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 Table 1. Device summary Order code Package LED7706 Packaging Tube VFQFPN-24 4x4 (exposed pad) LED7706TR February 2008 Tape and reel Rev 1 1/31 www.st.com 31 Contents LED7706 Contents 1 Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3 2.1 Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1 Maximum rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.2 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.3 Recommended operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6 Operation description - boost section . . . . . . . . . . . . . . . . . . . . . . . . . 11 2/31 6.1 Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.2 Enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.3 Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.4 Over voltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 6.5 Switching frequency selection and synchronization . . . . . . . . . . . . . . . . . 15 6.6 System stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6.6.1 Loop compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6.6.2 Slope compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.7 Boost current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.8 Thermal protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 LED7706 7 8 Contents Backlight driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 7.1 Current generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 7.2 PWM dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Fault management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 8.1 FAULT pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 8.2 MODE pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 8.3 Open LED fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 8.4 Shorted LED fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 9 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3/31 VIN- DIM EN FAULT AVCC MODE SW3 Ccomp Rcomp Css Rf sw Cldo5 Cav cc AVCC Rf ilt AVCC SS COMP MODE DIM EN FAULT LDO5 AVCC SW2 FSW 24 1 5 20 21 22 7 6 23 SYNC PM6600 LED7706 Rrilim FSW 4 8 VIN Cin 2 L 19 THPD ROW6 ROW5 ROW4 ROW3 ROW2 ROW1 PGND SLOPE OVSEL LX 4/31 RILIM Rbilim 25 16 15 14 13 12 11 17 9 18 D C13 C10 R1 R2 Rslope Cout VBOOST Application circuit 3 Figure 1. BILIM Typical application circuit SGND 1 10 VIN+ Typical application circuit LED7706 LED7706 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. 5/31 Pin settings 2.2 LED7706 Pin description Table 2. Pin functions N° Pin 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. 4 FSW 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. 5 MODE Current generators fault management selector. It allows to detect and manage LEDs failures. See Section 8.2 on page 25 for details. 6 AVCC +5 V analog supply. Connect to LDO5 through a simple RC filter. 7 LDO5 Internal + 5V LDO output and power section supply. Bypass to SGND with a 1 µF ceramic capacitor. 8 VIN 9 SLOPE 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. 10 SGND Signal ground. Supply return for the analog circuitry and the current generators. 11 ROW1 Row driver output #1. 12 ROW2 Row driver output #2. 13 ROW3 Row driver output #3. 14 ROW4 Row driver output #4. 15 ROW5 Row driver output #5. 16 ROW6 Row driver output #6. 17 PGND Power ground. Source of the internal Power MOSFET. 18 OVSEL Over-voltage selection. Used to set the desired OV threshold by an external divider. See Section 6.4 on page 15 for details. 19 LX Switching node. Drain of the internal Power MOSFET. 20 DIM Dimming input. Used to externally set the brightness by using a PWM signal. 21 EN 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. Input voltage. Connect to the main supply rail. 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). 22 6/31 Function 23 SYNC 24 SS Synchronization output. Used as external synchronization output. Soft-start. Connect a capacitor to SGND to set the desired soft-start duration. LED7706 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 PTOT Unit -0.3 to 40 Maximum LX RMS current 2.0 A Power dissipation @ TA = 25 °C 2.3 W ±2000 V Maximum withstanding voltage range test condition: CDF-AEC-Q100-002- “Human Body Model” acceptance criteria: “Normal Performance” 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 Thermal data Parameter Value Unit 42 °C/W RthJA Thermal resistance junction to ambient TSTG Storage temperature range -20 to 125 °C Junction operating temperature range -50 to 150 °C TJ 7/31 Electrical data 3.3 LED7706 Recommended operating conditions Table 5. Recommended operating conditions Value Symbol VIN VBST 8/31 Parameter Input voltage range Unit Min Max 4.5 36 V Boost section output voltage 36 FSW sync input Duty-Cycle 40 % fSW Switching frequency 250 1000 kHz Irowx rows output current 5 30 mA LED7706 Electrical characteristics 4 Electrical characteristics Table 6. Electrical characteristics (VIN = 12 V; TA = 0 °C to 85 °C and LDO5 connected to AVCC if not otherwise specified) Symbol Parameter Test condition Min Typ Max Unit 4.4 5 5.5 V Supply section 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. 1 IIN,SHDN Operating current in shutdown EN low 20 30 VUVLO,ON LDO5 Under Voltage Lock Out upper threshold 3.8 4.0 VUVLO,OFF LDO5 Under Voltage Lock Out lower threshold VAVCC IIN,Q mA µA V 3.3 3.6 LDO linear regulator Line regulation 6 V ≤ VIN ≤ 28 V, ILDO5 = 30 mA LDO dropout voltage VIN = 4.3 V, ILDO5 = 10 mA LDO maximum output current VLDO5 > VUVLO,ON 25 25 VLDO5 < VUVLO,OFF 80 120 40 60 20 30 mV mA Boost section tON,min Minimum switching on-time fSW Default switching frequency FSW connected to AVCC 570 Minimum FSW sync frequency 660 200 ns 750 kHz 210 FSW sync input threshold 240 FSW sync input hysteresis 300 mV 20 FSW sync min ON time SYNC output Duty-Cycle FSW connected to AVCC (Internal oscillator selected) SYNC output high level ISYNC = 10 µA SYNC output low level ISYNC = -10 µA 34 270 ns 40 % VAVCC -20V mV 20 Power switch KB RDSon LX current coefficient Internal MOSFET on-resistance RBILIM = 600 kΩ 5E5 6E5 7E5 V 280 500 mΩ 9/31 Electrical characteristics Table 6. LED7706 Electrical characteristics (continued) (VIN = 12 V; TA = 0 °C to 85 °C and LDO5 connected to AVCC if not otherwise specified) Symbol Parameter Test condition Min Typ Max Unit OC and OV protections VTH,OVP Over voltage protection reference threshold (OVSEL) 1.190 1.234 1.280 VTH,FRD Floating channel detection threshold 1.100 1.145 1.190 ∆VOVP,FRD V Voltage gap between OVP and FRD thresholds 90 mV Soft-start and power management EN, Turn-On level threshold 1.4 EN, Turn-off level threshold 0.8 DIM, high level threshold 1.0 1.15 DIM, low level threshold 0.8 EN, pull-up current 1.6 1.3 1.09 2.5 SS, charge current SS, end-of-startup threshold SS, reduced switching frequency release threshold V 4 5 6 2.0 2.4 2.8 µA V 0.8 Current generators section TDIM- Minimum dimming on-time ON,min ƒIrowx,1 rows current accuracy (1) ƒIrowx,2 mismatch(2) rows current VIFB Feedback regulation voltage Vrowx, Shorted LED fault detection threshold FAULT TMASK LED short circuit detection masking time VFAULT, FAULT pin Low-Level voltage LOW 500 RRILIM = 51 kΩ KR = 987 Irowx,nom = KR/RRILIM ns ±2.0 +4.0 400 MODE tied to SGND 3.4 MODE connected to AVCC 6.0 mV V µs 100 IFAULT,SINK = 4 mA 200 % 350 mV Thermal shutdown TSHDN Thermal shutdown turn-off temperature 150 Thermal shutdown hysteresis 30 1. Current accuracy calculated as ∆Irowx,1 = (Irowx-Irowx,NOM) / Irowx,NOM 2. Current Mismatch calculated as ∆Irowx,2 = |Irowx-Irowy| / Irowx,NOM 10/31 °C LED7706 Block diagram 5 Block diagram Figure 3. Functional and block diagram VIN SLOPE Current Sense LDO5 +5V LDO Ramp Generator + + + UVLO Detector Boost Control Logic _ UVLO + gm _ COMP LX ZCD 0.4V PGND Boost_EN BILIM SS Current Limit _ FRD + _ OVP Min Voltage Selector Soft Start CTRL6 ÷2 1.234V Current Generator 6 ROW6 Current Generator 5 ROW5 Current Generator 4 ROW4 Current Generator 3 ROW3 Current Generator 2 ROW2 VROW5 CTRL5 Ext Sync Detector OVSEL VROW6 Prot_EN SYNC 1.172V + VROW4 OSC CTRL4 VROW3 FSW CTRL3 VROW2 Prot_EN Boost_EN AVCC UVLO EN MODE CTRL2 CONTROL LOGIC CTRL6 CTRL5 CTRL4 CTRL3 CTRL2 VTH,FLT 6V 3.4V MODE CTRL1 LOGIC VROW1 ROW1 OVP FAULT FRD I to V + _ DIM I to V Thermal Shutdown Current Generator 1 1.2V RILIM SGND 11/31 Operation description - boost section LED7706 6 Operation description - boost section 6.1 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 LED7706 4R7 AVCC 1u 100n 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 NROWS mLEDS VOUT = max ( i=1 Σ VF,j ) + 400mV j=1 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 6.2 Operation description - boost section 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 µA 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 µA 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 LED7706 100n SGND 13/31 Operation description - boost section 6.3 LED7706 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 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 SS pin voltage AVCC Protections turn active 2.3V 4 1.2V 0.8V Nominal switching frequency release tss Current limit 100% 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 µA and tSS is the desired soft-start duration. 14/31 LED7706 6.4 Operation description - boost section 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 MΩ) 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 Figure 7. R2 R1 OVP threshold setting VIN VOUT LX R1 COUT OVSEL LED7706 R2 CF SGND 15/31 Operation description - boost section 6.5 LED7706 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 kΩ-400 kΩ 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 µs 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 SLAVE MASTER AVCC Sync Out FSW SYNC LED7706 RFSW SGND FSW SYNC SYNC 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 Operation description - boost section Figure 9. External sync waveforms 270 250ns minimum FSW pin voltage (ext. sync) 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 µH-15 µH 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 LED7706 Equation 6 fU ≤ 0.2 ⋅ FSW Equation 7 2 ⎛ VIN,min ⎞ ⎛ VOUT ⎜ ⎟ ⎜ ⎜ V ⎟ ⎜ 2 M R OUT ⎠ ⎝ IOUT ⎝ = 0. 2 ⋅ fU ≤ 0.2 ⋅ 2π ⋅ L 2π ⋅ L ⎞ ⎟ ⎟ ⎠ 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 ⎜ 2π ⋅ fU ⋅ C ⎝ VIN _ MAX Once the output capacitor has been chosen, the ⎞ ⎟ ⎟ ⎠ RCOMP can be calculated as: Equation 9 R COMP = 2π ⋅ fU ⋅ C GM ⋅ gEA ⋅ M Where GM=2.7 S and gEA=375 µS 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 = 1 2π ⋅ fZ ⋅ R COMP Where fZ=fU/5. 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 ⎞ M R ⎟ ⋅ RM ⎟ 1 + sRC ⎠ A simple technique to optimize different applications is to replace RCOMP with a 20 kΩ 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 6.8μH VIN= 6V C VBST=30÷36V IN 4.7μF MLCC FSW OVSEL LX LDO5 SLOPE VIN AVCC +5V ROW1 RILIM PGND ROW6 MODE SGND SYNC ROW5 EN ROW4 COMP FAULT VBST 50mA ROW3 LED7706 SS DIM 6.6.2 RL = ROW2 BILIM Up to 10 WLEDs per row 500Hz 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 LED7706 Figure 12. Main loop and current loop diagram VIN ROWx LX SGND PWM COMP gm Minimum voltage drop selector RILIM 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 ≤ 20/31 2 ⋅ K SLOPE ⋅ L ⋅ (VOUT − VIN − VBE ) (VOUT − VIN ) LED7706 Operation description - boost section Figure 13. Effect of slope compensation on small inductor current perturbation (D > 0.5) Inductor current (CCM) Programmed inductor peak current with slope compensation (SE) 0.35·TSW ITRIP ∆IL Inductor current perturbation t TSW 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 = KB R BILIM where K B = 6 ⋅ 10 5 V The maximum allowed current limit is 5 A, resulting in a minimum setting resistor RBILIM > 120 kΩ. 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). 21/31 Operation description - boost section LED7706 Equation 15 a IMOS,rms = VIN ⋅ D D FSW ⋅ L 3 Equation 15 b IMOS,rms = IOUT 6.8 2 ⎞ ⎛ D ⎞ VOUT 1 ⎛ ⎜ ⎜ ⎟ (D(1 − D))3 ⎟ + ⎟ ⎟ ⎜ (1 − D)2 12 ⎜ I ⎝ OUT ⋅ fSW ⋅ L ⎠ ⎠ ⎝ 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. 22/31 LED7706 Backlight driver section 7 Backlight driver section 7.1 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 KR R RILIM Where KR = 987 V. 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 SWF OVSEL LDO5 SLOPE LX VIN AVCC VIN ROW1 ROW2 BILIM RILIM ROW3 LED7706 PGND MODE ROW6 SYNC SGND EN ROW5 FAULT ROW4 DIM SS COMP High Current WLEDs Dimming Fault Faults Management Selection Enable Sync Output 23/31 Backlight driver section 7.2 LED7706 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. 24/31 LED7706 8 Fault management 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 softstart 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 MODE to GND MODE to VCC Internal MOSFET over current FAULT pin HIGH Power MOS turned OFF Output over voltage FAULT pin LOW Device turned OFF, latched condition Thermal shutdown FAULT pin LOW. Device turned OFF. Automatic restart after 30 C temperature drop. Shorted led FAULT pin LOW Device turned OFF, latched condition (Vth = 3.4 V) FAULT pin LOW Faulty row(s) disconnected (Vth = 6 V) OPEN row(s) FAULT pin LOW Device turned OFF at first occurrence, latched condition FAULT pin HIGH Faulty row(s) disconnected 25/31 Fault management 8.3 LED7706 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. 26/31 LED7706 9 Package mechanical data 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. 27/31 Package mechanical data Table 8. LED7706 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 Figure 16. Package dimensions 28/31 0.40 0.50 0.08 LED7706 Package mechanical data Table 9. VFQFPN-24 4 mm x 4 mm footprint mm Dim. Min Typ X 0.28 Y 0.69 ADmax=AEmax GDmin=GEmin Max 2.78 2.93 ZDmax=ZEmax 4.31 D2’=E2’ 2.63 Figure 17. Footprint 29/31 Revision history 10 LED7706 Revision history Table 10. 30/31 Document revision history Date Revision 08-Feb-2008 1 Changes Initial release. LED7706 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. 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