Technical Note

6/11

BD5650AFVM

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2011.11 - Rev.B

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Explanation for circuit working

1. Constant voltage control

(1-1) Output voltage

Voltage feedback system is composited from error amplifier, resistance R1 / R2 and optocoupler connected to OUT

terminal.

Output voltage “VOUT” is defined by expression (1).

VOUT = VREF × (R1+R2)/R2

(1)

VOUT is free setting from R1 / R2, but a potential of OUT terminal is not over VCC.

In addition, it is recommended that resistance R1 / R2 has high impedance not to have heavy load at output. But an input

bias current is 50nA(typ.) in VCT terminal, you need to select a resistance value that flow over 10uA not to influence the

ratio of resistance in (1).

We show a reference value below.

When R1=160kΩ, R2=51kΩ, Vout=5.00V

(1-2) Frequency response of error amplifier

In BD5650AFVM, shunt regulation executes constant voltage control. Monitoring an alteration of output voltage in VCT

terminal, through error amplifier, finally respond as sink current in OUT terminal. A frequency response of

transconductance, a change at output current against an change at input voltage, is shown in Fig.17. In case that frequency

is higher over 200kHz, a response of GAIN is lower, error amplifier is losing its function little by little.

Fig.17

Fig.18

It is needed that your application circuit connects external capacitance and resistance between OUT terminal and VCT

terminal for phase compensation regarding constant voltage control. But you need to decide external values sufficiently

considering static characteristics, transient characteristics and IC’s unevenness to keep working application margin.

+

-

1.21V

OUT

VCC

BD5650FVM

51ｋ

160ｋ

CH1

～

OSC

50

VCT

G= CH1[A]

OSC[V]

GND

Error Amplifier