UCC2817, UCC2818, UCC3817, UCC3818

BiCMOS POWER FACTOR PREREGULATOR

SLUS395J - FEBRUARY 2000 - REVISED MARCH 2009

6

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pin descriptions (continued)

IAC: (current proportional to input voltage) This input to the analog multiplier is a current proportional to

MOUT: (multiplier output and current amplifier inverting input) The output of the analog multiplier and the

inverting input of the current amplifier are connected together at MOUT. As the multiplier output is a current, this

is a high-impedance input so the amplifier can be configured as a differential amplifier. This configuration

improves noise immunity and allows for the leading-edge modulation operation. The multiplier output current

is limited to 2

I

IAC

. The multiplier output current is given by the equation:

I

MOUT +

I

IAC

(V

VAOUT *

1)

V

VFF

2

K

where K

+ 1

V

is the multiplier gain constant.

OVP/EN: (over-voltage/enable) A window comparator input that disables the output driver if the boost output

voltage is a programmed level above the nominal or disables both the PFC output driver and resets SS if pulled

below 1.9 V (typ).

PKLMT: (PFC peak current limit) The threshold for peak limit is 0 V. Use a resistor divider from the negative side

of the current sense resistor to VREF to level shift this signal to a voltage level defined by the value of the sense

resistor and the peak current limit. Peak current limit is reached when PKLMT voltage falls below 0 V.

RT: (oscillator charging current) A resistor from RT to GND is used to program oscillator charging current. A

resistor between 10 k

Ω and 100 kΩ is recommended. Nominal voltage on this pin is 3 V.

with a current source. This voltage is used as the voltage error signal during start-up, enabling the PWM duty

discharges to disable the PWM.

Note: In an open-loop test circuit, grounding the SS pin does not ensure 0% duty cycle. Please see the

application section for details.

VAOUT: (voltage amplifier output) This is the output of the operational amplifier that regulates output voltage.

The voltage amplifier output is internally limited to approximately 5.5 V to prevent overshoot.

VCC: (positive supply voltage) Connect to a stable source of at least 20 mA between 10 V and 17 V for normal

operation. Bypass VCC directly to GND to absorb supply current spikes required to charge external MOSFET

exceeds the upper under-voltage lockout voltage threshold and remains above the lower threshold.

pole external filter. At low line, the VFF voltage should be 1.4 V.

VSENSE: (voltage amplifier inverting input) This is normally connected to a compensation network and to the

boost converter output through a divider network.

VREF: (voltage reference output) VREF is the output of an accurate 7.5-V voltage reference. This output is

capable of delivering 20 mA to peripheral circuitry and is internally short-circuit current limited. VREF is disabled

ceramic capacitor for best stability. Please refer to Figures 8 and 9 for VREF line and load regulation

characteristics.