CS2842A Datasheet, PDF (7/10 Pages) Cherry Semiconductor Corporation – Off-Line Current Mode PWM Control Circuit with Undervoltage Lockout

CS2842A, CS3842A, CS2843A, CS3843A
ǒ Ǔ tc + RTCT
ln
VREF * Vlower
VREF * Vupper
ǒ Ǔ td + RTCT
ln
VREF * IdRT * Vlower
VREF * IdRT * Vupper
Substituting in typical values for the parameters in the
above formulas:
V_REF = 5.0 V
V_upper = 2.7 V
V_lower = 1.0 V
I_d = 8.3 mA
t_c ≈ 0.5534R_TC_T
ǒ Ǔ td + RTCT
ln
2.3 * 0.0083RT
4.0 * 0.0083RT
The frequency and maximum duty cycle can be
determined using the Typical Performance Characteristic
graphs.
Grounding
High peak currents associated with capacitive loads
necessitate careful grounding techniques. Timing and
bypass capacitors should be connected close to GND pin in
a single point ground.
The transistor and 5.0 kΩ potentiometer, shown in the test
circuit, are used to sample the oscillator waveform and apply
and adjustable ramp to Sense.
V_REF
OSC
GND
Timing Parameters
V_upper
V_lower
t_c
Sawtooth Mode
Large R_T (≈ 10 kΩ)
Triangular Mode
Small R_T (≈ 700 kΩ)
R_T
C_T
t_d
V_OSC
Internal Clock
V_OSC
Internal Clock
Figure 7. Oscillator Timing Network and
Parameters
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