LED7707TR STMicroelectronics, LED7707TR Datasheet

Page 1

... PDAs panel backlight ■ GPS panel backlight Table 1. Device summary Order codes LED7707 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 ...

Page 2

Contents Contents 1 Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...

Page 3

LED7707 6 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...

Page 4

Typical application circuit 1 Typical application circuit Figure 1. Application circuit VIN +5V 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 ...

Page 5

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 ...

Page 6

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 ...

Page 7

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, ...

Page 8

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 ...

Page 9

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 ...

Page 10

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. Figure 3 provides an overview of the internal blocks of the ...

Page 11

LED7707 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 ...

Page 12

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 ...

Page 13

LED7707 Figure 7. External sync waveforms 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 ...

Page 14

Operation description reacts by increasing the output voltage. When it reaches the floating row detection (FRD) threshold (which coincides with the OVP threshold, see managed according to 2.4 V threshold, the start-up finishes and all the protections turn active. The ...

Page 15

LED7707 Equation 4 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 R FSW The ...

Page 16

Operation description Figure 11. External sync waveforms FSW pin voltage (ext. sync) Slave SYNC pin voltage Slave LX pin voltage 5.1.6 Slope compensation The constant frequency, peak current-mode topology has the advantage of very easy loop compensation with output ceramic ...

Page 17

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 ...

Page 18

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 LED7707 improves the reliability of the ...

Page 19

LED7707 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 and to sink maximum current, ...

Page 20

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 ...

Page 21

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 ...

Page 22

Operation description 5.3.1 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 ...

Page 23

LED7707 Figure 16 shows an example of open channel detection in case of MODE connected to AVCC. At the point marked as “1” in point on the output voltage is increased as long as the output voltage reaches the floating ...

Page 24

Operation description At the point marked as “1” in 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 ...

Page 25

LED7707 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 ...

Page 26

Application information Equation 13b 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 ...

Page 27

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 ...

Page 28

Application information 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 17 where T is the ambient temperature, R ...

Page 29

LED7707 Equation 22 is the number of active rows, ΔV where n ROWs voltage and n LEDs d) LDO losses, due to the dissipation of the 5 V linear regulator: Equation 23 The LED7707 is housed leads ...

Page 30

Application information 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 ...

Page 31

LED7707 6.3.3 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 ...

Page 32

Application information Equation 25 where D is the duty-cycle defined as: Equation 26 whereas R is: 0 Equation 27 and Equation 28 The output voltage in the above calculations is considered as the maximum value (LED with the maximum forward ...

Page 33

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 ...

Page 34

Application information T can be calculated as: OFF Equation 35 defining D as: 2 Equation 36 The worst case for the output voltage ripple is when input voltage is lower (V A simple way to select the C In order ...

Page 35

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 ...

Page 36

Application information 6.4.9 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 ...

Page 37

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 ...

Page 38

Application information 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) ...

Page 39

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 ...

Page 40

Application information Figure 24. LED7707 demonstration board schematic Table 9. LED7707 demonstration board component list Component Description Ceramic X5R C2,C3 Ceramic X7R Ceramic ...

Page 41

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 ...

Page 42

Electrical characteristics 7 Electrical characteristics Figure 25. Efficiency versus DIM duty cycle rows, 10 white LEDs IN (60 mA) in series, F Figure 27. Efficiency versus DIM duty cycle rows, ...

Page 43

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 ...

Page 44

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 ...

Page 45

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 ...

Page 46

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, ...

Page 47

... 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. ...