CS2842A Datasheet, PDF (6/10 Pages) Cherry Semiconductor Corporation – Off-Line Current Mode PWM Control Circuit with Undervoltage Lockout

CS2842A, CS3842A, CS2843A, CS3843A
4.7 kΩ
1.0 kΩ
ERROR AMP
ADJUST
4.7 kΩ
2N2222
100 kΩ
5.0 kΩ
Sense
ADJUST
R_T
COMP
V_FB
Sense
OSC
V_REF
V_CC
0.1 μF
V_OUT
GND
A
1.0 kΩ
1.0 W
0.1 μF
V_REF
V_CC
V_OUT
C_T
Figure 4. Test Circuit
GND
CIRCUIT DESCRIPTION
V_CC
ON/OFF Command
to reset of IC
V_ON
V_OFF
CSX842A
16 V
10 V
CSX843A
8.4 V
7.6 V
I_CC
< 15 mA
< 1.0 mA
V_ON V_OFF
V_CC
Figure 5. Typical Undervoltage Characteristics
Undervoltage Lockout
During Undervoltage Lockout (Figure 5), the output
driver is biased to a high impedance state. The output should
be shunted to ground with a resistor to prevent output
leakage current from activating the power switch.
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 6). 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.
OSC
OSC
RESET
EA Output
Switch
Current
V_CC
I_O
V_O
Figure 6. Timing Diagram for Key CS2841B
Parameters
When the power supply sees a sudden large output current
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 internal oscillator
waveform provides the maximum duty cycle clamp as
programmed by the selection of oscillator components.
Setting the Oscillator
Oscillator timing capacitor, C_T, is charged by V_REF
through R_T and discharged by an internal current source.
During the discharge time, the internal clock signal blanks
out the output to the Low state, thus providing a user selected
maximum duty cycle clamp. Charge and discharge times are
determined by the formula:
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