UCC28050, UCC28051

UCC38050, UCC38051

SLUS515F−SEPTEMBER 2002 − REVISED MARCH 2009

6

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BLOCK DIAGRAM

UDG−02008

1

2

5

6

7

8

3

4

40 k

W

5 pF

+

+

+

x

x

MULT

2.5 V

2.3 V

+

+

OVP

ENABLE

2.7/2.5 V

gm VOL.

ERROR AMP

0.67/0.57 V

0.23/0.15 V

PWM

SQ

Q

R

+

TIMER

+

VREFAND

BIAS REG

VREF

INT. BIAS

1.7/1.4 V

UVLO

VREF

GOOD

OVP

V0_SNS

COMP

MULTIN

CS

ZCD

GND

DRV

VCC

REF

ZERO

POWER

DETECT

PIN DESCRIPTIONS

VO_SNS (Pin 1): This pin senses the boost regulator output voltage through a voltage divider. Internally, this

pin is the inverting input to the transconductance amplifier (with a nominal value of 2.5 V) and also is input to

the OVP comparator. Additionally, pulling this pin below the ENABLE threshold turns off the output switching,

ensuring that the gate drive is held off while the boost output is pre-charging and also ensuring no runaway if

feedback path is open.

COMP (Pin 2): Output of the transconductance error amplifier. Loop compensation components are connected

between this pin and ground. The output current capability of this pin is 10-

μA under normal conditions, but

increases to about 1-mA when the differential input is greater than the specified values in the specifications

table. This voltage is one of the inputs to the multiplier, with a dynamic input range of 2.5 V to 3.8 V. During zero

power or overvoltage conditions, this pin goes below 2.5 V nominal. When it goes below 2.3 V, the zero power

comparator is activated which prevents the gate drive from switching.

MULTIN (Pin 3): This pin senses the instantaneous boost regulator input voltage through a voltage divider. The

voltage acts as one of the inputs to the internal multiplier. Recommended operating range is 0 V to 2.5 V at high

line.

PIN DESCRIPTIONS (continued)

CS (Pin 4): This pin senses the instantaneous switch current in the boost switch and uses it as the internal ramp

for PWM comparator. The internal circuitry filters out switching noise spikes without requiring external

components. In addition, an external R-C filter may be required to suppress the noise spikes. An internal clamp

on the multiplier output terminates the switching cycle if this pin voltage exceeds 1.7 V. Additional external

filtering may be required. CS threshold is approximately equal to: