DN38

Design Note 38
Issue 1 June 1996
The ZR2431 Low Voltage Shunt Regulator in
SMPS Control Loops
D1
+ve
+ve
OPT1
R5
* Note 1
Input
(From Converter
Transformer)
R1
220
R3
160k
C1
22nF
IC1
ZR2431
Output
C2
220uF (3.0V)
R2
10k
C4
* Note 1
C3
* Note 1
-ve
R4
120k
-ve
Figure 1
The ZR2431 used within a SMPS Control/Feedback Loop.
Note 1: C3, C4 and R5 are optional components. See text.
Switch-Mode Power Supply
Controller
The circ uit s hown in Figure 1 is
commonly used in the control loop of
sw itch-mode pow er suppl ies. The
output voltage is sensed via R3/R4 by the
ZR2431 and the IC controls the current
passed through an opto-coupler and
hence feeds back output voltage status
to the switching regulator. The
minimum supply voltage on which this
popular circuit can be used is set by the
m i n i m u m c a t h o d e v o l ta ge o f th e
regulator IC plus the forward voltage
drop of the opto-coupler LED.
Previously, the voltage drop of old
references possessing a minimum
DN 38 - 1
Design Note 38
Issue 1 June 1996
cathode voltage of 2.5V, added to an
opto-coupler voltage drop of around
1.2V meant that the circuit was not viable
below 4V. Unlike old versions of the 431,
the reference voltage of the ZR2431 is
1.23V and its minimum cathode voltage
fractionally below this. Consequently,
the ZR2431 allows supplies with outputs
down to 2.5V to utilise this circuit
topology.
The capacitor C1 is not required for
correct operation of the ZR2431. It has
been included as it is frequently needed
to stabilise the overall control loop of the
switch-mode power supply of which
Figure 1 is only a part. The ZR2431 is
normally stable without additional
components in the circuit shown but if
any problems are observed, one or more
of the following steps should provide a
cure:1) Keep the operating current of the
ZR2431 above 2mA. This can be done by
selecting a s uitable load at the
photo-transistor end of the opto-coupler
or by adding R5 with the value 470 ohms.
2 ) A d d C 4 , w i t h a v a l u e o f 1µF.
Connected to this low impedance point,
the capacitor should have little if any
effect on the control loop of the power
supply yet will guarantee stability of the
ZR2431.
3) Some supplies use a compensation
resistor wired in series with capacitor C1
(in this case R2). If such a resistor has
been included, a small capacitor (1nF)
located in the position C3 can be an
effective stabilising component and may
be a preferable alternative to adding the
capacitor C4.
DN38 - 2
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