LED7706 STMicroelectronics, LED7706 Datasheet
LED7706
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... LED7706TR April 2009 6-rows 30 mA LEDs driver with boost regulator Description The LED7706 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 2/46 Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Switching frequency selection and synchronization . . . . . . . . . . . . . . . 15 Slope compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Boost current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Thermal protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Current generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 PWM dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 FAULT pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 MODE pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Open LED fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Shorted LED fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 LED7706 ...
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... LED7706 6 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 6.1 System stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 6.1.1 6.2 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.3 Component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.3.1 6.3.2 6.3.3 6.4 Design example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 7 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 8 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Loop compensation ...
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... Internal MOS OCP Rows current selection Dimming Fault Enable 4/46 L Slope Compensation OVP selection Switching Frequency selection LDO5 ROW1 BILIM ROW2 ROW3 RILIM LED7706 ROW4 SS ROW5 COMP SGND ROW6 Faults Management Selection Sync Output LED7706 V OUT MLCC WLEDs per row AM00594v1 ...
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... LED7706 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 AM00595v1 5/46 ...
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... Fault signal output. Open drain output. The pin goes low when a fault condition Section 5.3.1 on page 21 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 15 for details. for details. for details. for details. for details). LED7706 ...
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... LED7706 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 ° LED7706 (a) Min. Typ. Max. Unit 4 4 μ 3.8 4.0 V 3.3 3 ...
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... LED7706 Table 5. Electrical characteristics (continued) Symbol Parameter Power switch K LX current coefficient B R Internal MOSFET on-resistance DSon OC and OV protections Over voltage protection V TH,OVP reference threshold (OVSEL) Floating channel detection V TH,FRD threshold Voltage gap between OVP and ΔV OVP,FRD FRD thresholds Soft-start and power management ...
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... V V 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 LED7706 LX LX PGND PGND OVSEL OVSEL ROW6 ROW6 ROW5 ROW5 ROW4 ROW4 ROW3 ROW3 ROW2 ROW2 ROW1 ROW1 AM00596v1 ...
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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 (400 mV typ. see input voltage range is from 4 addition, the LED7706 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 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 ...
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... This voltage becomes the inverting input of the error amplifier through the minimum voltage drop DIM DIM BAS69 BAS69 EN EN LED7706 LED7706 100nF 100nF 220kΩ 220kΩ Floating ROWs detection Floating ROWs detection 95% of 95% of ...
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... V threshold. When the voltage on the OVSEL pin exceeds the OV threshold, the FAULT pin is tied low and the device is turned off; this condition is latched and the LED7706 is restarted by togglin g 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) ...
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... Equation 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. Figure 10. Multiple device synchronization ...
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... The SYNC pin is a synchronization output and provides (typ.) duty-cycle clock when the LED7706 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 LED7706 (master operation) ...
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... LED7706 The SLOPE pin allows to properly set the amount of slope compensation connecting a simple resistor R starts at 35% (typ.) of each switching period and its slope is given by the following equation: Equation 6 ⋅ Where KS = 5.8 To avoid sub-harmonic instability, the compensating slope should be at least half the slope of the inductor current during the off-phase when the duty-cycle is greater than 50% ...
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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 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 ...
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... Backlight driver section 5.2.1 Current generators The LED7706 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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... The output voltage can be considered almost constant because of the relatively slow discharge of the output capacitor. During the start-up sequence (see Section 5.1.3 on page 13 regardless of the applied dimming signal. Figure 14. PWM dimming waveforms 20/46 = ⋅ D 500 ns f DIM , min DIM ) the dimming duty-cycle is forced to 100% to detect floating rows LED7706 ...
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... 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. device detects and handles the internal protections related to the boost section (over- current, over-temperature and over-voltage) and to the current generators section (open and shorted LEDs) ...
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... 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 LED7706 behaves in a different manner: the open row is excluded from the control loop and the device continues to work properly with the remaining rows ...
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... 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 The LED7706 simply disconnects the rows whose voltage is higher than the threshold and the FAULT pin is forced low ...
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... Application information 6.1 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 (400 mV typ) ...
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... LED7706 Equation 14b Where V is the minimum input voltage and I IN,min 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 ...
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... Figure 17. Poor phase margin (a) and properly damped (b) load transient responses a) Figure 18. Load transient response measurement set- +5V 26/46 6.8μH LDO5 ROW1 BILIM ROW2 LED7706 ROW3 RILIM ROW4 SS ROW5 COMP SGND ROW6 LED7706 b) V BOOST 4.7μF MLCC 500Hz WLEDs per row V BST 50mA AM00607v1 ...
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... LED7706 6.2 Thermal considerations In order to prevent the device from exceeding the thermal shutdown threshold (150 °C important to estimate the junction temperature through the following equation: Equation 18 where T is the ambient temperature ambient and P D,tot The R measured on the application demonstration board (described in th, ° ...
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... The switching frequency of the LED7706 can be set in the 200 kHz-1 MHz range, allowing a wide selecting room for the inductance value. Low switching frequencies takes to high inductance value, resulting in significant DCR and size. On the other hand, high switching frequencies result in significant core losses. The suggested range is 4.7-22 µ ...
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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 LED7706 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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... Application information 6.4 Design example In order to help the design of an application using the LED7706, in this section a simple step-by-step design example is provided. A possible application could be the LED backlight in a 15” LCD panel using the LED7706. Here below the possible application conditions are listed: ● ...
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... LED7706 Equation 27 whereas R is: 0 Equation 28 and Equation 29 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 30 Considering the input voltage range, the lower L the condition to assure the DCM operation becomes: Equation 31 An inductor value of 6.8 µ ...
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... ⎨ 285 R ⎩ 0 ⎧ 125 @ V = OUT ⎨ 083 @ V ⎩ OFF ⎧ 348 ⋅ = ⎨ OFF SW 2 398 ⎩ ) ⋅ OUT OFF ` C OUT ) calculated as min = @ max = 488 @ min = @ max = min = max = @ min = @ max LED7706 t AM00608v1 ...
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... LED7706 Equation 37 The worst case for the output voltage ripple is when input voltage is lower (V A simple way to select the C In order to affect as less as possible the current generators, it would be better to fix the maximum ripple lower than the typical voltage across the generators. ...
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... Therefore, using equation (8) and choosing I Equation 41 34/46 = ⋅ nLED V 400 OUT , OVP , F max = 510 kΩ leads kΩ. 2 Section 6.4.4 ): BOOST, PEAK 240 BILIM I BOOST , PEAK LED7706 = 21.89 kΩ. The closest 2 Section 5.3.4 ). Section 6.1 are always valid. Figure 23 ) are usually will be: BILIM Ω k ...
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... LED7706 6.4.9 Power dissipation estimate As explained in Section 6.2 Neglecting the power dissipated by the LDO (surely less significant compared with the other contributions), equation (19), (21), (22) and (23) help to estimate the overall power dissipation. Before starting the power dissipation estimate it is important to highlight that the following calculations are considering the worst case (the actual value of the dissipated power would require measurements) ...
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... Therefore P is significantly lower and the overall efficiency is typically around 90 %. GEN 36/46 = ⋅ DISS , Diode , F Diode IN = ⋅ ≅ DCR I DCR DISS , Ind Ind , RMS = + DISS , TOTAL , D tot DISS , Diode − η DISS , TOT P IN ⋅ ⋅ 559 DISS , Ind = . 0 845 LED7706 ...
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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 LED7706 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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... Application information Figure 21. Demonstration board layout (top view) Figure 22. Demonstration board layout (bottom view) Figure 23 shows the LED7706 demonstration board application circuit, whereas the used components and their value. 38/46 LED7706 Table 9 lists ...
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... LED7706 Figure 23. LED7706 demonstration board schematic Application information AM00609v1 39/46 ...
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... Application information Table 9. LED7706 demonstration board component list Component Description Ceramic, 35V C1 X5R, 20% C2,C3 Ceramic, 50V X7R, 20 Ceramic, 25V C9 X5R, 20% C10 C11 C12 C13 Chip resistor 0 R9, R10 Chip resistor 0 R11 R12 R13 L1 6u8, 75 mΩ, 2 Schottky Red LED Signal Schottky ...
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... LED7706 7 Electrical characteristics Figure 24. Efficiency versus DIM duty cycle rows, 10 white LEDs IN (20 mA) in series Figure 26. Efficiency versus DIM duty cycle rows, 10 white LEDs IN (20 mA) in series Figure 28. Soft-start waveforms (EN, SS, and V monitored) OUT Electrical characteristics Figure 25. Efficiency versus DIM duty cycle, ...
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... Electrical characteristics Figure 30. Dimming waveforms (F = 200 Hz) DIM Figure 32. Dimming waveforms (F Figure 34. Dimming waveforms ( µs) DIM,ON 42/46 Figure 31. Dimming waveforms ( kHz) Figure 33. Dimming waveforms (F DIM Figure 35. Dimming waveforms with the minimum dimming on time (500 ns) LED7706 = 1 kHz) DIM = 20 kHz) DIM ...
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... LED7706 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. Package mechanical data ® 43/46 ...
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... Table 10. VFQFPN- mechanical data Dim ddd Figure 36. Package dimensions 44/46 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 LED7706 Max. 1.00 0.05 0.30 4.15 2.60 4.15 2.60 0.50 0.08 ...
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... LED7706 9 Revision history Table 11. Document revision history Date 08-Feb-2008 09-Apr-2009 Revision 1 Initial release Added: Chapter 3 on page page 7 2 Updated: Chapter 3 on page page 7, Table 4, Table Table 9 Revision history Changes 7, Chapter 3 on page 7 and Chapter Chapter 3 on page 7, Chapter Figure 3, Figure ...
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