TC170 Datasheet, PDF (8/16 Pages) TelCom Semiconductor, Inc – CMOS CURRENT MODE PWM CONTROLLER

TC170
FIGURE 3-4:
TRANSFORMER
ISOLATED CURRENT
SENSE
x 3.15 Current Sense
Amplifier
4
+
N
V_S
3
–
TC170
V_S
=
I_S
• RS
N
1
I_S
3.6 Undervoltage Lockout
The undervoltage lockout circuit forces the TC170
outputs OFF (low) if the supply voltage is below 7.7V.
Threshold hysteresis is 0.75V and guarantees clean,
jitter-free turn-on and turnoff points. The hysteresis
also reduces capacitive filtering requirements at the
PWM controller supply input (pin 15).
3.7 Circuit Synchronization
Current-mode-controlled power supplies can be
operated in parallel with a common load. Paralleled
converters will equally share the load current. Voltage-
mode controllers unequally share the load current,
decreasing system reliability.
Two or more TC170 controllers can be slaved together
for parallel operation. Circuits can operate from a
master TC170 internal oscillator with an external driver
(Figure 3-5). Devices can also be slaved to an external
oscillator (Figure 3-6). Disable internal slave device
oscillators by grounding pin 8. Slave controllers derive
an oscillator 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
eliminates 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 3-5). 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 3-5:
MASTER/SLAVE
PARALLEL OPERATION
9
R_O
TC170
8 _CO
Master
SYNC
10
CMPTR
7
1/2 TC4427
V_DD
2
9 _RO ^VREF
8
C_O
Slave
10
SYNC
TC170
7
CMPTR
FIGURE 3-6:
EXTERNAL CLOCK
SYNCHRONIZATION
V_DD
External*
Oscillator
*Pulse Width of
Oscillator is = T_D
15
1/2
V_IN
TC4427 _{10 SYNC TC170}
C_O ⁸
V_REF
R_O
2
9
VS+
15
V_IN
10 _{SYNC TC170 CO}
V_REF
R_O
2
9
DS21395B-page 8
© 2002 Microchip Technology Inc.