ANP031_AP431

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.
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ANP031 – App. Note 1
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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.
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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.
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ANP031 – App. Note 1
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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.
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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
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JANUARY 2010
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 Diodes Incorporated
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