TC18C46-1 Datasheet, PDF (8/10 Pages) TelCom Semiconductor, Inc – CMOS CURRENT MODE PWM CONTROLLER

TC18C46
TC28C46
TC38C46
CMOS CURRENT MODE
PWM CONTROLLERS
VDD
PIN 8
1/FO
TD
0.5V 3.5V
ICHARGE
Ï·
3.5V
RO_RT
8
9
10
SYNC
RT
2mA
DISCHARGE
CT
CURRENT
3.5V
TCHG
PIN 10
OUTPUT DEAD TIME (TD)
Figure 6. Oscillator Circuit
Selection of Timing Capacitor and Resistor
(C_T & R_T)
1) First determine the frequency of operation F_O and
the desired " dead" time", T_D (see Fig.6 graph).
We need to choose R_T and C_T.
Substituting and rearranging,
C_T = T_D (.002 - 3.5)
3
R_T
Knowing T_D choosing R_T permits calculation of C_{T .}
2) The current mirror in Fig. 6 , shows:
I_CHG = 3.5 V . Also,
R_T
I_CHG = C_T ^∆V Where,
∆T
and
∆V = 3.5 - 0.5
= 3.0 Volts
∆T = T_CHG
3) During discharge (T_D), the discharge current pulls
2 m A out of C_T, minus the I_CHG that keeps trying to
charge it:
2 m A - I_CHG = C_T ^{∆V ,where ∆V= 3.0 Volts}
∆T
∆T= T_D
4-108
TELCOM SEMICONDUCTOR, INC.