ANP031 Application Note AP431 Signal Response In Switch-Mode Power Supply Contents 1. Switch-Mode Power Supply Controller 2. AP431 Vref Minimum Operating Voltage 3. AP431 Response for Coupling Signal This application note contains new product information. Diodes, Inc. reserves the right to modify the product specification without notice. No liability is assumed as a result of the use of this product. No rights under any patent accompany the sale of the product. 1/5 ANP031 – App. Note 1 JANUARY 2010 www.diodes.com Diodes Incorporated ANP031 Application Note AP431 Signal Response In Switch-Mode Power Supply 1. Switch-Mode Power Supply Controller D1 12V Vout Input from converter transformer OPT1 R3 1k R1 17.8k C1 22nF CATHODE ANODE C2 220uF R4 10k REF C3 optional AP431 R2 4.7k GND Figure 1: SMPS control/feedback loop The circuit shown in Figure 1 is commonly used in the control loop of switch-mode power supplies. The output voltage is sensed via R1/R2 by the AP431 and the IC controls the current passing through an opto-coupler and hence feeds back output voltage status to the switching regulator. The minimum supply voltage at which this popular circuit can be used is set by the minimum cathode voltage of the regulator IC plus the forward voltage drop of the opto-coupler LED. With AP431's minimum cathode voltage of 2.5V, and 1.2V voltage drop of the opto-coupler, the circuit is not viable below 4V output voltage. The capacitor C1 is not required for correct operation of the AP431. It has been included as it is frequently needed to stabilize the overall control loop of the switch-mode power supply. This application note contains new product information. Diodes, Inc. reserves the right to modify the product specification without notice. No liability is assumed as a result of the use of this product. No rights under any patent accompany the sale of the product. 2/5 ANP031 – App. Note 1 JANUARY 2010 www.diodes.com Diodes Incorporated ANP031 Application Note AP431 Signal Response In Switch-Mode Power Supply 2. AP431 Vref Minimum Operating Voltage Q2 Q4 Q11 Figure 2: AP431 schematic 2.1 The simplified schematic of AP431 is illustrated in fig.2. The path of Q2, Q4, Q11 creates an internal bias circuit for IC normal operations. If REF voltage is below 3*Vbe (by Q2, Q4, and Q11), the internal biasing circuit ceases to operate. Therefore, the minimum required REF voltage for normal regulation is 3*Vbe, typically 2.1V at room temp. 2.2 For stable operation in switch-mode power supply with 12V output(fig. 1), output ripple voltage is around 100mV ripple voltage. REF voltage is regulated to 2.5V with about 20mV ripple voltage. Since REF voltage is over than 2.1V, the control circuit works normally to regulated output to target voltage. This application note contains new product information. Diodes, Inc. reserves the right to modify the product specification without notice. No liability is assumed as a result of the use of this product. No rights under any patent accompany the sale of the product. 3/5 ANP031 – App. Note 1 JANUARY 2010 www.diodes.com Diodes Incorporated ANP031 Application Note AP431 Signal Response In Switch-Mode Power Supply 3. AP431 Response for Coupling Signal 3.1 For some undesirable conditions that couple REF to below the minimum operating voltage, i.e., 2.1V, such as by large external pulses, power supply system could encounter slow response, and eventually result in system shutdown by UVP protection. 3.2 In the test circuit in fig. 3, Vpulse of around 4V magnitude causes REF voltage to drop below 2V, resulting in undesirable slow response on VKA. The circuit returns to normal regulation (i.e., VKA returns to 7V) when REF voltage recovers to over 2.1V. (Figure 4) 3.3 Possible solutions to prevent or alleviate the above undesirable situations are listed below: 3.3.1 Add cap 1nF to REF pin for filtering. (Figure 5). The transient voltage coupled to REF pin can be greatly reduced by the low pass filtering. 3.3.2 Reduce feedback resistors R1/R2 from 30k/20k to 15k/12k (Figure 6), can similarly reduce voltage swing coupled to Vref. 3.3.3 Reduce R3 resistor value from 3.9k to 100 to support more current to Cathode node for faster loop response (Figure 7). R3 3.9k VKA 8.5V R1 30k Cathode REF anode R4 VREF AP431 Vpulse R2 20k 49k Figure 3: AP431 test circuit for signal response This application note contains new product information. Diodes, Inc. reserves the right to modify the product specification without notice. No liability is assumed as a result of the use of this product. No rights under any patent accompany the sale of the product. 4/5 ANP031 – App. Note 1 JANUARY 2010 www.diodes.com Diodes Incorporated ANP031 Application Note AP431 Signal Response In Switch-Mode Power Supply VKA: AP431 Cathode to anode voltage VREF: REF voltage normal response on VKA Vpulse Vpulse slow response on VKA VKA VKA VREF VREF Figure 5: REF pin is added with 1nF Figure 4: AP431 Vka response to Vpulse input normal response on VKA Vpulse Vpulse VKA VKA normal response on VKA VREF VREF Figure 6: R1/R2=15K/12K Figure 7: R3=100 This application note contains new product information. Diodes, Inc. reserves the right to modify the product specification without notice. No liability is assumed as a result of the use of this product. No rights under any patent accompany the sale of the product. 5/5 ANP031 – App. Note 1 JANUARY 2010 www.diodes.com Diodes Incorporated