Tiny white LEDs
are capable of delivering ample white light without the fragility
problems and costs associated with ﬂuorescent backlights. They do pose a
problem however in that their forward voltage can be as high as 4 V,
precluding them being from powered directly from a single Li-Ion cell.
Applications requiring more white LEDs or higher efficiency can use an LT1615 boost converter to drive a series connected array of LEDs. The high efficiency circuit (about 80%) shown here can provide a constant-current drive for up to eight LEDs. Driving eight white LEDs in series requires at least 29 V at the output and this is possible thanks to the internal 36-V, 350-mA switch in the LT1615.
The constant-current design of the circuit guarantees a steady current through all LEDs,
regardless of the forward voltage differences between them. Although
this circuit was designed to operate from a single Li-Ion battery (2.5V
to 4.5V), the LT1615 is also capable of operating from inputs as low as 1
V with relevant output power reductions. The Motorola MBR0520 surface
mount Schottky diode (0.5 A 20 V) is a good choice for D1 if the output
voltage does not exceed 20 V. In this application however, it is better
to use a diode that can withstand higher voltages like the MBR0540 (0.5
A, 40 V). Schottky diodes, with their low forward voltage drop and fast
switching speed, are the best match.
Many different manufacturers make equivalent parts, but make sure
that the component is rated to handle at least 0.35 A. Inductor L1, a
4.7-µH choke, is available from Murata, Sumida, Coilcraft, etc. In order
to maintain the constant off-time (0.4 ms) control scheme of the
LT1615, the on-chip power switch is turned off only after the 350-mA (or
100-mA for the LT1615-1) current limit is reached. There is a 100-ns
delay between the time when the current limit is reached and when the
switch actually turns off. During this delay, the inductor current
exceeds the current limit by a small amount. This current overshoot can
be beneﬁcial as it helps increase the amount of available output current
for smaller inductor values.
This will be the peak current passed by the inductor (and the diode)
during normal operation. Although it is internally current-limited to
350 mA, the power switch of the LT1615 can handle larger currents
without problems, but the overall efficiency will suffer. Best results
will be o btained when IPEAK is kept well
below 700 mA for the LT1615.The LT1615 uses a constant off-time control
scheme to provide high efficiencies over a wide range of output current.
The LT1615 also contains circuitry to provide protection during
start-up and under short-circuit conditions.
When the FB pin voltage is at less than approximately 600 mV, the
switch off-time is increased to 1.5 ms and the current limit is reduced
to around 250 mA (i.e., 70% of its normal value). This reduces the
average inductor current and helps minimize the power dissipation in the
LT1615 power switch and in the external inductor L1 and diode D1. The
output current is determined by Vref/R1, in this case, 1.23V/68 = 18
mA). Further information on the LT1615 may be found in the device
datasheets which may be downloaded from http://www.linear-tech.com/pdf/16151fa.pdf
Author: D. Prabakaran
Copyright: Elektor Electronics