10

This refers to the determination of the gain frequency characteristic of the shunt regulator as an error amplifier.

Taking the configuration in figure 16, the error amplifier characteristic is as shown in figure 17.

AC Characteristic Determination:

Figure 17. GM431 Error Amplification Characteristic

G1

G2

When R = 0

5

F1

F2

When R = 0

5

In Figure 17, the following formulas are obtained:

Gain

G = G

50 dB to 60 dB (determined by shunt regulator)

10

G =

2

Corner frequencies

f = 1/(2p C G R)

11

0

3

f = 1/(2p C R)

2

1

5

G is the shunt regulator open-loop gain; this is given by the reciprocal of the reference voltage fluctuation

0

DVref/DV

, and is approximately 50 dB.

KA

Practical Example

Consider the example of a photocoupler, with an internal light emitting diode V = 1.05 V and I = 2.5 mA,

F

F

power supply output voltage V = 5 V, and bias resistance R current of approximately 1/5 I at 0.5 mA. If

22

F

the shunt regulator V = 3 V, the following values are found.

K

Next, assume that R = R = 10 kW. This gives a 5 V output. If R = 3.3 kW and C = 0.022 µF, the

34

5

1

following values are found.

G = 3.3 kW / 10 kW = 0.33 times (–10 dB)

2

1

2

R =

=316W

1

5V - 1.05V - 3V

2.5mA + 0.5mA

R =

=2.1 kW

2

1.05V

0.5mA

F

Frequency f (Hz)

osc

*

: PWM switching frequency

A

A

MICROELECTRONICS

GM431

2.5V ADJUSTABLE SHUNT REGULATOR

Power Management