CMOS CURRENT MODE
PWM CONTROLLER
TC170
×
3.15 CURRENT–
SENSE AMPLIFIER
+
–
TC170
4
+
VS
–
I • RS
VS = S
N
9
N
1
IS
RO
TC170
3
8 C
O
MASTER
SYNC
10
CMPTR
7
1/2 TC4427
Figure 3
Transformer Isolated Current Sense
VDD
Undervoltage Lockout
The undervoltage lockout circuit forces the TC170 out-
puts OFF (low) if the supply voltage is below 7.7V. Threshold
hysteresis is 0.75V and guarantees clean, jitter-free turn-on
and turn-off points. The hysteresis also reduces capacitive
filtering requirements at the PWM controller supply input
(pin 15).
2
9 R
O
8
CO
SLAVE
VREF
10
SYNC
7
CMPTR
TC170
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 TC170 controllers can be slaved to-
gether for parallel operation. Circuits can operate from a
master TC170 internal oscillator with an external driver
(Figure 4). Devices can also be slaved to an external
oscillator (Figure 5). Disable internal slave device oscilla-
tors by grounding pin 8. Slave controllers derive an oscilla-
tor from the bidirectional synchronization output signal 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
IN
. Since V
IN
must be above 8.25V 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
VIN
10
CO
SYNC TC170
VREF
RO
2
+
VS
15
*PULSE WIDTH OF
OSCILLATOR IS = TD
VIN
10
SYNC
TC170
VREF
RO
2
9
CO
9
8
Figure 5.
External Clock Synchronization
4-124
TELCOM SEMICONDUCTOR, INC.
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
