LED7707 6-rows 85 mA LEDs driver with boost regulator for LCD panels backlight Features ■ ■ Boost section – 4.
Contents LED7707 Contents 1 Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 2.1 Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Electrical data . . . . . .
LED7707 6 Contents Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 6.1 System stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 6.1.1 Loop compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 6.2 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.3 Component selection . . . . . . . . . . . . . . . . .
Typical application circuit LED7707 1 Typical application circuit Figure 1.
LED7707 Pin settings 2 Pin settings 2.1 Connections 19 18 24 COMP LX DIM EN FAULT SYNC SS Pin connection (through top view) 1 OVSEL RILIM PGND BILIM ROW6 LED7706 FSW ROW5 MODE ROW4 13 6 ROW3 ROW2 ROW1 SGND 12 SLOPE 7 VIN AVCC LDO5 Figure 2.
Pin settings 2.2 LED7707 Pin description Table 2. 6/47 Pin functions N° Pin Function 1 COMP Error amplifier output. A simple RC series between this pin and ground is needed to compensate the loop of the boost regulator. 2 RILIM Output generators current limit setting. The output current of the rows can be programmed connecting a resistor to SGND. 3 BILIM Boost converter current limit setting. The internal MOSFET current limit can be programmed connecting a resistor to SGND.
LED7707 Electrical data 3 Electrical data 3.1 Maximum rating Table 3. Absolute maximum ratings (1) Symbol Parameter Value VAVCC AVCC to SGND -0.3 to 6 VLDO5 LDO5 to SGND -0.3 to 6 PGND to SGND -0.3 to 0.3 VIN VIN to PGND -0.3 to 40 VLX LX to SGND -0.3 to 40 LX to PGND -0.3 to 40 RILIM, BILIM, SYNC, OVSEL, SS to SGND V -0.3 to VAVCC + 0.3 EN, DIM, SW, MODE, FAULT to SGND -0.3 to 6 ROWx to PGND/ SGND -0.3 to 40 VIN - 0.3 to VIN + 6 SLOPE to VIN SLOPE to SGND -0.
Electrical characteristics 4 LED7707 Electrical characteristics VIN = 12 V; TJ = 25 °C and LDO5 connected to AVCC if not otherwise specified (a) Table 5. Electrical characteristics Symbol Parameter Test condition Min. Typ. Max. Unit 36 V 36 V 5.5 V Supply section VIN Input voltage range 4.5 VBST Boost section output voltage VLDO5 LDO output and IC supply voltage EN high ILDO5 = 0 mA Operating quiescent current RRILIM = 51 kΩ, RBILIM = 220 kΩ, RSLOPE = 680 kΩ DIM tied to SGND.
LED7707 Table 5. Electrical characteristics Electrical characteristics (continued) Symbol Parameter Test condition Min. Typ. Max. Unit Power switch KB RDS(on) LX current coefficient RBILIM = 600 kΩ 1⋅106 1.2⋅106 1.4⋅106 280 Internal MOSFET on-resistance 500 V mΩ OC and OV protections VTH,OVP Over-voltage protection reference threshold 1.145 V Soft-start and power management EN, Turn-on threshold 1.6 EN, Turn-off threshold 0.8 DIM, high level threshold 1.
Operation description 5 LED7707 Operation description The device can be divided into two sections: the boost section and the backlight driver section. These sections are described in the next paragraphs. Figure 3 provides an overview of the internal blocks of the device. Figure 3. Simplified block diagram VIN SLOPE Current Sense +5V LDO + + UVLO Detector UVLO + 0.
LED7707 Operation description 5.1 Boost section 5.1.1 Functional description The LED7707 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 (700 mV typ.). The input voltage range is from 4.
Operation description LED7707 A dedicated circuit automatically selects the lowest voltage drop among all the rows and provides this voltage to 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.
LED7707 Operation description Figure 7. External sync waveforms DIM BAS69 EN 220k LED7707 100n SGND AM00584v1 5.1.3 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 μA (typ.
Operation description LED7707 reacts by increasing the output voltage. When it reaches the floating row detection (FRD) threshold (which coincides with the OVP threshold, see Section 5.1.4), the floating rows are managed according to Table 6 (see Section 5.3 on page 21). 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 to equation 2. Equation 2 C SS ≅ ISS ⋅ t SS 2 .
LED7707 Operation description Equation 4 RFSW = FSW 2.5 In addition, when the FSW pin is tied to AVCC, the LED7707 uses a default 660 kHz fixed switching frequency, allowing to save a resistor in minimum component-count applications. Figure 10. Multiple device synchronization SLAVE MASTER AVCC Sync Out FSW SYNC LED7707 RFSW SGND FSW SYNC SYNC LED7707 SGND AM00587v1 The FSW pin can also be used as synchronization input, allowing the LED7707 to operate both as master or slave device.
Operation description LED7707 Figure 11. External sync waveforms 270ns minimum FSW pin voltage (ext. sync) 270mV threshold Slave SYNC pin voltage Slave LX pin voltage AM00588v1 5.1.6 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.
LED7707 Operation description Equation 6 RSLOPE ≤ 2 ⋅ K S ⋅ L ⋅ (VOUT − VIN − VBE ) (VOUT − VIN ) Figure 12. Effect of slope compensation on small inductor current perturbation (D > 0.5) Inductor current (CCM) Programmed inductor peak current with slope compensation (SE) 0.
Operation description 5.1.7 LED7707 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 LED7707 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.
LED7707 Operation description 5.2 Backlight driver section 5.2.1 Current generators The LED7707 is a LEDs driver with six channels (rows); each row is able to drive multiple LEDs in series (max. 36 V) and to sink up to 85 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.
Operation description LED7707 Figure 14.
LED7707 5.2.2 Operation description 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. 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 10 μs.
Operation description 5.3.1 LED7707 FAULT pin The FAULT pin is an open-collector output, (with 4 mA current capability) active low, which gives information regarding faulty conditions eventually detected. This pin can be used either to drive a status LED or to warn the host system. The FAULT pin status is strictly related to the MODE pin setting (see Table 6 for details). 5.3.
LED7707 Operation description Figure 16 shows an example of open channel detection in case of MODE connected to AVCC. At the point marked as “1” in Figure 16, the row opens (row current drops to zero). From this point on the output voltage is increased as long as the output voltage reaches the floating row detection threshold (see Section 5.1.3 on page 13). Then (point marked as “2”) the faulty row is disconnected and the device keeps on working only with the remaining rows. Figure 16.
Operation description LED7707 At the point marked as “1” in Figure 17 one LED fails becoming a short-circuit. The voltage across the current generator of the channel where the failed LED is connected increases by an amount equal to the forward voltage of the faulty LED. Since the voltage across the current generator is above the threshold (4 V), the device is turned off and the fault pin is set low (point “2”).
LED7707 Application information 6 Application information 6.1 System stability The boost section of the LED7707 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 (700 mV typ).
Application information LED7707 Equation 13b R= VOUT IOUT Where VIN,min is the minimum input voltage and IOUT is the overall output current. Note that, the lower the inductor value (and the higher 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.
LED7707 Application information Figure 18. Poor phase margin (a) and properly damped (b) load transient responses a) b) Figure 19. Load transient response measurement set-up 6.8μH VIN= 12V C VBOOST IN 2 x 4.
Application information 6.2 LED7707 Thermal considerations In order to prevent the device from exceeding the thermal shutdown threshold (150 °C), it is important to estimate the junction temperature through the following equation: Equation 17 T J = T A + R th ,JA ⋅ PD ,tot where TA is the ambient temperature, Rth,JA is the equivalent thermal resistance junction to ambient and PD,tot is the power dissipated by the device.
LED7707 Application information Equation 22 PGEN = IROW ⋅ (nROWs − 1) ⋅ (VIFB + ΔVf,LEDs ⋅ nLEDs ) ⋅ DDIM where nROWs is the number of active rows, ΔVf,LEDs is the spread of the LEDs forward voltage and nLEDs is the number of LEDs per row. d) LDO losses, due to the dissipation of the 5 V linear regulator: Equation 23 PD,LDO = (VIN − VLDO ) ⋅ ILDO The LED7707 is housed in a 24 leads 4x4-VFQFPN package with exposed pad that allows good thermal performance.
Application information LED7707 6.3 Component selection 6.3.1 Inductor selection Being the LED7707 mostly dedicated to backlighting, real-estate applications dictate severe constrain in selecting the optimal inductor. The inductor choice must take into account different parameters like conduction losses (DCR), core losses (ferrite or iron-powder), saturation current and magnetic-flux shielding (core shape and technology).
LED7707 6.3.3 Application information Flywheel diode selection The flywheel diode must be a Schottky type to minimize the losses. This component is subject to an average current equal to the output one and must sustain a reverse voltage equal to the maximum output rail voltage. Considering all the channels sinking 75 mA each (i.e. 450 mA output current) and the maximum output voltage (36 V), the STP1L40M (If,ave = 1 A, Vr = 40 V) diode is a good choice.
Application information LED7707 Equation 25 R 0 ⋅ D ⋅ (1 − D) 2 ⋅ FSW 2 LB = where D is the duty-cycle defined as: Equation 26 D = 1− ⎧ 0.59 @ VIN,min = 10.8V VIN =⎨ VOUT ⎩0.50 @ VIN,max = 13.2V whereas R0 is: Equation 27 R0 = VOUT = 74Ω IOUT and Equation 28 IOUT = 6 ⋅ IROW = 360mA The output voltage in the above calculations is considered as the maximum value (LED with the maximum forward voltage connected to the leading generator): Equation 29 VOUT,max = 7 ⋅ VF,LEDs,max + 700mV = 26.
LED7707 6.4.4 Application information Output capacitor choice The choice of the output capacitor is mainly affected by the desired output voltage ripple. Since the voltage across the LEDs can be considered almost constant, this ripple is transferred across the current generators, affecting their dynamic response.
Application information LED7707 TOFF can be calculated as: Equation 35 ⎧ 569.7ns @ VIN,min = 10.8V TOFF = TSW ⋅ D 2 = ⎨ ⎩618.2ns @ VIN,max = 13.2V defining D2 as: Equation 36 D2 = 2 ⋅ FSW ⋅ L ⋅ M ⎧ 0.376 @ VIN,min = 10.8V =⎨ R 0 ⋅ (M − 1) ⎩0.408 @ VIN,max = 13.2V The worst case for the output voltage ripple is when input voltage is lower (VIN,min = 10.8 V). A simple way to select the COUT value is fixing a maximum voltage ripple.
LED7707 6.4.6 Application information Over-voltage protection divider setting The over-voltage protection (OVP) divider provides a partition of the output voltage to the OVSEL pin. The OVP divider setting not only fixes the OVP threshold, but also the openchannel detection threshold. The proper OVP divider setting can be calculated by the equation (3): Equation 38 R 2 = R1 ⋅ 1.145 V VOUT,MAX + 4V − 1.
Application information 6.4.9 LED7707 Power dissipation estimate As explained in section 5.2, there are several contributions to the total power dissipation. Neglecting the power dissipated by the LDO (surely less significant compared with the other contributions), equation (18), (20), (21) and (22) help to estimate the overall power dissipation.
LED7707 Application information In order to estimate also the efficiency, other contributions to the power dissipation must be added to PD, tot (which represents only the power dissipated by the device), that is: Equation 47 PDISS,Diode = VF,Diode ⋅ IIN ⋅ D 2 = 133mW where VF, Diode = 0.4 V Equation 48 2 2 PDISS,Ind = DCR ⋅ IInd ,RMS ≅ DCR ⋅ IIN = 63mW where DCR = 80 mΩ (typical DCR of the recommended inductors).
Application information 6.5 LED7707 Layout consideration 1. A careful PCB layout is important for proper operation. In this section some guidelines are provided in order to achieve a good layout. 2. The device has two different ground pins: signal ground (SGND) and power ground (PGND). The PGND pin handles the switching current related to the boost section; for this reason the PCB traces should be kept as short as possible and with adequate width. 3.
LED7707 Application information Figure 22. Demonstration board layout (top view) Figure 23. Demonstration board layout (bottom view) Figure 24 shows the LED7707 demonstration board application circuit, whereas Table 9 lists the used components and their value.
Application information LED7707 Figure 24. LED7707 demonstration board schematic Table 9. LED7707 demonstration board component list Component Description Package Part number MFR Value C1 Ceramic, 35 V X5R, 20 % SMD 1210 UMK325BJ106KM-T Taiyo Yuden 10 µF SMD 1206 GRM31CR71H475KA88B C2,C3 C4 Ceramic, 50 V X7R, 20 % 4.7 µF Murata SMD 1206 GRM31CR71H225KA88B N.M. C5 1 µF C6 100 nF C7 3.3 nF C8 C9 SMD 0603 Standard N.M. C10 220 pF C11 4.7 nF C12 N.M.
LED7707 Table 9. Application information LED7707 demonstration board component list (continued) Component Description Package Part number MFR Value R8 680 kΩ R9, R10 100 kΩ R11 1.2 kΩ R12 N.M. R13 N.M. L1 6u8, 60 mΩ, 5.8 A 7x7 mm XPL7030-682ML Coilcraft 6.8 µF D1 Schottky, 40 V, 1 A DO216-AA STPS1L40M ST STPS1L40M D2 Red LED, 3 mA SMD 0603 Standard D3 Signal Schottky SOD-523 BAS69 N.M.
Electrical characteristics 7 LED7707 Electrical characteristics Figure 25. Efficiency versus DIM duty cycle, VIN = 12 V, 6 rows, 10 white LEDs (60 mA) in series, FSW = 660 kHz Figure 26. Efficiency versus DIM duty cycle, VIN = 18 V, 6 rows, 10 white LEDs (60 mA) in series, FSW = 660 kHz Figure 27. Efficiency versus DIM duty cycle, VIN = 24 V, 6 rows, 10 white LEDs (60 mA) in series, FSW = 825 kHz Figure 28.
LED7707 Electrical characteristics Figure 29. Soft-start waveforms (EN, SS, and VOUT monitored) Figure 30. Boost section switching signals (LX, SYNC and inductor current monitored), VIN = 12 V, 10 LEDs Figure 31. Dimming waveforms (FDIM = 200 Hz) Figure 32.
Package mechanical data 8 LED7707 Package mechanical data In order to meet environmental requirements, ST offers these devices in different grades of ECOPACK® packages, depending on their level of environmental compliance. ECOPACK® specifications, grade definitions and product status are available at: www.st.com. ECOPACK is an ST trademark.
LED7707 Package mechanical data Table 10. VFQFPN-24 4 mm x 4 mm mechanical data mm Dim. Min Typ Max A 0.80 0.90 1.00 A1 0.00 0.02 0.05 A3 0.20 b 0.18 0.25 0.30 D 3.85 4.00 4.15 D2 2.40 2.50 2.60 E 3.85 4.00 4.15 E2 2.40 2.50 2.60 e L 0.50 0.30 ddd 0.40 0.50 0.08 Figure 33.
Revision history 9 LED7707 Revision history Table 11.
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