TC18C43 Datasheet, PDF (5/7 Pages) TelCom Semiconductor, Inc – BiCMOS CURRENT MODE PWM CONTROLLERS

BiCMOS CURRENT MODE
PWM CONTROLLERS
1 _TC18C43
TC28C43
TC38C43
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
TCx8C43
VON = 8.4V
VOFF = 7.6V
IS
1mA
TCx8C43
170µA
8.4 V
7.6V
VDD
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.
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
R
RS
C
ISENSE
5
0.4V
–+
PWM
COMPARATOR
+
–
GND
9
2R
ERROR AMP
R
LIMIT BUFFER
+
1.4V
–
Current is sensed through voltage drop across resistor
2 ^{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.
3
SHUTDOWN METHODS
TCx38C43
TCx38C43
14
VREF
5k
SHUTDOWN
ISENSE
5.1k
5
1
CMPTR
ANALOG
9 ^GND
4
SHUTDOWN
TO CURRENT
SENSE RESISTOR
5
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
6
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-
7 ^{mined the operating characteristics of the current mode}
controller.
PEAK CURRENT (IS) FORMULA:
0.95V
IS = _RS
TELCOM SEMICONDUCTOR, INC.
8
4-97