TC170 Datasheet, PDF (6/8 Pages) TelCom Semiconductor, Inc – CMOS CURRENT MODE PWM CONTROLLER

TC170
CMOS CURRENT MODE
PWM CONTROLLER
× 3.15 CURRENT–
SENSE AMPLIFIER
4
+
N
VS
3
–
TC170
VS
=
IS • RS
N
1
IS
Figure 3 Transformer Isolated Current Sense
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).
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.
9 _RO
8 _CO
MASTER
TC170
SYNC
10
CMPTR
7
1/2 TC4427
VDD
2
9 _RO ^VREF
8
CO
SLAVE
10
SYNC
TC170
7
CMPTR
Figure 4. Master/Slave Parallel Operation
VDD
EXTERNAL*
OSCILLATOR
15
1/2
TC4427 ₁₀
VIN
SYNC ^TC170
8
CO
VREF
RO
2
9
VS+
*PULSE WIDTH OF
OSCILLATOR IS = TD
15
VIN
10 _SYNC
TC170
CO
VREF
RO
2
9
Figure 5. External Clock Synchronization
4-124
TELCOM SEMICONDUCTOR, INC.