CMOS CURRENT MODE
PWM CONTROLLERS
1
2
3
4
5
6
7
8
TC18C46
TC28C46
TC38C46
9
8
× 3.15 CURRENT–
R
C
T
SENSE AMPLIFIER
N
1
4
3
+
+
–
TCx8C46
V
I
S
S
–
SYNC
10
CMPTR
7
T
MASTER
TCX8C46
I
• RS
S
=
V
S
N
1/2 TC4427
V
Figure 3. Transformer Isolated Current Sense
DD
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
10
7
V
SYNC
CMPTR
REF
9
8
R
C
T
TCx8C46
T
SLAVE
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.
Figure 4. Master/Slave Parallel Operation
V
DD
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.
15
1/2
TC4427
EXTERNAL*
OSCILLATOR
10
8
TCx8C46
C
T
SYNC
V
R
T
9
REF
2
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 VS. Since VS must be above 7V for the
undervoltage lockout to be disabled, a CMOS or open-
collector TTL driver should be used.
V
DD
15
*PULSE WIDTH OF
OSCILLATOR IS = T
D
10
TCx8C46
C
SYNC
V
T
R
T
REF
2
9
Figure 5. External Clock Synchronization
TELCOM SEMICONDUCTOR, INC.
4-107
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
