BICMOS PWM CONTROLLERS
TC25C25
TC35C25
OSCILLATOR FREQUENCY AND OUTPUT DEAD TIME
1/FO
T
V
D
DD
3V
PIN 5
.5V
1.5V
+
–
0.5V 3V
7
ON-TIME
(T
1.5V
I
≈
)
CHARGE
5
CHG
R
T
6
PIN 4
3
DISCH
SYNC
R
D
OUTPUT DEAD TIME (T )
D
C
R
T
T
1
The oscillator frequency (FO) =
T
CHG + TD
I
+
S
V
IN
TO OUTPUT
SHUTDOWN
TCHG is the charging duration of CT. One of the PWM
Controller output drivers is ON during charging of CT. TD is
the output dead time when both of the output drivers are
inactive. Resistor (RT) sets the Capacitor (CT) charging
current.
1mA
V
V
= 9.2V
ON
= 7V
OFF
75µA
V
IN
7V 9.2V
To choose an oscillator frequency (FO), first select the
period of dead time (TD) required. Calculate the capacitor
charge time (TCHG).
I
= STARTUP CURRENT
S
1 – FO x TC
TCHG
=
FO
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
CT. See graph in Typical Characteristic Curve. Calculate
capacitor charging current (ICHG).
2.5 x CT
TCHG
ICHG
=
CT in Farads, TCHG in seconds, ICHG in amperes, and
RT in ohms.
1.5
ICHG
RT =
The resistor (RD) controls the period of dead time (TD).
During dead time this resistor (RD) current is the sum of the
CT discharge current and the ICHG current. The value for RD
can range from >1Ω to <900Ω. Dead time increases when
RD is increased. See graph in Typical Characteristic Curve
for dead time resistor value.
4-116
TELCOM SEMICONDUCTOR, INC.
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
