CS2842A Datasheet, PDF (5/6 Pages) Cherry Semiconductor Corporation – Off-Line Current Mode PWM Control Circuit with Undervoltage Lockout

Circuit Description: continued
former saturation in some power supplies, the internal
oscillator waveform provides the maximum duty cycle
clamp as programmed by the selection of oscillator com-
V_OSC
ponents.
OSC
RESET
EA Output
Switch
Current
V_CC
I_OUT
V_OUT
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:
( ) ^{tc = RTCT ln}
V_REF - V_lower
V_REF - V_upper
Figure 2: Timing Diagram for key CS2841B parameters
( ) ^{td = RTCT ln}
V_REF - I_dR_T - V_lower
V_REF - I_dR_T - V_upper
V_REF
R_T
OSC
C_T
Gnd
Timing parameters
V_upper
V_lower
t_c
Sawtooth Mode
LARGE R_T (≈10kΩ)
t_d
V_OSC
Substituting in typical values for the parameters in the
above formulas:
V_REF = 5.0V, V_upper = 2.7V, V_lower = 1.0V, I_d = 8.3mA
t_c ≈ 0.5534R_TC_T
( ) ^{td = RTCT ln}
2.3 - 0.0083 R_T
4.0 - 0.0083 R_T
The frequency and maximum duty cycle can be deter-
mined using the Typical Performance Characteristic
graphs.
Grounding
High peak currents associated with capacitive loads neces-
sitate careful grounding techniques. Timing and bypass
capacitors should be connected close to Gnd pin in a sin-
gle point ground.
The transistor and 5kΩ potentiometer, shown in the test
circuit, are used to sample the oscillator waveform and
apply an adjustable ramp to Sense.
Triangular Mode
SMALL R_T (≈700kΩ)
Internal Clock
V_REF
Internal Clock
Figure 3: Oscillator Timing Network and parameters
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