BICMOS PWM CONTROLLERS
TC25C25
TC35C25
OSCILLATOR FREQUENCY AND OUTPUT DEAD TIME
1/FO
VDD
PIN 5
TD
3V
.5V
ON-TIME
(TCHG)
PIN 4
0.5V 3V
+
ICHARGE
≈
5
1.5V
RT
6
1.5V
–
7
3
DISCH
SYNC
RD
CT
RT
OUTPUT DEAD TIME (TD)
The oscillator frequency (F
O
) =
1
T
CHG
+ T
D
V+
IN
TO OUTPUT
SHUTDOWN
1mA
VON = 9.2V
VOFF = 7V
75µA
IS
T
CHG
is the charging duration of C
T
. One of the PWM
Controller output drivers is ON during charging of C
T
. T
D
is
the output dead time when both of the output drivers are
inactive. Resistor (R
T
) sets the Capacitor (C
T
) charging
current.
To choose an oscillator frequency (F
O
), first select the
period of dead time (T
D
) required. Calculate the capacitor
charge time (T
CHG
).
T
CHG
=
1 – F
O
x T
C
F
O
VIN
7V
9.2V
IS = STARTUP CURRENT
UNDERVOLTAGE LOCKOUT SECTION
The typical turn on threshold is 9.2V for operation of this
family of PWM Controllers. When supply voltage at pin 15
drops below 7V, after normal operation above 9.2V, lockout
occurs and both output drives to pin 11 and 14 are termi-
nated.
Select a capacitor in the range of 100 pF to 1000 pF for
C
T
. See graph in Typical Characteristic Curve. Calculate
capacitor charging current (I
CHG
).
I
CHG
=
2.5 x C
T
T
CHG
C
T
in Farads, T
CHG
in seconds, I
CHG
in amperes, and
R
T
in ohms.
R
T
=
1.5
I
CHG
The resistor (R
D
) controls the period of dead time (T
D
).
During dead time this resistor (R
D
) current is the sum of the
C
T
discharge current and the I
CHG
current. The value for R
D
can range from >1Ω to <900Ω. Dead time increases when
R
D
is increased. See graph in Typical Characteristic Curve
for dead time resistor value.
4-116
TELCOM SEMICONDUCTOR, INC.
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
