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White LED Camera-Flash
Boost Converter
V IN
2.6V TO 5.5V
L1
4.7 μ H
2
IN
LX
10
D1
R3
100k Ω
C1
22 μ F
1
GND
MAX1583X
C2
10 μ F
D2, D3, D4
POWER-OK
5
POK
WHITE LEDs
6
EN1
OUT
8
MODE CONTROL
7
EN2
C3
0.1 μ F
R1
6.04k Ω
1%
R2
4
STB
LED
9
3.01k Ω
Figure 1. Typical Application Circuit
1%
3 MOV
C RES = LED BOOST STROBE
Design Procedure
Adjusting the LED Current
The LED current in the strobe and movie modes is pro-
grammable using external resistors. To set the LED cur-
rent during strobe mode, connect a resistor (R STB ) from
STB to GND. V STB is regulated to 0.6V and the current
sourced by STB is mirrored through LED with a gain of
1000. R STB is calculated as:
R STB = 600 / I LED(DESIRED)
To set the LED current during movie mode, connect a
resistor (R MOV ) from MOV to GND. V MOV is regulated
to 0.6V and the current sourced by MOV is mirrored
through LED with a gain of 100. R MOV is calculated as:
R MOV = 60 / I LED(DESIRED)
Reservoir Capacitance vs. Current Limit
During strobe mode, the current demands on the boost
converter to maintain the desired LED strobe current
may cause the MAX1583 to reach the preset current
limit. The MAX1583 is available with three factory-pre-
set current limits: 250mA, 500mA, and 1A. If the current
limit is reached, the internal MOSFET terminates its ON
cycle early, causing the output current to drop. See the
nect a reservoir capacitor from OUT to GND to provide
the extra LED current. The value of this capacitor is cal-
culated as:
( I ? I ) × t
24 V ? ( n × V F )
where:
I LED is the desired strobe current,
I BOOST is the maximum current from the boost convert-
er obtained from the Typical Operating Characteristics,
t STROBE is the time duration of the strobe,
n is the number of LEDs, and
V F is the forward voltage of the LEDs.
For example, assume a minimum input voltage of 3.2V
with the MAX1583Y (500mA current limit). The required
strobe current is 100mA for 30ms through two series
LEDs with a 4V forward voltage. The reservoir capacitor
is determined as follows:
I BOOST = 75mA from the Typical Operating
Characteristics .
Typical Operating Characteristics for the Maximum
Boost I OUT vs. V IN for each current limit. If the required
strobe current exceeds the maximum boost I OUT , con-
C RES =
( 100 mA ? 75 mA ) × 30 ms
24 V ? ( 2 × 4 V )
=
25 mA × 30 ms
16 V
= 47 μ F
8
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