4

UC2855A/B

UC3855A/B

ELECTRICAL CHARACTERISTICS:Unless otherwise specified: VCC = 18V, RVS = 23k, CT = 470pF, CI = 150pF, VRMS

= 1.5V, IAC = 100

µA, ISENSE = 0V, CAO = 4V, VAOUT= 3.5V, VSENSE = 3V. –40°C to 85°C (UC2855A/B), 0°Cto70°C

(UC3855A/B).

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX

UNITS

Gate Driver Output

Output High Voltage

lOUT = −200mA, VCC = 15V

12

12.8

V

Output Low Voltage

lOUT = 200mA

1

2.2

V

Output Low Voltage

lOUT = 10mA

300

500

mV

Output Low (UVLO)

lOUT = 50mA, VCC = 0V

0.9

1.5

V

Output RISE/FALL Time

CLOAD = 1nF

35

ns

Output Peak Current

CLOAD = 10nF

0.5

1.5

A

ZVT

ZVS Threshold

2.3

2.6

2.9

V

Input Bias Current

V = 2.5V, VCT =0

6

20

µA

Propagation Delay

Measured at ZVTOUT

100

ns

Maximum Pulse Width

400

ns

Output High Voltage

lOUT = −100mA, VCC = 15V

12

12.8

V

Output Low Voltage

lOUT = 100mA

1

2.2

V

lOUT = 10mA

300

900

mV

Output Low (UVLO)

lOUT = 50mA, VCC = 0V

0.9

1.5

V

Output RISE/FALL Time

CLOAD = 1nF

35

ns

Output Peak Current

CLOAD = 10nF

0.25

0.75

A

Current Synthesizer

ION to CS Offset

VION = 0V

30

50

mV

Cl Discharge Current

IAC = 50

µA

105

118

140

µA

IAC = 500

µA5

µA

IAC Offset Voltage

0.3

0.65

1.1

V

ION Buffer Slew Rate

10

V/

µs

ION Input Bias Current

VION =2V

2

15

µA

RVS Output Voltage

23k from RVS to GND

2.87

3

3.13

V

Note 1: Gain constant (K) =

IAC VA

V

VIMO

OUT

RMS

•

•

(– . )

()

15

2

at VRMS = 1.5V, VAOUT = 5.5V.

PIN DESCRIPTIONS

CA

This is the inverting input to the current amplifier.

Connect the required compensation components be-

tween this pin and CAOUT. The common mode operating

range for this input is between

−0.3V and 5V.

CAO: This is the output of the wide bandwidth current

amplifier and one of the inputs to the PWM duty cycle

comparator. The output signal generated by this amplifier

commands the PWM to force the correct input current.

The output can swing from 0.1V to 7.5V.

CI: The level shifted current sense signal is impressed

upon a capacitor connected between this pin and GND.

The buffered current sense transformer signal charges

the capacitor when the boost switch is on. When the

switch is off, the current synthesizer discharges the ca-

pacitor at a rate proportional to the dI/dt of the boost in-

ductor current. In this way, the discharge current is

approximately equal to

3

4

V

RRVS

IAC

–

.

Discharging the CI capacitor in this fashion, a “recon-

structed” version of the inductor current is generated us-

ing only one current sense transformer.