IN-PLUG

© Copyright 2005-2007 - ASIC Advantage, Inc. – All rights reserved

- Revision 11 – April 02, 2007

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ROLLOFF (Pin 8 – BYPASS)

A capacitor connected between the BYPASS pin and GND is used to create a voltage which allows the chip to sense

the peak/rms input voltage and adjust on and off times of the GDRV pin so as to maintain a constant current for the

LEDs. The suggested value of this capacitor is 1uF, with a 16V or greater rating.

CURRENT SENSE (Pin 2 – IS)

A low-value resistor R9 is used in the application to sense the inductor current. The voltage across this resistor is

sensed through resistor R2 from the IS pin to R9. Note that resistor R9 is NOT sensing just the FET current. Also

note that in the voltage being generated is a NEGATIVE voltage, below GND level. The waveform will be a sawtooth,

with an inductive ramp in both directions. The suggested series sense resistor R2 value is 1k ohms R9 should be

sized to give a peak voltage of about -1.2 volts. As a conceptual guide, two simulation results showing the voltage

across R9 are shown below. Vertical is the voltage and horizontal is time. The time scale is different between the two

pictures. The first picture shows the “critical conduction” mode nature of the design, where the current goes between

a peak value and zero (zero volts being the first major horizontal grid line from the top). The second picture shows

that the peak current follows the rectified AC line. In an actual circuit, the waveform would have switching noise

superimposed on it. A small capacitor from IS to GND (where C x R2 << T) may be useful to improve the circuit

behavior. Note that the IS pin is used to determine both when the peak current has been reached and the FET should

be turned off, and, when the current has reached zero and the FET should be turned on.

Fig. 8

Fig. 9

LED CURRENT SENSE AND COMPENSATION (Pin 6 – LEDSNS, Pin 7 - COMP)

Two capacitors and a resistor (C1, C2, and R1 shown in Fig.10 ) provide a network around the feedback amplifier

located inside the chip to stabilize the system performance. Recommended values are C1 = 100nF, C2 = 220nF, and

R1 = 24k ohms.

The LED current is sensed through a sense resistor (RS in Fig. 1 and Fig. 2 page 2) and fedback to LEDSNS through

resistor R1. The sense resistor RS should have a value of 0.50 volts divided by the desired peak string diode current.

The application circuit (Fig.10) also shows a suggested method for implementing output overvoltage protection by

connecting a zener diode (or string of diodes) from the output to the LED current sense point. If the output voltage

was to exceed the breakdown voltage of the zener, this will pull-up the LED current sense voltage and shut-off the

gate drive.