BiCMOS CURRENT MODE
PWM CONTROLLERS
TC18C43
TC28C43
TC38C43
Current is sensed through voltage drop across resistor
RS. A small RC filter may be required to suppress switching
transient. This voltage enters PWM Controller at I
SENSE
,
pin 5. A voltage of 0.4 V is added before this is fed into PWM
Comparator (+) input. The PWM Comparator (–) input
senses the voltage feedback error amp output with a limit
buffer that limits this voltage to 1.4V maximum. This limit
buffer limits the peak current across RS to a maximum of
0.95V. In normal operation, the error amplifier controls the
current limit threshold.
SHUTDOWN METHODS
1
Dead Time
The value of R
T
has an effect on the discharge rate but
the primary consideration is the value of C
T
. The time
required to discharge the capacitor is approximately 1000
C
T
.
UNDERVOLTAGE LOCKOUT RANGE
VIN
2
3
4
IS
TCx8C43
VON = 8.4V
VOFF = 7.6V
1mA
TCx8C43
14
TCx38C43
TCx38C43
1
VREF
CMPTR
170µA
8.4 V
7.6V
VDD
5k
SHUTDOWN
9
5.1k
5
ISENSE
ANALOG
GND
Undervoltage Lockout Range
The TCx8C43 PWM Controller is used where wide
ranges of input voltage is not required. The range from
starting V
in
voltage threshold to under voltage threshold is
approximately 9.5% of the starting voltage. The typical start-
up voltage is 8.4V and dropout voltage is 7.6V. This range
is used most in DC-to-DC converter applications.
SHUTDOWN
TO CURRENT
SENSE RESISTOR
5
6
7
Duty Cycle Limit
The TCx8C43 PWM Controller has a duty cycle limit
maximum of 99%. The oscillator is running at the same
frequency as the output.
CURRENT SENSE CIRCUIT
ISENSE
R
5
0.4V
PWM
COMPARATOR
Shutdown can be accomplished by either pulling I
SENSE
above 1 volt or pulling CMPTR, pin 1 to GND. This will set the
PWM latch so that the output will remain low until the next
clock pulse after the shutdown condition is removed.
BENCH TEST OPERATIONAL SIMULATION
The timing ramp (R
T
/C
T
) is buffered by the emitter
fullpower and fed back to the I
SENSE
input. This ramp
simulates the dI/dT current ramp which would flow through
the primary of the transformer. The output voltage of the
power supply is simulated by feeding some of the reference
voltage into V
FB
. The combination of the two levels deter-
mined the operating characteristics of the current mode
controller.
LIMIT BUFFER
2R
–
RS
C
GND
9
+
+
–
+
–
1.4V
ERROR AMP
R
PEAK CURRENT (IS) FORMULA:
IS =
0.95V
RS
8
4-97
TELCOM SEMICONDUCTOR, INC.
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
