CS3845B Datasheet, PDF (5/8 Pages) Cherry Semiconductor Corporation – Current Mode PWM Control Circuit with 50% Max Duty Cycle

V_OSC
OSC
RESET
Toggle
F/F Output
EA Output
Switch
Current
V_CC
I_O
V_O
Figure 2: Timing Diagram
V_upper
V_lower
t_on
t_C
t_{on =} t_C
t_{off =} t_C+2t_d
Figure 3: Timing Parameters.
Circuit Description: continued
Setting the Oscillator
The parameters T_c and T_d can be determined as follows:
( ) ^{tc = RTCT ln}
V_REF - V_lower
V_REF - V_upper
( ) ^{td = RTCT ln}
V_REF - I_dR_T - V_lower
V_REF - I_dR_T - V_upper
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,
then
t_c ≈ 0.5534R_TC_T
( ) ^{td = RTCT ln}
2.3 - 0.0083 R_T
4.0 - 0.0083 R_T
For better accuracy R_T should be ≥10kΩ.
Grounding
t_off
High peak currents associated with capacitive loads neces-
sitate careful grounding techniques. Timing and bypass
t_d
capacitors should be connected close to Gnd in a single
point ground.
The transistor and 5kΩ potentiometer are used to sample
the oscillator waveform and apply an adjustable ramp to
Sense.
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