Application Note 1029 Design Consideration with AP3606/AP3607 Prepared by You Qing Ling System Engineering Dept. Additionally, 1MHz high switching frequency enables the use of small external flying capacitors. 1. Introduction The AP3606 and AP3607 are step-up DC/DC converters based on 1x/1.5x charge pump for white LED backlight application. The AP3606(AP3607) integrates 4(6) channels current sink and automatic mode selection charge pump. High efficiency is achieved by utilizing a 1x/1.5x fractional charge pump in combination with low dropout current sink. The supply voltage ranges of AP3606 and AP3607 are from 2.7V to 5.5V which make them ideally suit for applications powered by Li-ion battery. Internal soft-start circuitry prevents excessive inrush current during start-up and mode transition. D1 8 C2- 2 10 D4 C2+ 3 9 NC C1+ 4 Exposed PAD, Connected to AGND D2 VOUT2 7 D3 D1 NC 6 11 VOUT2 VOUT1 5 1 16 15 14 13 12 D3 11 D4 EP 10 D5 9 5 6 7 8 EN 4 C1- VIN 3 C1+ EP 12 D2 PGND C2+ 13 AGND 2 14 EN C2- 15 VIN 1 16 PGND C1- Pin 1 Dot by Marking AGND Pin 1 Dot by Marking VOUT1 The AP3606 and AP3607 are available in a tiny package of QFN-3×3-16. Its operating temperature range is from -40°C to 85°C. The pin configuration and the representative block diagrams of the AP3606 and AP3607 are respectively shown in Figure 1 and Figure 2: The AP3606(AP3607) supports up to 4(6) white LEDs and regulates a constant current for brightness control. The LED current can be changed from 1.25mA to 20mA by applying a pulse signal to the EN pin. There are totally 16 steps for current control. D6 Exposed PAD, Connected to AGND Note: Pin15 should be connected with Pin 16 on PCB Board. Note: Pin14 should be connected with Pin 16 on PCB Board. AP3606 AP3607 Figure 1. Pin Configuration of AP3606/AP3607 (Top View) Nov. 2008 Rev. 1. 1 BCD Semiconductor Manufacturing Limited 1 Application Note 1029 VIN C1- C1+ 1 4 7 C22 8 3 16 1x /1.5x Charge Pump VOUT1 5 AGND 6 PGND 14 VOUT2 Current Limitation 1MHz OSC EN C2+ OVP Soft Start Shutdown Delay OTSD UVLO 16 Steps Pulse Dimming Controller 4 Channels Current Sink 13 D1 12 D2 11 D3 10 D4 AP3606 C2- C2+ C1- C1+ 1 VIN 7 4 3 2 16 1x /1.5x Charge Pump 5 1MHz OSC Current Limitation 6 EN Shutdown Delay UVLO OTSD 16 Steps Pulse Dimming Controller AGND OVP Soft Start 8 VOUT1 15 6 Channels Current Sink 14 13 12 11 10 PGND VOUT2 9 D1 D2 D3 D4 D5 D6 AP3607 Figure 2. Functional Block Diagram of AP3606/AP3607 Nov. 2008 Rev. 1. 1 BCD Semiconductor Manufacturing Limited 2 Application Note 1029 channel is not used, it should be connected to VIN directly. It should not be floating. 1.1 Soft Start The AP3606(AP3607) has an internal soft start circuit to limit the inrush current during startup and charge pump mode transition. It prevents possible drop of the input voltage if a high impedance power source is connected to the input of the AP3606(AP3607). In mobile phone applications, the AP3606(AP3607) is powered by Li-ion battery. The soft start function prevents excessive output current from leaving the battery, which can cause excessive voltage drop. It is of benefit to other circuits powered by Li-ion battery directly. 1.5 Mode Selection The AP3606(AP3607) can automatically switch between 1x mode and 1.5x mode for optimizing the efficiency. An internal current sensing circuit monitors WLED current to keep it constant when the battery voltage fluctuates. The AP3606(AP3607) switches to 1.5x mode whenever the dropout condition is detected from any of the 4(6) current sinks and returns to 1x mode whenever the dropout condition releases. 1.2 Enable and Dimming Control The AP3606(AP3607) has an EN pin to enable/disable the IC or control the brightness of white LEDs. When a logic high signal is added to the EN pin, the AP3606(AP3607) starts up and operates normally. When a logic low signal is added to the EN pin and remains more than 1ms, the IC will shut down. The shutdown current is very small. It can minimize the power dissipation and extend the battery life. 1.6 Under Voltage Lockout The AP3606(AP3607) provides an under voltage lockout (UVLO) circuit to prevent it from undefined status when startup. The UVLO circuit shuts down the device when VIN drops below a typical threshold of 2.2V. The UVLO circuit has 250mV hysteresis, which means the device starts up again when VIN rises to 2.45V or above. The UVLO hysteresis can prevent AP3606(AP3607) from unstable state when startup. If adding a pulse signal to the EN pin, the AP3606(AP3607) will operate in dimming mode. The LED current can be changed from 1.25mA to 20mA, in 16 steps totally. The detailed operation of brightness dimming please refer to 4.3. 1.7 Over Voltage Protection The AP3606(AP3607) has an internal over voltage protection circuit. When the white LEDs are disconnected from the circuit or fail to open, the voltage between input and output will be clamped at 5.5V. 1.3 Current Matching Current matching is most important for WLED backlights. The AP3606(AP3607) current sink block implements the current matching function. It can achieve less than ±3% current matching between 4 channels for AP3606 and 6 channels for AP3607. 1.8 Over Temperature Protection The AP3606(AP3607) is also equipped with a thermal protection and shutdown circuit that continuously monitors the IC junction temperature. When some abnormal operations occur, the internal junction temperature rises. If the thermal protection circuit senses the junction temperature exceeding approximately 160°C, it will disable AP3606(AP3607) and protect the device from damaged. The thermal protection and shutdown circuit has 20°C of system hysteresis. As we know, WLED is current control device whose forward voltage has a wide distribution. If the LED lot number is different, the distribution is much wider. A highlight feature of AP3606(AP3607)) is that it’s current matching precision is independent on LED forward voltage. It can guarantee ±3% current matching precision for all types of WLEDs. 2. Components Selection 2.1 Capacitor Selection Careful selection of the four external capacitors CIN, COUT and CFLY (C1 and C2) is very important because they will affect charge pump capability, output voltage ripple, input voltage ripple and power efficiency. Polarized capacitor such as tantalum or 1.4 White LED Connection The AP3606 supports up to 4 white LEDs that are connected from VIN to D1, D2, D3 and D4 channels respectively. The AP3607 supports up to 6 white LEDs which are connected from VIN to D1, D2, D3, D4, D5 and D6 channels respectively. If a certain Nov. 2008 Rev. 1. 1 BCD Semiconductor Manufacturing Limited 3 Application Note 1029 Ceramic Capacitors have features of very low ESR, low cost, small PCB footprint, and are non-polarized. All the above are of benefit to the charge pump. The main materials of ceramic capacitor include NPO (COG), X7R, X5R, Y5U, Z5U and Y5V. MLCC X7R and X5R dielectrics are much more desirable because of their capacitance, precision, operating temperature range, temperature stability and cost (compare with NPO ceramic capacitor). Table 1 gives the key features of X7R and X5R ceramic capacitors. aluminum is not recommended for using with the AP3606(AP3607). For the two flying capacitors C1 and C2, it is important to use low ESR capacitors to avoid unnecessary efficiency losses. To reduce the input and output ripple effectively, low ESR capacitors on VIN and VOUT are recommended. To get the optimum performance, low ESR ceramic capacitor will be the best option for CIN, COUT and CFLY. The recommended value for each capacitor is 1µF typical. Table 1. Key Features of X7R and X5R Ceramic Capacitors Ceramic Capacitor X7R X5R Operating Temperature Range (℃ ) -55 to 125 -55 to 85 Precision Capacitance ± 15% ± 15% 150pF to 4.7µF 0.47µF to 10µF Temperature Stability -5% to 5% -15% to 5% 2.2 White LED Selection The AP3606(AP3607) is specifically intended for driving white LEDs with forward voltage from 0V to 4.0V. Since the D1 to D6 current sinks are independent of the voltage, the LED brightness will be matched regardless of the specific LED forward voltage (VF) levels. The AP3606(AP3607) is designed especially for white LED driver with current matching. Figure 3, 4, 5 and 6 shows the typical application circuits. In these applications, the LED current can be changed from 1.25mA to 20mA by applying a pulse signal to the EN pin. There are totally 16 steps of current can be set by users. In a word, the AP3606(AP3607) can drive most white LEDs for backlighting with good current matching. If the LED is not used, the LED pin should be connected to VIN directly. Figure 3 and Figure 5 show the connection in 3 WLEDs and 5 WLEDs applications. 3. Typical application C1 1µF C2 1µF D1 U1 7 Li-ion Battery CIN 1µF 1 4 2 3 C1- C1+ C2- C2+ D2 D3 VIN Pulse Input 8 14 16 COUT 5 EN AP3606 D1 VOUT2 D2 VOUT1 D3 AGND PGND D4 13 12 11 10 6 1µF Figure 3. Typical Application of AP3606 (3 WLEDs) Nov. 2008 Rev. 1. 1 BCD Semiconductor Manufacturing Limited 4 Application Note 1029 Figure 4. Typical Application of AP3606 (4 WLEDs) C1 1µF C2 1µF D1 7 Li-ion Battery CIN 1 4 2 C1- C1+ C2- VIN D2 8 1µF 15 16 COUT C2+ D1 Pulse Input 5 EN AP3607 D3 VOUT2 D4 VOUT1 D5 PGND AGND 1µF D2 D3 D4 D5 3 D6 14 13 12 11 10 9 6 Figure 5. Typical Application of AP3607 (5 WLEDs) Nov. 2008 Rev. 1. 1 BCD Semiconductor Manufacturing Limited 5 Application Note 1029 C1 1 F C2 1 F D1 7 Li-ion Battery CIN 1 4 2 3 C1- C1+ C2- C2+ VIN D1 Pulse Input 1 F D2 8 15 16 COUT 5 EN AP3607 D3 VOUT2 D4 VOUT1 D5 AGND PGND 1 F D6 D2 D3 D4 D5 D6 14 13 12 11 10 9 6 Figure 6. Typical Application of AP3607 (6 WLEDs) (AP3607) can be calculated through formula 2 and formula 3. 4. Application Hints 4.1 Output Ripple Low ESR ceramic capacitor for COUT and CFLY and high switching frequency of AP3606(AP3607) minimize the output voltage ripple. η LED = Formula 1 can be used to calculate the output voltage ripple. 1 1 ID × × f 2 + 2 × I × ESR VRIPPLE = D COUT C OUT --------------------------------------(1) η LED = VD × I D V ×I V (1x mode) ≈ D D = D VIN × I IN VIN × I D VIN --------------------------------------(3) With a fully charged battery, the charge pump operates in 1x mode. The efficiency will increase along with the decrease of battery voltage. When the battery voltage drops to about 3.5V (VF=3.2V), the efficiency will reach the top point which is about 91%. As the discharge of battery ceaselessly run out, leading to the current sinks no longer have enough voltage overhead to maintain a constant current regulation, the charge pump will switch into 1.5x mode. The conversion efficiency is lowest at this crossover. As the battery discharges further, the efficiency again increases and reaches a second maximum at about 2.7V VIN. ID ---equal to the sum of LED current f --- operating frequency, 1MHz COUT --- output capacitor ESRCOUT --- the equivalent serial resistor of output capacitor The current sink is powered by charge pump. Because of its high bandwidth, the output voltage ripple is filtered by the current sink. So, there is little current ripple in the current sink. It is of benefit to the LED. 4.2 Power Efficiency 4.3 Dimming Control The AP3606(AP3607) implements a pulse dimming mechanism to control the brightness of white LEDs. The power conversion efficiency of the AP3606 Nov. 2008 VD × I D VD × I D VD (1.5x mode) ≈ = VIN × I IN VIN × 1.5 I D 1.5VIN --------------------------------------(2) Rev. 1. 1 BCD Semiconductor Manufacturing Limited 6 Application Note 1029 When AP3606(AP3607) is powered on, the WLED is in full brightness. Its current is about 20mA. The dimming control can be achieved by applying a pulse to EN pin. When the low level duration time of pulse is between TLOmin and TLOmax, and the high-level duration time is larger than THImin, the LED current will decrease 1/16. If the low level duration time is larger than TSHDNmax, the IC will be turned off. When AP3606/AP3607 is powered on, the WLED is in full brightness. And it will keep maximum current until the pulse is detected. After 15 pulses the WLED current decreases to 1/16 of full brightness. It will increase to full brightness if a pulse is added to EN pin then. Please refer to Figure 7 and Figure 8 for detailed information and example. Figure 7. Dimming Control by Pulse Signal Figure 8. WLED Current vs. Dimming Signal (AP3607) Nov. 2008 Rev. 1. 1 BCD Semiconductor Manufacturing Limited 7 Application Note 1029 4.4 PCB Layout Consideration Due to high switching frequency and large transient currents produced by AP3606(AP3607), careful PCB layout is necessary. For best performance, all external capacitors should be located as close as possible to the device. A true ground plan and short connection is highly recommended. In detail, COUT has the first priority to be located closely to VOUT and GND pin which is advantageous to reduce noise coupling from charge pump to LEDs and reduce the output ripple. CFLY has the second priority and CIN has the third priority. Furthermore, the traces from pins to flying capacitor should be short and wide to reduce parasitic resistance. VIN traces should be wide to minimize inductance and handle the large current. The exposed pad, PGND and AGND should be connected to a strong ground plane for heat sinking and noise prevention. These rules are adopted to minimize the LED current ripple, improve the drive capability and efficiency, and minimize the input voltage ripple to prolong the battery life. AP3607 Figure 9. AP3606/AP3607 PCB Layout Examples 4.5 The Layout Compatibility of AP3606 and AP3607 There is a good layout compatibility between AP3606 and AP3607. It is convenient for user to use AP3606 or AP3607 as backlight drive in the same PCB board. In other word, the PCB board used for AP3606 also suits AP3607. Figure 10 illustrates how to apply AP3606 and AP3607 to the same PCB. Figure 9 gives an layout example of the AP3606(AP3607). In this example, the package of AP3606(AP3607) is QFN-3x3-16 and the package of CIN, COUT and CFLY is 0805. VIN LED1 LED2 R COUT 16 C1 C2 15 14 13 1 12 2 11 EP 3 LED5 9 6 7 LED4 10 4 5 LED3 LED6 8 VIN Pulse input CIN AP3606 Nov. 2008 Figure 10. Rev. 1. 1 BCD Semiconductor Manufacturing Limited 8 Application Note 1029 In case AP3607 is used, please remove R as Figure 12 shows. If AP3606 is applied, please: 1) Remove LED1 & LED6 2) Set R to 0Ω VIN The recommended schematics please refer to Figure 11. 12 D2 C2- 2 11 D3 C2+ 3 10 D4 C1+ 4 9 NC AP3606 7 LED3 LED4 C2 D2 D1 VOUT2 12 D3 LED3 C2- 2 11 D4 LED4 C2+ 3 10 D5 LED5 C1+ 4 9 D6 LED6 AP3607 5 6 7 8 Pulse input CIN 8 EN 6 VIN AGND 5 C1 LED2 13 EN 1 14 VIN 13 15 1 D1 14 C1- PGND C2 15 16 C1- AGND 16 C1 VOUT2 NC VOUT1 COUT LED1 PGND R=0 COUT VOUT1 LED1 LED2 VIN Pulse input CIN Figure 12. Figure 11. Nov. 2008 Rev. 1. 1 BCD Semiconductor Manufacturing Limited 9