Test Circuit
VREF
RT
2N2222
VCC
A
100kΩ
COMP
VFB
VREF
4.7kΩ
0.1µF
0.1µF
1kΩ
Error Amp
Adjust
VCC
1kΩ
1W
CS3844B
5kΩ
Sense
Adjust
4.7kΩ
VOUT
VOUT
Gnd
Sense
OSC
Gnd
CT
Circuit Description
Undervoltage Lockout
During Undervoltage Lockout (Figure 1), the output driv-
er is biased to sink minor amounts of current. The output
should be shunted to ground with a resistor to prevent
activating the power switch with extraneous leakage cur-
rents.
VCC
ON/OFF Command
to reset of IC
CSX844B
PWM Waveform
VON
VOFF
16V
10V
To generate the PWM waveform, the control voltage from
the error amplifier is compared to a current sense signal
which represents the peak output inductor current (Figure
2). An increase in VCC causes the inductor current slope to
increase, thus reducing the duty cycle. This is an inherent
feed-forward characteristic of current mode control, since
the control voltage does not have to change during
changes of input supply voltage.
ICC
When the power supply sees a sudden large output cur-
rent increase, the control voltage will increase allowing
the duty cycle to momentarily increase. Since the duty
cycle tends to exceed the maximum allowed to prevent
transformer saturation in some power supplies, the inter-
nal oscillator waveform provides the maximum duty
cycle clamp as programmed by the selection of OSC com-
ponents.
<15mA
<600µA
VCC
VON VOFF
Figure 1: Typical Undervoltage Characteristics
4