EVALED7706 STMicroelectronics, EVALED7706 Datasheet
EVALED7706
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EVALED7706 Summary of contents
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Features ■ Boost section – 4 input voltage range – Internal power MOSFET – Internal +5 V LDO for device supply – output voltage – Constant frequency peak current-mode control – 250 ...
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Contents Contents 1 Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...
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LED7706 6 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...
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Typical application circuit 1 Typical application circuit Figure 1. Application circuit VIN +5V 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 ...
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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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Pin settings 2.2 Pin description Table 2. Pin functions N° Pin 1 COMP 2 RILIM 3 BILIM 4 FSW 5 MODE 6 AVCC 7 LDO5 8 VIN 9 SLOPE 10 SGND 11 ROW1 12 ROW2 13 ROW3 14 ROW4 15 ...
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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, ...
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Electrical characteristics 4 Electrical characteristics °C and LDO5 connected to AVCC if not otherwise specified IN J Table 5. Electrical characteristics Symbol Parameter Supply section V Input voltage range IN V Boost section ...
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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 ...
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Operation description 5 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. overview of the internal blocks of the device. Figure 3. Simplified ...
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LED7706 5.1 Boost section 5.1.1 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 ...
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Operation description 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 ...
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LED7706 Figure 7. Enable pin driven by dimming signal 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. ...
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Operation description selector (see Figure 5 reacts by increasing the output voltage. When it reaches the floating row detection (FRD) threshold (95% of the OVP threshold), the floating rows are managed according to (see Section 5.3 on page 21 finishes ...
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LED7706 Figure 9. OVP threshold setting 5.1.5 Switching frequency selection and synchronization The switching frequency of the boost converter can be set in the 250 kHz-1 MHz range by connecting the FSW pin to ground through a resistor. Calculation of ...
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Operation description Keeping the FSW pin voltage lower than 270 mV for more than 4.8 µs results in a stop of the device switching activity. Normal operation is resumed as soon as FSW rises above the mentioned threshold and the ...
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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 ...
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Operation description 5.1.7 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 ...
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LED7706 5.2 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, ...
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Operation description 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 ...
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LED7706 5.3 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 soft- start sequence, all protections turn ...
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Operation description 5.3.3 Open LED fault In case a row is not connected or a LED fails open, the device has two different behaviors according to the MODE pin status. Connecting the MODE pin to SGND, as soon as an ...
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LED7706 5.3.4 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 ...
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Application information 6 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 ...
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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 ...
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Application information 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 ...
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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 ...
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Application information Equation 23 is the number of active rows, Δ V where n ROWs voltage and n LEDs d) LDO losses, due to the dissipation of the 5 V linear regulator: Equation 24 The LED7706 is housed in a ...
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LED7706 best trade off between the different loss contributions varies from manufacturer to manufacturer. A 6.8 µH inductor has been experimentally found as the most suitable for applications running at a 660 kHz switching frequency. Table 7. Recommended inductors Manufacturer ...
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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 ...
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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 ...
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Application information Equation 32 where I is the inductor peak current (see L, peak Equation 33 whereas D, working in DCM, is: Equation 34 defining M as: Equation 35 Figure 20. Inductor current in DCM operation ...
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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 ...
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Application information Equation 40 R can be chosen is in the order of hundreds of kilo-ohms to reduce the leakage current in 1 the resistor divider. For example, setting R standard commercial value is R The correct selection of the ...
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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 ...
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Application information 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 48 where V = 0.4 V ...
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LED7706 6.5 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 ...
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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 ...
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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 ...
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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 ...
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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. ...
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Package mechanical data 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 ...
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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, ...
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