CS3845B Datasheet, PDF (4/6 Pages) Cherry Semiconductor Corporation – Current Mode PWM Control Circuit with 50% Max Duty Cycle

Test Circuit
4.7kΩ
1kΩ
Error Amp
Adjust
4.7kΩ
R_T
2N2222
100kΩ
COMP
V_REF
5kΩ
Sense
Adjust
V_FB
V_CC
CS-3845B
Sense
V_OUT
OSC
Gnd
C_T
A
0.1µF
1kΩ
0.1µF
1W
V_REF
V_CC
V _{O UT}
Gnd
Circuit Description
V_CC
V_ON
V_OFF
ON/OFF Command
to reset of IC
CSX845B
8.4V
7.6V
I_CC
<15mA
<1mA
V_ON V_OFF
V_CC
Figure 1: Startup voltage for CS3845B.
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.
PWM Waveform
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 V_CC 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.
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 compo-
nents.
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