AN44 Datasheet, PDF (1/6 Pages) Zetex Semiconductors – A high power LED driver for low voltage halogen replacement

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, T_ON
The ZXSC300 turns on Q1 until it senses 19mV (nominal) on the I_SENSE pin.
The current in Q1 to reach this threshold is therefore 19mV/R1, called I_PEAK.
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 V_F. 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 R_DS(on) and R1
always drops less than 19mV, as this is the turn-off threshold for Q1.
V_IN = V_F + V(L1)
T_ON = I_PEAK x L1/ V(L1)
Issue 2 - July 2006
1
© Zetex Semiconductors plc 2006
www.zetex.com