5

UCC1570

UCC2570

UCC3570

Latched Shutdown

If CURLIM rises above 0.6V, or COUNT rises to 4V, the

shutdown latch will be set. This will force OUT low, dis-

proximately 1mA. When, and if, VCC falls below 9V, the

ing the circuit to restart. If VCC remains above 9V, an al-

ternate restart will occur if VFWD is momentarily reduced

below 1V. External shutdown commands from any source

may be added into either the COUNT or CURLIM pins.

Deadtime Control

The voltage waveform on RAMP has independently con-

trolled rising and falling edges. At the start of the clock

period, RAMP is at 1V and rises to 4V. It then discharges

back to 1V and awaits the next clock period. OUT can

only be high during the rising part of the waveform, while

it is positively blanked off during the falling portion. Set-

ting the –dV/dt slope by R4 from ISET to GND estab-

lishes a minimum deadtime as:

td

R

=•

•

03

4

.

50pF. In order to have a pulse at OUT in the next clock

period, RAMP must fall to 1V prior to the end of the cur-

rent period. If it does not, OUT will remain low for the en-

tire next clock period.

Voltage Feedforward

The +dV/dt on RAMP is made proportional to line volt-

age. The slope is:

()

dV

dt

VFWD

=•

•

10

3

where VFWD is line voltage scaled by R1 and R2. There-

fore, a changing line voltage will accomplish an immedi-

ate proportionate pulse width change without any action

from the feedback amplifier. This will result in constant

volt-second drive to the power transformer providing both

international voltage operation, and excellent dynamic

line regulation. VFWD is intended to operate over a 4:1

range (1V to 4V) with undervoltage and overvoltage sen-

sors designed to drive OUT low if this range is exceeded.

Choose R3 between 20k and 200k.

APPLICATION INFORMATION (cont.)

Figure 1. UCC1570 typical application.