4

UCC1857

UCC2857

UCC3857

PIN DESCRIPTIONS

AGND: Reference point of the internal reference and all

thresholds, as well as the return for the remainder of the

device except for the output drivers.

CA–: Inverting input of the inner current loop error ampli-

fier.

CAO: Output of the inner current loop error amplifier.

This output can swing between approximately 0.2V and

6V. It is one of the inputs to the PWM comparator.

VAO: This is the output of the voltage loop error ampli-

fier. It is internally clamped to approximately 5.6V by the

UCC3857 and can swing as low as approximately 0.1V.

Voltages below 0.5V on VAO will disable the MOSDRV

output and force the IGDRV1 and IGDRV2 outputs to a

zero overlap condition.

CRMS: A capacitor is connected between CRMS and

ground to average the AC line voltage over a half cycle.

CRMS is internally connected to the RMS detection cir-

cuitry.

CT: A capacitor (low ESR, ESL) is tied between CT and

ground to set the ramp generator switching frequency in

conjunction with RT. The ramp generator frequency is ap-

proximately given by:

f

RC

SW

TT

≈

0.67

•

.

DELAY: A resistor to VREF and a capacitor to AGND are

connected to DELAY to set the overlap delay time for the

MOSDRV output stage. The overlap delay function can

be disabled by removing the capacitor to AGND.

IAC: A resistor is connected to the rectified AC input line

voltage from IAC. This provides the internal multiplier

and the RMS detector with instantaneous line voltage in-

formation.

IGDRV1: Driver output for one of the two external IGBT

power switches.

IGDRV2: Driver output for one of the two external IGBT

power switches.

MOSDRV: Driver output for the external power MOSFET

switch.

MOUT: Output of the analog multiply and divide circuit.

The output current from MOUT is fed into a resistor to

the return leg of the input bridge. The resultant waveform

forms the sine reference for the current error amplifier.

PKLMT: Inverting input of the peak current limit com-

parator. The threshold for this comparator is nominally

set to 0V. The peak limit comparator terminates the

MOSDRV, IGDRV1 and IGDRV2 outputs when tripped.

PGND: Return for all high level currents, internally tied to

the output driver stages of the UCC3857.

the charging current for the internal ramp generator. The

UCC3857 provides a temperature compensated 3.0V at

Current out of RT should be limited to 250

µA for best

performance.

VA–: This is the feedback input for the outer voltage con-

trol loop. An external opto isolator circuit provides the

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX

UNITS

Soft Start

Charge Current

10

µA

Shutdown Comparator Threshold

Measured on SS

0

0.4

V

Multiplier

Output Current, IAC Limited

–200

µA

Output Current, Power Limited

–200

µA

Output Current, Zero

–2

0

2

µA

Gain Constant

2.5

1/V

Zero Power, Peak Current

Zero Power Comparator Threshold

Measured on VAO

0.5

V

Peak Current Limit Comparator

Threshold

Measured on PKLMT

0

V