
4-107
TELCOM SEMICONDUCTOR, INC.
7
6
5
4
3
1
2
8
Figure 5. External Clock Synchronization
Figure 4. Master/Slave Parallel Operation
Figure 3. Transformer Isolated Current Sense
Under Voltage Lockout
The under voltage lockout circuit forces the PWM con-
troller outputs OFF (LOW) if the supply voltage is below 7V.
Threshold hysteresis is 0.8V and guarantees clean, jitter-
free turn-ON and turn-OFF points. The hysteresis also
reduces capacitive filtering requirements at the PWM con-
troller supply input (pin 15).
Circuit Synchronization
Current-mode-controlled power supplies can be oper-
ated in parallel with a common load. Paralleled converters
will equally share the load current. Voltage-mode control-
lers unequally share the load current, decreasing system
reliability.
Two or more of these PWM controllers can be slaved
together for parallel operation. Circuits can operate from a
master PWM controller internal oscillator with an external
driver (Figure 4). Devices can also be slaved to an exter-
nal oscillator (Figure 5). Disable internal slave device os-
cillators by grounding pin 8. Slave controllers derive an
oscillator from the bidirectional synchronization output sig-
nal at pin 10.
Pin 10 is bidirectional in that it is intended to be both a
sync output and input. This is accomplished by making the
output driver "weak." This is advantageous in that it elimi-
nates an additional pin from the package but does not
enable the device to directly drive another device. In order
to make it an effective driver, a buffer is required (Figure 4).
In order to use pin 10 as a sync input, it is necessary to
overcome the internal driver. This requires a pulse with an
amplitude equal to V
S
. Since V
S
must be above 7V for the
undervoltage lockout to be disabled, a CMOS or open-
collector TTL driver should be used.
4
3
3.15 CURRENT–
SENSE AMPLIFIER
TCX8C46
VS
–
N
1
IS
VS=ISN
+
–
+
MASTER
RT
SYNC
CMPTR
7
SLAVE
9
8
9
8
2
10
7
1/2 TC4427
VDD
TCx8C46
10
CT
VREF
CT
RT
CMPTR
SYNC
TCx8C46
1/2
TC4427
TCx8C46
TCx8C46
SYNC
VREF
SYNC
EXTERNAL*
OSCILLATOR
CT
2
9
2
9
10
15
15
VDD
8
10
VDD
*PULSE WIDTH OF
OSCILLATOR IS = TD
CT
VREF
RT
RT
CMOS CURRENT MODE
PWM CONTROLLERS
TC18C46
TC28C46
TC38C46