4-124

TELCOM SEMICONDUCTOR, INC.

CMOS CURRENT MODE

PWM CONTROLLER

TC170

4

3

3.15 CURRENT–

SENSE AMPLIFIER

TC170

VS

N

1

I S

VS =

IS • RS

N

×

+

–

+

–

Figure 5.

External Clock Synchronization

Figure 4.

Master/Slave Parallel Operation

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

the undervoltage lockout to be disabled, a CMOS or open-

collector TTL driver should be used.

MASTER

RO

SYNC

CMPTR

SLAVE

9

8

9

8

210

7

7

1/2 TC4427

VDD

TC170

10

CO

VREF

CO

RO

CMPTR

SYNC

TC170

1/2

TC4427

TC170

TC170

SYNC

SYNC

EXTERNAL*

OSCILLATOR

CO

29

29

10

15

15

8

10

VS

+

VDD

*PULSE WIDTH OF

OSCILLATOR IS = TD

CO

VREF

RO

RO

VREF

VIN

VIN