Datasheet
Pin Descriptions
f [
ǒ
0.6
RT CT
Ǔ
I
MOUT
+
I
IAC
(V
VAOUT
* 1)
V
VFF
2
K
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UCC2818-EP
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........................................................................................................................................................................................... SLUS716 – DECEMBER 2008
CAI: Current amplifier noninverting input. Place a resistor between this pin and the GND side of current sense
resistor. This input and the inverting input (MOUT) remain functional down to and below GND.
CAOUT: Current amplifier output. This is the output of a wide bandwidth operational amplifier that senses line
current and commands the PFC pulse-width modulator (PWM) to force the correct duty cycle. Compensation
components are placed between CAOUT and MOUT.
CT: Oscillator timing capacitor. A capacitor from CT to GND sets the PWM oscillator frequency according to:
The lead from the oscillator timing capacitor to GND should be as short and direct as possible.
DRVOUT: Gate drive. The output drive for the boost switch is a totem-pole MOSFET gate driver on DRVOUT.
Use a series gate resistor to prevent interaction between the gate impedance and the output driver that might
cause the DRVOUT to overshoot excessively. See characteristic curve (Figure 13 ) to determine minimum
required gate resister value. Some overshoot of the DRVOUT output is always expected when driving a
capacitive load.
GND: Ground. All voltages measured with respect to ground. V
CC
and REF should be bypassed directly to GND
with a 0.1- µ F or larger ceramic capacitor.
IAC: Current proportional to input voltage. This input to the analog multiplier is a current proportional to
instantaneous line voltage. The multiplier is tailored for very low distortion from this current input (I
IAC
) to
multiplier output. The recommended maximum I
IAC
is 500 µ A.
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:
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 V
REF
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 k Ω is recommended. Nominal voltage on this pin is 3 V.
SS: Soft-start. V
SS
is discharged for V
VCC
low conditions. When enabled, SS charges an external capacitor with a
current source. This voltage is used as the voltage error signal during start-up, enabling the PWM duty cycle to
increase slowly. In the event of a V
VCC
dropout, the OVP/EN is forced below 1.9 V (typ), SS quickly 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.
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