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AN44 Datasheet(PDF) 1 Page - Zetex Semiconductors

Part No. AN44
Description  A high power LED driver for low voltage halogen replacement
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Maker  ZETEX [Zetex Semiconductors]
Homepage  http://www.diodes.com/
Logo ZETEX - Zetex Semiconductors

AN44 Datasheet(HTML) 1 Page - Zetex Semiconductors

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Issue 2 - July 2006
1
www.zetex.com
© Zetex Semiconductors plc 2006
AN44
A high power LED driver for low voltage halogen replacement
Introduction
LED lighting is becoming more popular as a replacement technology for Halogen low voltage
lighting, primarily because of the low efficiency, reliability and lifetime issues associated with
Halogen bulbs.
Discussed below is a novel approach for driving high power LED's as a replacement for low
voltage halogen lighting systems.
A typical schematic diagram is shown in Figure 1.
Figure 1
Schematic diagram
Operation
Please refer to the typical schematic diagram in Figure 1.
On period, TON
The ZXSC300 turns on Q1 until it senses 19mV (nominal) on the ISENSE pin.
The current in Q1 to reach this threshold is therefore 19mV/R1, called IPEAK.
With Q1 on, the current is drawn from the battery and passes through C1 and LED in parallel.
Assume the LED drops a forward voltage VF. The rest of the battery voltage will be dropped across
L1 and this voltage, called V(L1) will ramp up the current in L1 at a rate di/dt = V(L1)/L1, di/dt in
Amps/sec, V(L1) in volts and L1 in Henries.
The voltage drop in Q1 and R1 should be negligible, since Q1 should have a low RDS(on) and R1
always drops less than 19mV, as this is the turn-off threshold for Q1.
VIN = VF + V(L1)
TON = IPEAK x L1/ V(L1)


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