TC170

DS21395B-page 8

© 2002 Microchip Technology Inc.

FIGURE 3-4:

TRANSFORMER

ISOLATED CURRENT

SENSE

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.

lockout to be disabled, a CMOS or open-collector TTL

driver should be used.

FIGURE 3-5:

MASTER/SLAVE

PARALLEL OPERATION

FIGURE 3-6:

EXTERNAL CLOCK

SYNCHRONIZATION

4

3

N1

N

+

–

+

–

x 3.15 Current Sense

Amplifier

TC170

Master

SYNC

CMPTR

Slave

9

8

9

8

210

7

7

1/2 TC4427

10

CMPTR

SYNC

TC170

TC170

1/2

TC4427

SYNC

External*

Oscillator

29

29

10

15

15

8

10

VS

+

*Pulse Width of

TC170

TC170

SYNC