Semiconductor Components Industries, LLC, 2004
November, 2004 − Rev. 0
1 Publication Order Number:
AND8171/D
AND8171/D
NCP1421/2 Reference
Designs for High−Power
White LED Flash
Applications
Prepared by: Jim Hill
ON Semiconductor
Abstract
The attached design illustrates how the NCP1421/2 boost
converters can be configured as a current regulator for
biasing high current white LED’s. Typical boost converters,
such as these, have a reference voltage of 1.2 V. Since this
is a current sourcing application, the more straightforward
approach of directly sensing the boost converter’s reference
voltage (V
ref
), which is 1.2 V, across a sense resistor would
dissipate too much power at the currents required to drive
high−power White LED’s. Also, the lot−to−lot forward
voltage variation is too high to simply regulate at a fixed
voltage with a current limiting resistor. Therefore, this paper
describes a technique that reduces both the power loss in the
sense resistor and the lot−to−lot variation effect of the LED.
This applications shows two implementations of this
concept. Figure 1 shows a simple boost converter configured
at various current levels and uses the Lumileds LXHL−
WW06 white LED. Figure 5 shows a circuit that switches
between a low current for focus lighting and high current for
the flash and uses the Lumileds LXCL−PWF1 white LED.
Overview
The NCP1421 and NCP1422 are monolithic boost
converter IC’s uniquely suited to power higher current
portable applications (600 − 800 mA maximum). Their high
switching frequency (up to 1.2 MHz) allows for a low
profile, small sized inductor and output capacitor to be used.
Also an integrated disconnect switch provides “true cutoff”
by isolating the output from the battery during shutdown.
The NCP1421 comes in the 3x5 mm Micro−8 package, and
the NCP1422 comes in the 3x3 mm DFN package. Because
of these features the NCP1421/2 are well suited to provide
current regulation for biasing high current white LED’s in
portable flash applications. Figure 1 illustrates this circuit.
In summary the reference voltage is split between the
current sense resistor, R4, and a divided down voltage from
the white LED with resistors R2 and R3. This helps remove
some of the dependence of the NCP1421/2’s output voltage,
and thus current, on the LED’s forward voltage, V
F
. This
also helps prevent lot−to−lot V
F
variation affecting the LED
brightness.
Figure 1 shows a typical circuit which, with the Bill of
Materials shown in Table 1, can provide LED currents of 200,
600 and 800 mA. The 200 mA design uses the NCP1422
because of its smaller footprint, and the 600 mA and 800 mA
designs use the NCP1421 and NCP1422 respectively to
showcase the load current limits of each device.
The higher currents (600 and 800 mA) assume that the
LED will be pulsed and not run at steady state. 50 ms pulses
on the LBI/EN were used in the analysis of these circuits.
The NCP1421/2 takes 1.5 ms (nominal) to turn on after the
LBI/EN pin is driven high.
Figure 1. NCP1422 Configured to Drive High Current White LED
FB
1
LBI/EN
2
LBO
3
REF
4
BAT
5
GND
6
LX
7
8
R3
U1
NCP1422
R1
100k
C3
R4
C1
220 nF
D1
L1
R2
C2
6.8 H
22 F
22 F
V
OUT
50 ms Pulse
ON
OFF
V
IN
APPLICATION NOTE
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