AN4130 datasheet(24/41 Pages) STMICROELECTRONICS

24 / 41 page

Design guidance

AN4130

24/41

Doc ID 023315 Rev 1

4.7

DMG pin (R6, R7)

This single pin performs several functions; voltage limiting, zero current detection, and

correction for line voltage changes. Its operation is discussed thoroughly in the relevant

datasheet and is only summarized here. The internal circuit is also used in ST's HVLED805

and Altair chips. The datasheets and application notes for these parts offer additional insight

into the pin operation.

Design begins with compensation for the internal comparator's propagation delay. At high

line voltage the slope of FET current vs. time is higher, so for the same comparator

reference voltage the current overshoots more than at low line. This is compensated by

adding an offset proportional to line voltage to the comparator's reference input. R6, in

conjunction with an internal resistor, RFF, of about 45

Ω, decreases the peak current trip

point proportional to the line voltage. The values in the demo unit slightly overcompensate

line voltage.

The two resistors form a divider that normally sets the converter output voltage. The LED

driver uses constant-current mode - the constant-voltage circuit is used only for overvoltage

protection, such as an open load situation. On the positive swing of the output and auxiliary

windings the divided voltage is measured at the end of the transformer discharge time, and

compared to an internal 2.5 V reference by a transconductance op amp. The Vout setting

should equal the output filter capacitor's voltage rating.

The third function, zero current detection, uses the voltage at the DMG pin to measure the

duty cycle of the output diode conduction period. This is used for the internal current

regulator - the duty cycle measured here determines the FET cutoff current, indirectly

controlling output current. The divider's resistor values have very little effect on this function.

During product development, it is helpful to separate the functions. Choose a value for R7

that sets the overvoltage protection level very high (2X expected), and adjust R6 to give the

flattest current limit vs. line voltage characteristic. When the value for R6 is set, pick the

value for R7 to give the correct overvoltage protection.

4.8

Filter capacitor for Vcc

The dimming requirement sets a minimum size for this capacitor. During the dimmer's non-

conducting period the line is not present for the internal startup circuit to take over, so the

stored energy in this part is used to keep the chip alive.

At low conduction angles the capacitor is barely topped off, and it must hold the Vcc voltage

above the shut-off threshold for a half cycle. At the same time, the LEDs are operating on

very low current, and the reflected voltage is smaller than normal, and LED dependent.

The capacitor value therefore depends on the turns ratio of the auxiliary winding to the

output winding, the LED voltage at minimum conduction angle, and the shut-off threshold.

A small 10 µF low-voltage ceramic was tried, but the voltage coefficient of capacitance was

so high the part did not work in the dimming mode. Capacitance was too low to maintain

power to the chip between dimmed line pulses. High-K ceramic capacitance falls off

dramatically, well below the rated voltage. A 10 µF electrolytic was then selected - it works

well.

AN4130 Datasheet(PDF) 24 Page - STMicroelectronics

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