CMOS CURRENT MODE
PWM CONTROLLERS
TC18C46
TC28C46
TC38C46
1/FO
VDD
PIN 8
TD
0.5V 3.5V
ICHARGE
3.5V
RO
RT
+
–
3.5V
PIN 10
TCHG
10
SYNC
DISCHARGE
CURRENT
2mA
8
9
RT
OUTPUT DEAD TIME (TD)
CT
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
3
(.002 - 3.5)
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
R
T
I
CHG
= C
T
.
Also,
∆V
∆T
Where,
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
∆T
,where
∆V=
3.0 Volts
∆T=
T
D
4-108
TELCOM SEMICONDUCTOR, INC.
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
