UCC28521, UCC28528

SLUS608B − JANUARY 2005 REVISED JUNE 2005

2

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DESCRIPTION (CONTINUED)

The transient response of the circuit is enhanced by allowing a much faster charge/discharge of the voltage

amplifier output capacitance when the output voltage falls outside a certain regulation window. A number of

additional features such as UVLO circuit with selectable hysteresis levels, an accurate reference voltage for the

voltage amplifier, zero power detect, OVP/enable, peak current limit, power limiting, high-current output gate

driver characterize the PFC section.

The PWM section features peak current mode control (with a ramp signal available to add slope compensation),

programmable soft-start, accurate maximum duty cycle clamp, peak current limit and high-current output gate

driver. The PWM stage is suppressed until the PFC output has reached 90% of its programmed value during

startup. During line dropout and turn off, the UCC28521 allows the PWM stage to operate until the PFC output

has dropped to 71% of its nominal bulk output value. The UCC28528 on the other hand allows the PWM stage

to operate continuously until the bias falls below the UVLO turn-off threhold.

Both devices are available in 20-pin DW package.

SIMPLIFIED APPLICATION DIAGRAM

12

3

16

18

10

5

7

2

UCC28521/28

GT1

VSENSE

ISENSE1

IAC

GT2

VREF

VERR

RT

D1

VAC

15

14

13

11

17

19

20

PWRGND

VAOUT

1

6

VFF

MOUT

CAOUT

PKLMT

SS2

8

9

D_MAX

CT_BUFF

GND

ISENSE2

VCC

REF

PRIMARY

SECONDARY

+

−

VOUT

4

REF

BIAS

RECT

+

−

PWM

V−LOOP

Z

Z

Z