistor, according to the following equation:
Equation 14
IBOOST,PEAK = where
KB R BILIM
K B = 6 10 5 V
The maximum allowed current limit is 5 A, resulting in a minimum setting resistor RBILIM > 120 k. The maximum guaranteed RMS current in the power switch is 2 Arms. The current limitation works by clamping the COMP pin voltage proportionally to RBILIM. Peak inductor current is limited to the above threshold decreased by the slope compensation contribution. In a boost converter the r.m.s. current through the internal MOSFET depends on both the input and output voltages, according to equations 15 a (DCM) and 15 b (CCM).
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Operation description - boost section Equation 15 a
LED7706
IMOS,rms =
VIN D D FSW L 3
Equation 15 b
IMOS,rms = IOUT
2 D VOUT 1 (D(1 - D))3 + (1 - D)2 12 I OUT fSW L
6.8
Thermal protection
In order to avoid damage due to high junction temperature, a thermal shutdown protection is implemented. When the junction temperature rises above 150 C (typ.), the device turns off both the control logic and the boost converter and holds the FAULT pin low. The LDO is kept alive and normal operation is automatically resumed after the junction temperature has been reduced by 30 C.
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LED7706
Backlight driver section
7
7.1
Backlight driver section
Current generators
The LED7706 is a LEDs driver with six channels (rows); each row is able to drive multiple LEDs in series (max. 40 V) and to sink up to 30 mA maximum current, allowing to manage different kinds of LEDs. The LEDs current can be set by connecting an external resistor (RRILIM ) between the RILIM pin and ground. The voltage across the RILIM pin is internally set to 1.23 V and the rows current is proportional to the RILIM current according to the following equation:
Equation 16
IROWx Where KR = 987 V.
=
KR R RILIM
The maximum current mismatch between the rows is 2 % @ Irowx = 20 mA. Due to the spread of the LEDs' forward voltage, the total drop across the LED's strings will be different. The device will manage the unconnected rows according to the MODE pin setting (see Table 7). The LED7706 allows parallelizing different rows if required by the application. If the maximum current provided by a single row (30 mA) is not enough for the load, two or more current generators can be connected together, as shown in Figure 14. The connection between rows in parallel must be done as close as possible to the device in order to minimize parasitic inductance.
Figure 14. rows parallelization for higher current
VIN
AVCC
LDO5 BILIM RILIM SS COMP SGND DIM
SLOPE
OVSEL
SWF ROW1 ROW2 ROW3 ROW4 ROW5 ROW6 PGND
VIN FAULT
LED7706
LX
MODE
SYNC
Dimming Fault Enable Faults Management Selection Sync Output
EN
High Current WLEDs
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Backlight driver section
LED7706
7.2
PWM dimming
The brightness control of the LEDs is performed by a Pulse-Width Modulation of the rows current. When a PWM signal is applied to the DIM pin, the current generators are turned on and off mirroring the DIM pin behavior. Actually, the minimum dimming duty-cycle depends on the dimming frequency.
Figure 15. PWM dimming waveforms
The real limit to the PWM dimming is the minimum on-time that can be managed for the current generators; this minimum on-time is approximately 500 ns. Thus, the minimum dimming duty-cycle depends on the dimming frequency according to the following formula:
Equation 17
DDIM,min = 500ns fDIM
For example, at a dimming frequency of 20 kHz, 1% of dimming duty-cycle can be managed. During the off-phase of the PWM signal the boost converter is paused, the current generators are turned off and the output voltage is frozen across the output capacitor. During the start-up sequence the dimming duty-cycle is forced to 100 % to detect floating rows regardless of the applied dimming signal.
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LED7706
Fault management
8
Fault management
The main loop keeps the row having the lowest voltage drop regulated to about 400 mV. This value slightly depends on the voltage across the remaining active rows. After the softstar t sequence, all protections turn active and the voltage across the active current generators is monitored to detect shorted LEDs.
8.1
FAULT pin
The FAULT pin is an open-collector output, active low, which gives information regarding faulty conditions eventually detected. This pin can be used either to drive a status LED (with a series resistor to not exceed 4 mA current) or to warn the host system. The FAULT pin status is strictly related to the MODE pin setting (see Table 7 for details).
8.2
MODE pin
The MODE pin is a digital input and can be connected to AVCC or SGND in order to choose the desired fault detection and management. The LED7706 can manage a faulty condition in two different ways, according to the application needs. Table 7 summarizes how the device detects and handles the internal protections related to the boost section (OverCurrent, Over-Temperature and Over-Voltage) and to the current generators section (open and shorted LEDs).
Table 7. Faults management summary
FAULT Internal MOSFET over current Output over voltage Thermal shutdown MODE to GND MODE to VCC
FAULT pin HIGH Power MOS turned OFF FAULT pin LOW Device turned OFF, latched condition FAULT pin LOW. Device turned OFF. Automatic restart after 30 C temperature drop. FAULT pin LOW Device turned OFF, latched condition (Vth = 3.4 V) FAULT pin LOW Device turned OFF at first occurrence, latched condition FAULT pin LOW Faulty row(s) disconnected (Vth = 6 V) FAULT pin HIGH Faulty row(s) disconnected
Shorted led
OPEN row(s)
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Fault management
LED7706
8.3
Open LED fault
In case a row is not connected or a LED fails open, the device has two different behaviors according to the MODE pin status. Connecting the MODE pin to SGND, the LED7706 behaves in a different manner: as soon as an open row is detected, the FAULT pin is tied low and the device is turned off. The internal logic latches this status: to restore the normal operation, the device must be restarted by toggling the EN pin or performing a Power-On Reset (POR occurs when the voltage at the LDO5 pin falls below the lower UVLO threshold and subsequently rises above the upper one). If the MODE pin is high (i.e. connected to AVCC), the open row is excluded from the control loop and the device continues to work properly with the remaining rows. The FAULT pin is not affected. Thus, if less than six rows are used in the application, the MODE pin must be set high.
8.4
Shorted LED fault
When a LED is shorted, the voltage across the related current generator increases of an amount equal to the missing voltage drop of the faulty LED. Since the feedback voltage on each active generator is constantly compared with a fault threshold VTH,FAULT, the device detects the faulty condition and acts according to the MODE pin status. If the MODE pin is low, the fault threshold is VTH,FAULT = 3.4 V. When the voltage across a row is higher than this threshold, the FAULT pin is set low and the device is turned off. The internal logic latches this status until the EN pin is toggled or a POR is performed. In case the MODE pin is connected to AVCC, the fault threshold is set to 6 V. The LED7706 simply disconnects the rows whose voltage is higher than the threshold and the FAULT pin is forced low. This option is also useful to avoid undesired triggering of the shorted-LED protection simply due to the high voltage drop spread across the LEDs.
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LED7706
Package mechanical data
9
Package mechanical data
In order to meet environmental requirements, ST offers these devices in ECOPACK packages. These packages have a Lead-free second level interconnect. The category of second Level Interconnect is marked on the package and on the inner box label, in compliance with JEDEC Standard JESD97. The maximum ratings related to soldering conditions are also marked on the inner box label. ECOPACK is an ST trademark. ECOPACK specifications are available at: www.st.com.
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Package mechanical data
LED7706
Table 8.
VFQFPN-24 4 mm x 4 mm mechanical data
mm Dim. Min A A1 A3 b D D2 E E2 e L ddd 0.30 0.18 3.85 2.40 3.85 2.40 0.80 0.00 Typ 0.90 0.02 0.20 0.25 4.00 2.50 4.00 2.50 0.50 0.40 0.50 0.08 0.30 4.15 2.60 4.15 2.60 Max 1.00 0.05
Figure 16. Package dimensions
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LED7706
Package mechanical data
Table 9.
VFQFPN-24 4 mm x 4 mm footprint
mm Dim. Min X Y 0.69 2.78 2.93 4.31 2.63 Typ Max 0.28
ADmax=AEmax GDmin=GEmin ZDmax=ZEmax D2'=E2'
Figure 17. Footprint
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Revision history
LED7706
10
Revision history
Table 10.
Date 08-Feb-2008
Document revision history
Revision 1 Initial release. Changes
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LED7706
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