AN4129 datasheet(22/39 Pages) STMICROELECTRONICS

22 / 39 page

Circuit description and design guidance

AN4129

22/39

Doc ID 023314 Rev 1

R6 and R7 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's discharge time, and

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

The third function, zero current detection, uses the voltage at the DMG pin to determine 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 shutoff 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 shutoff threshold.

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

so high that 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 uF electrolytic was then selected and works

well. Its value can be increased quite a bit without affecting startup time - the charging of the

output capacitor dominates the time from power application to first light. But 10 µF is

adequate.

4.9

COMP pin capacitor

Because the HVLED815PF is used only in current limit mode when driving LEDs, the usual

loop stability compensation network is not needed. Voltage limiting is used only when the

LED string is open (think bench testing…) and it needs to be small so that overvoltage

response is quick. 1 nanofarad is a good value.

4.10

Current sense resistor

This resistor value is determined by the average LED current desired and the turns ratio of

the transformer, according to the following formula:

AN4129 Datasheet(PDF) 22 Page - STMicroelectronics

AN4129 Datasheet(HTML) 22 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Datasheet Download

Link URL