CS3842B Datasheet, PDF (5/8 Pages) Cherry Semiconductor Corporation – Off-Line Current Mode PWM Control Circuit Off-Line Current Mode PWM Control Circuit

V_OSC
OSC
RESET
EA Output
Switch
Current
V_CC
I_OUT
V_OUT
Figure 2: Timing Diagram for key CS-384XB parameters
Figure 3: Oscillator
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 oscillator timing
components.
Setting the Oscillator
The oscillator timing capacitor, C_T, is charged by V_REF
through R_T and discharged by an internal current source
(Figure 3). During the discharge time, the internal clock
signal blanks out the output to the Low state, thus provid-
ing a user selected maximum duty cycle clamp.
Charge and discharge times are determined by the general
formulas:
( ) ^{tc = RTCT ln}
V_REF – V_lower
V_REF – V_upper
( ) ^{td = RTCT ln}
V_REF – I_d R_T –V_lower
V_REF – I_d R_T – V_upper
V_REF
R_T
OSC
C_T
Gnd
V_upper
V_lower
t_c
Sawtooth Mode
LARGE R_T (≈10kΩ)
t_d
V_OSC
Triangular Mode
SMALL R_T (≈700kΩ)
Internal Clock
V_REF
Substituting in typical values for the parameters in the
above formulas:
then
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 from the Typical Performance Characteristics
graphs.
Grounding
High peak currents associated with capacitive loads neces-
sitate careful grounding techniques. Timing and bypass
capacitors should be connected close to ground in a single
point ground.
The transistor and 5kΩ potentiometer are used to sample
the oscillator waveform and apply an adjustable ramp to
Sense.
Internal Clock
Figure 3: Oscillator Timing Network and parameters
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