AP5724 (application) WHITE LED STEP-UP CONVERTER Power Management Business Unit PWM application note apply to device family: AP5724/25/26(SOT26) series AP5724/25/26pcb_V2(SOT26) Rev. 1 1 of 8 www.diodes.com JULY 2009 © Diodes Incorporated AP5724 (application) WHITE LED STEP-UP CONVERTER General Description Features • • • • • • • • Inherently Matched LED Current High Efficiency: 84% Typical Fast 1.2MHz Switching Frequency Current limit and UVLO protections Over Voltage Protect function built-in Soft-Start function built-in SOT26 and DFN2020C-6: Available in “Green” Molding Compound (No Br, Sb) Lead Free Finish/RoHS Compliant (Note 1) The AP5724 is a step-up DC/DC converter specifically designed to drive white LEDs with a constant current. The device can drive 2~6 LEDs in series from a Li-Ion cell. Series connection of the LEDs provides identical LED currents resulting in uniform brightness and eliminates the need for ballast resistors. The AP5724 switches at 1.2MHz that allows the use of tiny external components. A low 0.1V feedback voltage minimizes power loss in the current setting resistor for better efficiency. Applications • • • • • Cellular Phones PDAs, Hand help Computers Digital Cameras MP3 Players GPS Receivers AP5724/25/26pcb_V2(SOT26) Rev. 1 2 of 8 www.diodes.com JULY 2009 © Diodes Incorporated AP5724 (application) WHITE LED STEP-UP CONVERTER Functional Description Inductor Selection A 10µH~22µH inductor is recommended for most AP5724 applications. Although small size and high efficiency are major concerns, the inductor should have low core losses at 1.2MHz and low DCR (copper wire resistance). Capacitor Selection The small size of ceramic capacitors are ideal for AP5724 applications. X5R and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V or Z5U. A 1µF input capacitor and a 1µF output capacitor are sufficient for most AP5724 applications. Diode Selection Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for AP5724 applications. The forward voltage drop of a Schottky diode represents the conduction losses in the diode, while the diode capacitance (CT or CD) represents the switching losses. For diode selection, both forward voltage drop and diode capacitance need to be considered. Schottky diodes with higher current ratings usually have lower forward voltage drop and larger diode capacitance, which can cause significant switching losses at the 1.2MHz switching frequency of the AP5724. A Schottky diode rated at 100mA to 200mA is sufficient for most AP5724 applications. LED Current Control The LED current is controlled by the feedback resistor (RSET in Figure 1). The feedback reference is 0.1V. The LED current is 0.1V/ RSET. In order to have accurate LED current, precision resistors are preferred (1% is recommended). The formula and table for RSET selection are shown below. AP5724 RSET = 0.1V/ILED (See Table 1) AP5725 RSET = 0.25V/ILED AP5726 RSET = 0.31V/ILED Table 1. RSET Resistor Value Selection ILED (mA) 5 10 15 20 30 AP5724 RSET (Ω) 20 10 6.6 5 3.3 AP5724/25/26pcb_V2(SOT26) Rev. 1 AP5725 RSET (Ω) 50 25 16.6 12.5 8.3 AP5726 RSET (Ω) 62 31 20.6 15.5 10.3 3 of 8 www.diodes.com JULY 2009 © Diodes Incorporated AP5724 (application) WHITE LED STEP-UP CONVERTER Open-Circuit Protection In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback voltage will be zero. The AP5724 will then switch at a high duty cycle resulting in a high output voltage, which may cause the SW pin voltage to exceed the level of the over voltage protect function. The OVP pin can detect the output voltage and monitor if the output voltage reach to the protect voltage level (Figure 2). Once OVP is activated, SW pin stops switching. L1 22uH VIN D1 COUT 1uF C IN 1uF V IN SW x AP5724 OVP EN x FB GND RSET 5 Figure 2. LED Driver with Open-Circuit Protection Dimming Control There are four different types of dimming control circuits: 1. Using a PWM Signal to EN Pin With the PWM signal applied to the EN pin, the AP5724 is turned on or off by the PWM signal. The LEDs operate at either zero or full current. The average LED current increases proportionally with the duty cycle of the PWM signal. A 0% duty cycle will turn off the AP5724 and corresponds to zero LED current. A 100% duty cycle corresponds to full current. The typical frequency range of the PWM signal is below 2kHz. AP5724/25/26pcb_V2(SOT26) Rev. 1 4 of 8 www.diodes.com JULY 2009 © Diodes Incorporated AP5724 (application) WHITE LED STEP-UP CONVERTER 2. Using a DC Voltage For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The dimming control using a DC voltage is shown in Figure 3. As the DC voltage increases, the voltage drop on R2 increases and the voltage drop on RSET decreases. Thus, the LED current decreases. The selection of R2 and R3 will make the current from the variable DC source much smaller than the LED current and much larger than the FB pin bias current. For VDC range from 0V to 2V, the selection of resistors in Figure 3 gives dimming control of LED current from 0mA to 20mA. 3. Using a Filtered PWM Signal The filtered PWM signal can be considered as an adjustable DC voltage. It can be used to replace the variable DC voltage source in dimming control. AP5724 FB VDC R3 100k R2 5k RSET 5 Figure 3. Dimming Control Using a DC Voltage 4. Using a Logic Signal For applications that need to adjust the LED current in discrete steps, a logic signal can be used as shown in Figure 4. RSET sets the minimum LED current (when the NMOS is off). RSET sets how much the LED current increases when the NMOS is turned on. AP5724 FB RINC Logic Signal RSET Figure 4. Dimming Control Using a Logic Signal AP5724/25/26pcb_V2(SOT26) Rev. 1 5 of 8 www.diodes.com JULY 2009 © Diodes Incorporated AP5724 (application) WHITE LED STEP-UP CONVERTER Evaluation Board VIN vs. Efficiency 100 3 LEDs Efficiency(%) 90 4 LEDs 80 6 LEDs 70 60 50 40 2.5 3 3.5 VIN (V) 4 4.5 5 Specification Symbol Parameter System Supply Input VIN Operating Input Voltage UVLO Under Voltage Lockout Under Voltage Lockout Hysteretic IQ Quiescent Current ISD Shutdown Current Oscillator FOSC Operation Frequency Dmax Maximum Duty Cycle Reference Voltage VFB Feedback Voltage IFB FB Pin Bias Current MOSFET Rds(on) On Resistance of MOSFET IOCP Switching Current Limit Control and Protection EN Voltage High EN Voltage Low IEN EN Pin Pull Low Current OVP OVP Threshold θJA Thermal Resistance Junction-to-Ambient θJC Thermal Resistance Junction-to-Case Notes: Conditions Min Typ. Max Unit FB=0.2V, No Switching VEN < 0.4V 2.7 - 2.2 85 500 0.1 5.5 2.4 1 V V mV µA µA 1 86 1.2 90 1.4 - MHz % 0.09 10 0.1 45 0.11 100 V nA 0.5 - 0.75 750 1.0 - Ω mA 1.5 26 4 30 162 200 36 30 0.4 6 34 V V µA V o C/W o C/W o C/W o C/W Normal Operation ON OFF SOT26 (Note 3) DFN2020C-6 (Note 3) SOT26 (Note 3) DFN2020C-6 (Note 3) 3. Test condition for SOT26 and DFN2020C-6: Devices mounted on FR-4 substrate PC board, 2oz copper, with minimum recommended pad layout. AP5724/25/26pcb_V2(SOT26) Rev. 1 6 of 8 www.diodes.com JULY 2009 © Diodes Incorporated AP5724 (application) WHITE LED STEP-UP CONVERTER Schematic Test Circuit VOUT VIN L1 C1 1uF D2 D1 22uH B0540WS C2 1uF U1 LED 1 2 D3 R1 5ohm LED 3 SW GND Vin OVP FB EN 6 5 4 ON OFF AP5724 D4 LED D5 LED D6 LED D7 LED Bill of Materials Component Location Quantity Specification Mark Maker Part No. Size TAIYO YUDEN TMK212 B7105MG-T C0805 22uH/0.88A Wurth Electronics 744042220 4.8 X 4.8 X 1.6 22uH/0.56A GOTREND GLP3810PH220N 3.8 X 3.8 X 1.05 22uF/25V C1,C2 2 (Ceramic Capacitor) L1 R1 D1 U1 1 1 1 1 (1%) YAGEO 40V/0.5A ZETEX ZLLS400 SOD323 40V/0.5A DIODES B0540WS SOD323 40V/0.25A DIODES SDM20U40 SOD523 AP5724_SOT26 Diodes 5Ω AP5724/25/26pcb_V2(SOT26) Rev. 1 7 of 8 www.diodes.com R0805 SOT26 JULY 2009 © Diodes Incorporated AP5724 (application) WHITE LED STEP-UP CONVERTER PCB Layout Top Layer AP5724/25/26pcb_V2(SOT26) Rev. 1 Bottom Layer 8 of 8 www.diodes.com JULY 2009 © Diodes Incorporated