Rainbow Electronics MAX1583 - Manual (Page 7)

MAX1583
White LED Camera-Flash
Boost Converter
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Detailed Description
The MAX1583 provides power and control for up to five
white LEDs. The MAX1583 consists of a gated oscilla-
tor, a 24V boost converter, and an LED current-regula-
tion circuit, and operates from 2.6V to 5.5V input supply
voltages. The MAX1583 operates in one of four user-
selectable modes (
Table 1). The precharge mode
charges the output capacitor to 24V for quick transients
to strobe mode. The POK output indicates the output
has reached its regulation value of 24V and is ready for
a strobe pulse. In movie mode, the LED current is regu-
lated at a user-selectable level for constant lighting.
Strobe mode regulates the LED current to a user-selec-
table high-intensity level for still picture use. When the
MAX1583 is not in use, it can be put in shutdown mode,
reducing the input current to 0.01µA (typ).
Boost Converter
The heart of the MAX1583 is a gated oscillator boost
converter that is used to supply the LED current. The
boost converter is active when the feedback input volt-
age (V
LED ) is below the regulation threshold (0.6V) and
V
OUT
is below the overvoltage-lockout threshold (24V).
During switching cycles, the 94% (typ) duty cycle,
1MHz oscillator switches an internal power MOSFET.
On the rising edge of each clock cycle, the MOSFET is
turned on, ramping up current and storing energy in the
external inductor. On the falling edge of the oscillator,
the MOSFET is turned off, allowing the energy stored in
the inductor to be transferred to the output. Under nor-
mal operation, the MOSFET turns on or off only at the
beginning or end of the oscillator cycle. This ensures
there are no short-duration pulses. Exceeding the cur-
rent limit or thermal limit, or pulling EN1 and EN2 low,
immediately turns the MOSFET off.
Current Regulator
The MAX1583 controls the brightness by regulating the
current through the external LEDs to a user-selectable
level. In movie mode, the current into LED is regulated
to 100 times the current set by the MOV resistor, R
MOV .
In strobe mode, the current into LED is regulated to
1000 times the current set by the STB resistor, R
STB .
Precharge Mode
Drive EN1 low and EN2 high to enable precharge
mode. In this mode, the boost converter switches con-
tinuously until the output capacitor is charged to 24V.
Once V
OUT reaches 24V, POK goes high impedance
and the MAX1583 does not switch again until V OUT falls
below 23.5V. This results in a 500mV ripple on the out-
put during this mode. LED is high impedance in
precharge mode; therefore, the external LEDs are off.
POK Output
An open-drain, power-OK output (POK) is low until
V
OUT reaches 24V. Once the threshold is reached,
POK is latched in a high-impedance state. The latch is
reset in strobe, precharge, or movie mode.
Strobe Mode
Drive EN1 and EN2 high to enable strobe mode. In this
mode, the boost converter switches continuously to
supply maximum current for the LEDs unless limited by
overvoltage lockout. The current through the LEDs is
regulated to 1000 times the current set by the STB
resistor. See the
Adjusting the LED Current section for
details on setting the strobe-mode current. The time
duration of the strobe mode must be set using external
circuitry or software.
Movie Mode
Drive EN1 high and EN2 low to enable movie mode.
In this mode, the boost converter switches as neces-
sary to maintain V
LED at 0.6V, and the current regulator
regulates the current through LED to 100 times the cur-
rent set by the MOV resistor. See the Adjusting the LED
Current section for details on setting the movie-
mode current.
Shutdown Mode
Drive EN1 and EN2 low to place the MAX1583 into a
low-power shutdown mode. LED is high impedance
during shutdown, eliminating the DC conduction path
that exists in the typical boost topology.
EN1 EN2 MODE
0 0 Shutdown
0 1 Precharge
1 0 Movie
1 1 Strobe
Table 1. Enable Inputs