TFT-LCD DC/DC Converter with Integrated Backlight LED Driver

Transcription

1 TFT-LCD DC/DC Converter with Integrated Backlight LED Driver Description The is a step-up current mode PWM DC/DC converter (Ch-1) built in an internal 1.6A, 0.25Ω power N-channel MOSFET and integrated a step-up LED driver (Ch-2). Pin selectable fixed switching frequency (1.2MHz or 640kHz) and external compensation pin provide the user flexibility in setting the loop dynamic, allowing the use of tiny, low profile inductors and low value, low ESR ceramic output capacitors. The channel 2 is a step-up DC/DC converter specifically designed to drive white LEDs with a constant current. The device can drive two or three LEDs in series from a Li-ion cell. The is available in low profile TSSOP-16 exposed pad package. Features Operating Voltage from 2.6V to 5.5V Output Voltage from Input Voltage to 13V for Ch-1 (16V for Ch-2) 90% Efficiency for both Ch-1 and Ch-2 Pin Selectable Switching Frequency for Ch-1 (1.2MHz or 640kHz) 1.3MHz Switching Frequency for Ch-2 1.6A, 0.25Ω for Ch-1 Internal Power N-channel MOSFET 1.3A, 0.4Ω for Ch-2 Internal Power N-channel MOSFET Ch-1 Programmable Soft Start Ch-1 External Compensation Network 0.1uA Shutdown Current Analog/PWM Dimming by EN2 Pin Over Temperature Protection Built-in Over Voltage Protection for ch-2 TSSOP-16 Exposed Pad Package Applications Portable DVD Car TV Portable Instruments Pin Assignments TP Package (TSSOP-16 Exposed Pad) Ordering Information TR: Tape / Reel P: Green G: Green Package Type TP: TSSOP-16(Exposed Pad) Figure 1. Pin Assignment of 1

2 Typical Application Circuit Figure 2. Typical Application Circuit of 2

3 Functional Pin Description Pin Name Pin Function LX1 VIN1 FREQ1 SS1 EN2 OVP2 GND2 FB2 LX2 VIN2 PGND2 COMP1 FB1 EN1 GND1 PGND1 Switching Pin of ch-1. Power Input Pin of ch-1. Frequency Select Pin of ch-1. When FREQ connected to ground, the frequency is 640kHz. When FREQ connected to VIN, the frequency is 1.2MHz. Soft-Start Control Pin of ch-1. Connect an external capacitor to this pin to set soft start time. The pin sources 4uA constant current to charge the capacitor. Leaving it floating for not using soft-start. Enable and dimming control for ch-2. Connect EN2 low to turn off ch Analog dimming control : apply 0.9V to 1.5V DC voltage signal 2. Digital dimming control : apply external PWM pulse signal Over Voltage Input. OVP measures the output voltage for LED open circuit protection. Connect OVP to the output at the top of the LED string. Analog Ground of ch-2. Feedback Pin of ch-2. Reference Voltage is 0.2V. Connect cathode of the lowest LED and resistor here. Calculate the resistor value according to the formula R SET = 0.2V / I LED. Switching Pin of ch-2. Power Input Pin of ch-2 Power Ground of ch-2. Compensation Pin for Error Amplifier of ch-1. Connect a series RC from COMP to ground. Feedback Pin of ch-1. The typical reference voltage is 1.24V. Set V OUT = 1.24V (1+ R1/R2). Enable Pin of ch-1. Connect EN1 low to turn off ch-1. Analog Ground of ch-1. Power Ground of ch-1. 3

4 Block Diagram M M Figure 3. Block Diagram of 4

5 Absolute Maximum Ratings VIN1, VIN2 to GND V to +6V LX1 to GND V to +14V LX2, OVP2 to GND V to +20V FB1,FB2,EN1,EN2 to GND V to +6V FREQ1, COMP1, SS1 to GND V to +6V Power T A =25, TSSOP-16 Expose Pad (P D ) mW Package Thermal Resistance, TSSOP-16 Expose Pad (θ JA ) /W Junction Temperature C Storage Temperature Range C to +150 C Lead Temperature (Soldering, 10sec.) C Note1:Stresses beyond those listed under Absolute Maximum Ratings" may cause permanent damage to the device. Recommended Operating Conditions Supply Voltage, VIN1 & VIN V to 5.5V Operation Temperature Range C to +85 C 5

6 Electrical Characteristics (V IN1 =V IN2 =5V, V EN1 =V EN2 =5V, T A =25 C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Units INPUT Operating Voltage Range V IN1,V IN V VIN1 Under Voltage Lockout VIN2 Under Voltage Lockout Quiescent Current (ch-1 & ch-2) V UVLO1 V UVLO2 I IN-TOTAL V IN1 rising, typical hysteresis is 100mV V IN2 rising, typical hysteresis is 100mV V V V FB1 = V FB2 =1.3V, not switching µa V FB1 = V FB2 =0V, switching 1.8 ma Shutdown Current I SD V EN1 =V EN2 =0V 0.1 µa ERROR AMPLIFIER Feedback Voltage of FB1 V FB V Feedback Voltage of FB2 V FB V FB Input Bias Current of FB1 I FB1 V FB1 =1.24V 0.1 µa FB Input Bias Current of FB2 I FB2 V FB2 =0.2V 0.1 µa OSCILLATOR Frequency of Ch-1 Maximum Duty Cycle of Ch-1 F OSC1 T DUTY1 FREQ1=GND khz FREQ1=V IN khz FREQ1=GND % FREQ1=V IN1 84 % Frequency of Ch-2 F OSC khz Maximum Duty Cycle of Ch-2 T DUTY % N-CHANNEL SWITCH Current Limit of Ch-1 (Note2) I LIM1 1.6 A On-Resistance of Ch-1 (Note2) R ON Ω Current Limit of Ch-2 (Note2) I LIM2 1.3 A On-Resistance of Ch-2 (Note2) R ON2 0.4 Ω CONTROL INPUT EN1 High Level V IH1 0.7*V IN1 V EN1 Low Level V IL1 0.3*V IN1 V EN2 High Level V IH2 1.7 V EN2 Low Level V IL2 0.5 V EN2 Analog Dimming Voltage Range V DIM V PROTECTION OVP2 Threshold V OVP V OVP2 Input Resistance (Note2) R OVP 1.2 MΩ Thermal Shutdown Temperature (Note2) Note2:The specification is guaranteed by design, not production tested. T SD Typical 20 O C hysteresis 140 C 6

7 Typical Performance Curves Ch-1:Step-Up Converter FREQ=640Khz L=10uH Output Voltage (V) VOUT=10V VIN=3.3V L=10uH FREQ=640Khz Output Current (ma) Figure 4. Output Voltage vs. Output Current Efficiency (%) FREQ=1.2Mhz L=4.7uH VIN=3.3V VOUT=10V Output Current (ma) Figure 5. Efficiency vs. Output Current VIN=3.3V, VOUT=10V, L=10uH Freq=640Khz Frequency (Khz) VOUT (V) Junction Temperature ( o C) V IN =3.3V Junction Temperature ( o C) Figure 6. Frequency vs. Junction Temperature Figure 7. Output Voltage vs. Junction Temperature 200mA 10mA I OUT V EN1 V OUT V OUT I LX1 I LX1 V IN1 =3.3V, V OUT =10V, FREQ=640kHz, C OUT =33uF+0.1uF R C =120kΩ, C C1 =1000pF, C C2 =47pF Figure 8. Load transient Response V IN1 =3.3V, V OUT =10V, I OUT =10mA, L1=10uH, C SS =33nF, FREQ=640kHz, C OUT =33uF Figure 9. Start-up Waveform with Soft-start 7

8 Typical Performance Curves (Continued) V EN1 V EN1 V OUT V OUT I LX1 I LX1 V IN1 =3.3V, V OUT =10V, I OUT =200mA, L1=10uH, C SS =33nF, FREQ=640kHz, C OUT =33uF Figure 10. Start-up Waveform with Soft-start V IN1 =3.3V, V OUT =10V, I OUT =10mA, L1=10uH, without soft start, FREQ=640kHz, C OUT =33uF Figure 11. Start-up Waveform without Soft-start V LX1 V OUT V OUT I LX1 I LX1 V IN1 =3.3V, V OUT =10V, I OUT =200mA, FREQ=640kHz, L1=10uH, C OUT =33uF+0.1uF Figure 12. Switching Waveform V IN1 =3.3V, V OUT =10V, I OUT =500mA, FREQ=640kHz, L1=10uH Figure 13.OCP Waveform 8

9 Typical Performance Curves Ch-2:LED Driver Feedback Voltage (V) Frequency (MHz) Supply Voltage (V) Supply Voltage (V) Figure 14. Feedback Voltage vs. Supply Voltage Figure 15. Oscillator Frequency vs. Supply Voltage VIN2=2.5V VIN2=5V OVP Threshold (V) Feedback Voltage (V) Supply Voltage (V) Temperature ( o C) Figure 16. Over Voltage Protect vs. Supply Voltage Figure 17. Temperature vs. Feedback Voltage VIN2=2.5V VIN2=5V 0.20 Frequency (MHz) Feedback Voltage (V) Temperature ( o C) Figure 18. Temperature vs. Frequency Enable Voltage (V) Figure 19. Enable Voltage vs. Feedback Voltage (analog control) 9

10 Typical Performance Curves (Continued) V IN2 =3.3V Vin3p3V Vin5p0V V IN2 =5.0V I LX V LX2 V FB (V) V LED 0.16 V FB I OUT (ma) Figure 20. Load Regulation Figure 21. Operation Waveform V IN2 =5V, 3LEDs, I LED =100mA, C OUT =1uF Figure 22. Duty Cycle vs. Feedback Voltage 3.3Vi to 3LEDs (PWM control) Figure 23. Duty Cycle vs. Feedback Voltage 3.3Vi to 4LEDs (PWM control) Figure 24. Efficiency vs. LED Current (Different Input Voltage) Figure 25. Efficiency vs. Input Voltage (Different Inductor) 10

11 Typical Performance Curves (Continued) Figure 26. Efficiency vs. LED Current (Different Input Voltage) Figure 27. Efficiency vs. Input Voltage (Different Inductor) Figure 28. Efficiency vs. LED Current (Different Input Voltage) Figure 29. Efficiency vs. Input Voltage (Different Inductor) 11

12 Application Information Operation The is designed in a current mode, pulse-width modulation (PWM) architecture for fast-transient response and low-noise operation to provide the regulated voltages required by thin film transistor (TFT) LCD display and LED backlight of LCD panel. The functional diagram is shown in Figure 3. At light loads, the (ch-2) operates in PFM to maintain the highest efficiency. The operates well with a variety of external components. See the following sections to optimize external components for a particular application. Inductor Selection For most applications, a 10uH is recommended for general used. The inductor parameters, current rating, DCR and physical size, should be considered. The DCR of inductor affects the efficiency of the converter. The inductor with lowest DCR is chosen for highest efficiency. The saturation current rating of inductor must be greater than the switch peak current. These factors affect the efficiency, transient response, output load capability, output voltage ripple, and cost. Diode Selection For diode selection, both forward voltage and diode capacitance need to be considered. The output diode should be rated to the output voltage and peak switch current. Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for applications. Make sure the diode s peak current rating is at least IPK and its breakdown voltage exceeds V OUT. Capacitor Selection The ceramic capacitor is ideal for application. X5R or X7R types are recommended because they hold their capacitance over wide voltage and temperature ranges than other Y5V or Z5U types. The input capacitor can reduced peak current and noise at power source. The output capacitor is typically selected based on the output voltage ripple requirements. A higher or lower capacitance may be used depending on the acceptable noise level. Feedback Resistance Network An external resistor divider of (ch-1) is required to divide the output voltage down to the nominal reference voltage. Current drawn by the resistor network should be limited to maintain the overall converter efficiency. The maximum value of the resistor network is limited by the feedback input bias current and the potential for noise being coupled into the feedback pin. A resistor network in the order of 50kΩ is recommended. The boost converter output voltage is determined by the following relationship: R = + 1 V OUT VFB1 1 R2 where V FB1 =1.24V as specified. Over Voltage Protection The (ch-2) has an internal open-circuit protection circuit. In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail open circuit, OVP is clamped at 18V (typ). The will then switch, at a very low frequency to minimize input current. The OVP and input current during output open circuit are shown in the Typical Performance Curves. LED Current Setting The EN2 pin controls the feedback voltage as shown in the figure11. For EN2 higher than 1.7V, the feedback voltage is 200mV, which results in full LED current. In order to have accurate LED current, precision resistors are preferred (1% is recommended). The LED current can be programmed by : I LED =200mV / R FB Dimming Control There are three different types of dimming control circuits. The LED current can be set by modulating the EN2 pin with a DC voltage, PWM signal or a filtered PWM signal. (1) Using a DC Voltage The EN2 pin voltage can be modulated to set the dimming of the LED string. As the voltage on the EN pin increases from 0.9V to 1.5V, the LED current increases from 0mA to maximum current. As the EN2 pin voltage exceeds 1.5V, it has no effect on the LED current. Feedback voltage versus Enable voltage is given in the figure 19. Figure 30. Dimming Control Using a DC Voltage 12

13 Applications Information (Continued) (2) Using a PWM Signal Changing the LED forward current not only changes the intensity of the LEDs, but also changes the color. Controlling the intensity of the LEDs with a direct PWM signal allows dimming of the LEDs without changing the color. Dimming the LEDs via a PWM signal essentially involves turning the LED on and off. The LEDs operate at either zero or full current. The average of LED current increases proportionally with the duty cycle of the PWM signal. The color of the LEDs remains unchanged since the LED current value is either zero or a constant value. The typical frequency range of the PWM signal is 100Hz to 1kHz. Two way of PWM control dimming, drive EN2 directly or drive FB2 pin through a resistor. First, drive EN2 directly shown as figure 31(a). A 0% duty cycle will turn off the and corresponds to zero LED current. A 100% duty cycle corresponds to full current. The amplitude of the PWM signal should be higher than the minimum EN2 voltage. Second, drive FB2 pin through a resistor shown as figure 31(b). Increase of duty cycle will decrease LED average current. In this application, LED is dimmed by FB2 pin and turned off completely by EN2 pin. Vin PWM Cin 1uF VIN2 EN2 GND2 L1 -ch2 LX2 OVP FB2 Figure 31(a). Dimming Control Using a PWM Signal Figure 31(b). Dimming Control Using a PWM Signal D1 R1 Cout 1uF (3) Using a Filtered PWM Signal If audio noise is concerned or higher PWM frequency is used, a filtered PWM signal can be used to control the brightness of the LED string. The PWM signal is filtered by a RC network. The corner frequency of R, C should be much lower than the frequency of the PWM signal. RF needs to be much smaller than the internal impedance of the EN2 pin which is 600kΩ (typ). Figure 32 show the two dimming methods of using filtered PWM signal. R2 in figure 32(a) extends the available range of duty cycle. Figure 32 (a). Dimming Control Using a Filtered PWM Signal 0~5V PWM signal VIN2 EN2 GND2 -ch2 R4 400k LX2 OVP FB2 R3 80k C1 0.1uF R2 20k R1 1 Figure 32(b). Dimming Control Using a Filtered PWM Signal Layout Consideration The proper PCB layout and component placement are critical for all switching regulators. The careful attention should be taken to the high-frequency, high current loops to prevent electromagnetic interference (EMI) problems. Minimize the length and area of all traces connected to the LX1 and LX2 node. Keep the noise-sensitive feedback and compensation circuitry away from the switching node. Place Cout next to Schottky diode as possible. The exposed die plate, on the underneath of the package, should be soldered to an equivalent area of metal on the PCB. This contact area should have multiple via connections to the back of the PCB as well as connections to intermediate PCB layers, if available, to maximize thermal dissipation away from the IC. 13

14 Outline Information TSSOP-16 EP Package (Unit: mm) SYMBOLS DIMENSION IN MILLIMETER UNIT MIN MAX A A A b D E E e L D E Note: Followed From JEDEC MO-153-F. Life Support Policy Fitipower s products are not authorized for use as critical components in life support devices or other medical systems. 14