CMOS CURRENT MODE
PWM CONTROLLERS
TC18C46
TC28C46
TC38C46
1/F
O
V
DD
T
D
PIN 8
3.5V
R
+
–
0.5V 3.5V
T
CHG
3.5V
I
Ϸ
CHARGE
R
O
T
PIN 10
8
9
OUTPUT DEAD TIME (T )
D
10
R
SYNC
T
2mA
C
DISCHARGE
CURRENT
T
Figure 6. Oscillator Circuit
Selection of Timing Capacitor and Resistor
(CT & RT)
Substituting and rearranging,
CT = TD (.002 - 3.5)
1)
First determine the frequency of operation FO and
the desired " dead" time", TD (see Fig.6 graph).
3
RT
We need to choose RT and CT.
Knowing TD choosing RT permits calculation of CT .
2)
The current mirror in Fig. 6 , shows:
ICHG = 3.5 V
RT
.
Also,
∆V = 3.5 - 0.5
= 3.0 Volts
∆T = TCHG
∆V
∆T
ICHG = CT
Where,
and
3)
During discharge (TD), the discharge current pulls
2 m A out of CT, minus the ICHG that keeps trying to
charge it:
∆V ,where ∆V= 3.0 Volts
∆T ∆T= TD
2 m A - ICHG = CT
4-108
TELCOM SEMICONDUCTOR, INC.
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
