LED7707 STMicroelectronics, LED7707 Datasheet

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... LED7707TR April 2009 6-rows 85 mA LEDs driver with boost regulator Description The LED7707 consists of a high efficiency monolithic boost converter and six controlled current generators (rows) specifically designed to supply LEDs arrays used in the backlighting of LCD panels. The device can manage an output voltage ...

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... Fault management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5.3.1 5.3.2 5.3.3 5.3.4 2/47 Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Over-voltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Switching frequency selection and synchronization . . . . . . . . . . . . . . . 14 Slope compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Boost current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Thermal protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Current generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 PWM dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 FAULT pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 MODE pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Open LED fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Shorted LED fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 LED7707 ...

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... LED7707 6 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 6.1 System stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 6.1.1 6.2 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.3 Component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6.3.1 6.3.2 6.3.3 6.4 Design example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.4.1 6.4.2 6.4.3 6.4.4 6.4.5 6.4.6 6.4.7 6.4.8 6.4.9 6.5 Layout consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 7 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 8 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Loop compensation ...

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... LDO5 Internal MOS OCP BILIM Rows current selection RILIM SS COMP SGND Dimming Fault Enable 4/47 L Slope Compensation OVP selection Switching Frequency selection ROW1 ROW2 ROW3 LED7707 ROW4 ROW5 ROW6 Faults Management Selection Sync Output LED7707 V OUT MLCC WLEDs per row AM00579v1 ...

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... LED7707 2 Pin settings 2.1 Connections Figure 2. Pin connection (through top view COMP RILIM BILIM LED7706 FSW MODE AVCC 6 7 Pin settings 19 18 OVSEL PGND ROW6 ROW5 ROW4 ROW3 13 12 5/47 ...

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... Fault signal output. Open drain output. The pin goes low when a fault condition Section 5.3.1 on page 22 is detected (see Synchronization output. Used as external synchronization output. Soft-start. Connect a capacitor to SGND to set the desired soft-start duration. Function Section 5.1.5 on page 14 for details. for details. for details. for details. for details). LED7707 ...

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... LED7707 3 Electrical data 3.1 Maximum rating Table 3. Absolute maximum ratings Symbol V AVCC to SGND AVCC V LDO5 to SGND LDO5 PGND to SGND V VIN to PGND SGND PGND RILIM, BILIM, SYNC, OVSEL SGND EN, DIM, SW, MODE, FAULT to SGND ROWx to PGND/ SGND SLOPE to VIN SLOPE to SGND Internal switch maximum RMS current ...

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... LDO5 V > V LDO5 UVLO,ON V < V LDO5 UVLO,OFF FSW connected to AVCC FSW connected to AVCC (Internal oscillator selected μA I SYNC = -10 μA I SYNC from 0 °C to +85 °C. Specification over the 0 to +85 ° LED7707 (a) Min. Typ. Max. Unit 4 4 μ 4.0 4 ...

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... LED7707 Table 5. Electrical characteristics (continued) Symbol Parameter Power switch K LX current coefficient B R Internal MOSFET on-resistance DS(on) OC and OV protections Over-voltage protection V TH,OVP reference threshold Soft-start and power management EN, Turn-on threshold EN, Turn-off threshold DIM, high level threshold DIM, low level threshold EN, pull-up current ...

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... Generator 3 Generator ROW2 ROW2 Current Current CTRL2 CTRL2 Generator 2 Generator TH,FLT TH,FLT 4V 4V MODE MODE LOGIC LOGIC Current Current Generator 1 Generator 1 SGND SGND LED7707 LX LX PGND PGND OVSEL OVSEL ROW6 ROW6 ROW5 ROW5 ROW4 ROW4 ROW3 ROW3 ROW2 ROW2 ROW1 ROW1 ...

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... 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 addition, the LED7707 has an internal LDO that supplies the internal circuitry of the device and is capable to deliver mA. The input of the LDO is the VIN pin ...

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... Enable function The LED7707 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 ...

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... This voltage becomes the inverting input of the error amplifier through the minimum voltage drop selector (see Figure 5 DIM BAS69 LED7707 EN 220k 100n Floating ROWs detection Floating ROWs detection 95% of ...

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... Section 5.3 on page 21 ⋅ I ≅ the desired soft-start duration 145 = ⋅ OUT , MAX (typically in the 100 pF-330 pF range) may be required to F Figure OVSEL LED7707 SGND Section 5.1.4 ), the floating rows are ). After the SS voltage reaches − 145 OUT C OUT LED7707 AM00586v1 AM00586v1 ...

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... The SYNC pin is a synchronization output and provides (typ.) duty-cycle clock when the LED7707 is used as master or a replica of the FSW pin when used as slave used to connect multiple devices in a daisy-chain configuration or to synchronize other switching converters running in the system with the LED7707 (master operation) ...

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... L between the SLOPE pin and the output. The compensation ramp SLOPE ⎛ V ⎜ = OUT S K ⎜ ⎝ (typ.) and S BE 270ns minimum ⎞ − − ⎟ ⎟ ⎠ SLOPE is the slope ramp in [A/s]. E LED7707 AM00588v1 ...

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... LED7707 Equation 6 Figure 12. Effect of slope compensation on small inductor current perturbation (D > 0.5) Inductor current (CCM) Inductor current (CCM) ·T ·T 0.35 0. Inductor current Inductor current ΔI ΔI perturbation perturbation ⋅ ⋅ ⋅ − ≤ S OUT IN R SLOPE − OUT IN Programmed inductor peak current with ...

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... 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 ...

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... 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 and to sink maximum current, allowing to manage different kinds of LEDs. The LEDs current can be set by connecting an external resistor (R pin and ground ...

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... Operation description Figure 14. Rows parallelism for higher current V IN Dimming Fault Enable 20/47 VCC ROW0 ROW1 BILIM ROW2 RILIM LED7707 ROW3 SS ROW4 COMP SGND ROW5 Faults Management Selection Sync Output LED7707 High Current WLEDs AM00591v1 ...

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... LED7707 5.2.2 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 ...

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... Thus, if less than six rows are used in the application, the MODE pin must be set high. Connecting the MODE pin to SGND, the LED7707 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 ...

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... In case the MODE pin is connected to AVCC, the LED short-circuit protection is disabled. The LED7707 simply keeps on regulating the set current without affecting the FAULT pin. Despite the higher power dissipation, this option is 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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... Note that, once a new dimming cycle starts (point “3”), the device waits the masking time (approximately 100 μs) and then sets the FAULT pin low and turns off. Figure 17. Shorted LED detection (MODE to GND)   24/47 Figure 17 one LED fails becoming a short-circuit. The voltage masking time masking time LED7707 ...

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... 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) ...

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... COMP ⋅ π with a 10 kΩ trimmer adjusting its value to properly damp the output is the overall output current. OUT ) ⋅ OUT OFF OUT , max is the peak L, peak can be calculated as: COMP ⋅ ⋅ ⋅ capacitor is determined to COMP 1 ⋅ COMP COMP LED7707 = 4.7 nF ...

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... LED7707 Figure 18. Poor phase margin (a) and properly damped (b) load transient responses a) Figure 19. Load transient response measurement set- 12V +5V 6.8μH VCC ROW1 BILIM ROW2 LED7707 ROW3 RILIM ROW4 SS ROW5 COMP SGND ROW6 Application information b) V BOOST 2 x 4.7μF MLCC WLEDs per row ...

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... DS(on) = ⋅ cond DSon V = − OUT = ⋅ ⋅ OUT GEN , Master ROW ⋅ tot Section 6.5 ⋅ ⋅ DIM ⋅ ⋅ DIM 2 ⋅ ⋅ IFB DIM is the voltage across the IFB LED7707 ) is ...

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... LEDs d) LDO losses, due to the dissipation of the 5 V linear regulator: Equation 23 The LED7707 is housed leads 4x4-VFQFPN package with exposed pad that allows good thermal performance. However it is also important to design properly the demonstration board layout in order to assure correct heat dissipation. Figure 20 shows a picture of the LED7707 application demonstration board taken using an infrared camera. The chip temperature, in those application conditions, is kept below 50 ° ...

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... As in most of boost converters, the input capacitor is less critical, although it is necessary to reduce the switching noise on the supply rail. The input capacitor is also important for the internal LDO of the LED7707 and must be kept as close as possible to the chip. The rated voltage of the input capacitor can be chosen according to the supply voltage range µF X5R MLCC is recommended ...

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... Design example In order to help the design of an application using the LED7707, in this section a simple step-by-step design example is provided. A possible application could be the LED backlight in a 17” LCD panel using the LED7707. Here below the possible application conditions are listed: ● ...

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... ⋅ ⎧ − ⎨ ⎩ IN OUT Ω OUT OUT = ⋅ 360 mA OUT ROW = ⋅ 700 OUT , max , F LEDs , max will be at the lower input voltage. Hence < = μ min Section 6.4.4 LED7707 = min = max = the rated RMS current ...

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... LED7707 6.4.4 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. The output ripple can be estimated as (neglecting the contribution of ESR of C ...

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... SW ⎨ ⋅ − 408 ⎩ 0 value is fixing a maximum voltage ripple. OUT lower than 70 mV (i.e. the the voltage across the OUT ( ) − ⋅ > peak OUT C OUT Δ ⋅ OUT , max = @ min = @ max = @ min = @ max IN,min OFF = μ LED7707 = 10.8 V). ...

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... LED7707 6.4.6 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 open- channel detection threshold. The proper OVP divider setting can be calculated by the equation (3): ...

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... D sw OUT ⋅ GEN , Master ROW ( ( ) = ⋅ − ⋅ + Δ ROW ROWs IFB ≅ tot , D cond , D sw GEN = + Amb ⋅ 216 mW DIM + ) t ⋅ 233 mW DIM 2 ⋅ IFB DIM ) ⋅ ⋅ 630 LEDs LEDs DIM + = Master GEN = 25 °C: A ⋅ = ° tot LED7707 ...

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... LED7707 In order to estimate also the efficiency, other contributions to the power dissipation must be added to P (which represents only the power dissipated by the device), that is: D, tot Equation 47 where Diode Equation 48 where DCR = 80 mΩ (typical DCR of the recommended inductors). Therefore the total dissipated power is: ...

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... COMP pin in order to avoid noise issue and instability of the compensation. 7. Noise sensitive signals (i.e. feedbacks and compensation) should be routed as short as possible to minimize noise collection. The LED7707 pinout makes it easy to separate power components (e.g. inductor, diode) from signal ones. 8. The LX switching node should have and adequate width for high efficiency. ...

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... LED7707 Figure 22. Demonstration board layout (top view) Figure 23. Demonstration board layout (bottom view) Figure 24 shows the LED7707 demonstration board application circuit, whereas the used components and their value. Application information Table 9 lists 39/47 ...

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... Application information Figure 24. LED7707 demonstration board schematic Table 9. LED7707 demonstration board component list Component Description Ceramic X5R C2,C3 Ceramic X7R Ceramic X5R C10 C11 C12 C13 Chip resistor 0 Chip resistor R7 0 40/47 Package Part number SMD 1210 UMK325BJ106KM-T SMD 1206 GRM31CR71H475KA88B ...

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... LED7707 Table 9. LED7707 demonstration board component list (continued) Component Description R8 R9, R10 R11 R12 R13 L1 6u8, 60 mΩ, 5 Schottky Red LED Signal Schottky U1 Integrated circuit J2 PCB pad jumper J8 Header 8 SW1, SW2 Jumper 3 SW3 Push button Package Part number 7x7 mm XPL7030-682ML DO216-AA ...

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... Figure 27. Efficiency versus DIM duty cycle rows, 10 white LEDs IN (60 mA) in series, F 42/47 Figure 26. Efficiency versus DIM duty cycle, = 660 kHz SW Figure 28. Efficiency versus DIM duty cycle, = 825 kHz SW LED7707 rows, 10 white LEDs IN (60 mA) in series 660 kHz rows, 10 white LEDs IN (60 mA) in series 825 kHz ...

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... LED7707 Figure 29. Soft-start waveforms (EN, SS, and V monitored) OUT Figure 31. Dimming waveforms (F = 200 Hz) DIM Electrical characteristics Figure 30. Boost section switching signals (LX, SYNC and inductor current monitored Figure 32. Dimming waveforms ( LEDs = 1 kHz) DIM 43/47 ...

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... Package mechanical data 8 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 trademark. 44/47 LED7707 ® ...

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... LED7707 Table 10. VFQFPN- mechanical data Dim ddd Figure 33. Package dimensions mm Min Typ 0.80 0.90 0.00 0.02 0.20 0.18 0.25 3.85 4.00 2.40 2.50 3.85 4.00 2.40 2.50 0.50 0.30 0.40 Package mechanical data Max 1.00 0.05 0.30 4.15 2.60 4.15 2.60 ...

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... Revision history 9 Revision history Table 11. Document revision history Date 18-Sep-2008 20-Oct-2008 10-Apr-2009 46/47 Revision 1 Initial release Updated Table 3 and 2 Removed Table 4 Updated Table 4, Table 3 Table 9 Changes Table 5 5, Figure 3, Figure 4, Figure 8, LED7707 Figure 9 and ...

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... LED7707 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. ...