CMOS CURRENT MODE
PWM CONTROLLERS
TC18C46
TC28C46
TC38C46
1
×
3.15 CURRENT–
SENSE AMPLIFIER
+
–
TCX8C46
4
+
VS
–
I • RS
VS = S
N
9
N
1
IS
RT
TCx8C46
2
SYNC
10
CMPTR
7
1/2 TC4427
3
8 C
T
MASTER
Figure 3. Transformer Isolated Current Sense
VDD
3
7
CMPTR
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).
2
9 R
T
8
CT
SLAVE
VREF
10
SYNC
TCx8C46
4
5
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.
Figure 4. Master/Slave Parallel Operation
VDD
15
EXTERNAL*
OSCILLATOR
1/2
TC4427
10
TCx8C46
SYNC
VREF
2
VDD
15
*PULSE WIDTH OF
OSCILLATOR IS = TD
10 SYNC TCx8C46 C
T
VREF
2
RT
9
RT
9
8
CT
6
7
Figure 5. External Clock Synchronization
8
TELCOM SEMICONDUCTOR, INC.
4-107
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
