Max1583 white led camera-flash boost converter, Design procedure – Rainbow Electronics MAX1583 User Manual
Page 8
MAX1583
White LED Camera-Flash
Boost Converter
8
_______________________________________________________________________________________
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
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-
nect a reservoir capacitor from OUT to GND to provide
the extra LED current. The value of this capacitor is cal-
culated as:
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.
C
mA
mA
ms
V
V
mA
ms
V
F
RES
=
−
Ч
− Ч
=
Ч
= µ
(
)
(
)
100
75
30
24
2 4
25
30
16
47
C
I
I
t
V
n
V
RES
LED
BOOST
STROBE
F
=
−
Ч
− Ч
(
)
(
)
24
MAX1583X
V
IN
2.6V TO 5.5V
LED
EN2
EN1
POK
GND
IN
LX
OUT
STB
MOV
R3
100k
Ω
C1
22
µF
C3
0.1
µF
R1
6.04k
Ω
1%
R2
3.01k
Ω
1%
L1
4.7
µH
2
1
5
6
7
4
3
MODE CONTROL
POWER-OK
10
8
9
D2, D3, D4
WHITE LEDs
C2
10
µF
D1
Figure 1. Typical Application Circuit