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High-Efficiency Step-Up
Current Regulator for LEDs
10 μ H
V BATT = 3.3V
MBR0540
10 μ F
1 μ F
V CC = 3.3V
1 μ F
V CC
MAX1698
EXT
MAX1698A
SHDN
REF
CS
OPTIONAL
DZ
LEDs
ADJ
FB
GND
PGND
R FB
15 ?
15 ?
15 ?
15 ?
Figure 2. Typical Operating Circuit
Detailed Description
The MAX1698/MAX1698As' high efficiency and small
size make them ideally suited to drive LEDs. They oper-
ate as a boost DC-DC converter that controls output
current rather than voltage. Losses are minimized by a
low, 300mV current-sense threshold. In the standard
configuration, a feedback resistor, R FB , sets the current
through the primary chain of LEDs. Additional chains of
matching LEDs can be added with an equivalent resis-
tor. In matched LED arrays, the secondary chain cur-
rents closely track the primary chain. An optional zener
diode, D2, prevents overvoltage in the event that one of
the LEDs in the primary chain becomes an open circuit.
The LED brightness can be adjusted dynamically by a
voltage input at ADJ.
Soft-Start
The MAX1698/MAX1698A include a soft-start function
that eliminates input current surges at turn-on. They do
this by extending the external FET driver (EXT) mini-
mum off-time during start-up. During the first 512
switching cycles, the minimum off-time is 5μs. It is then
allowed to drop to 2μs for the next 1500 switching
cycles. After that time, the minimum off-time falls to the
1μs value used during normal operation. (See
Shutdown/Soft-Start Timing in the Typical Operating
Characteristics section.)
Design Procedure
Setting the Maximum LED Current
Resistor R FB sets the maximum current in the primary
Shutdown
In shutdown, the supply current is reduced below 1μA.
EXT goes low in shutdown, shutting off the external N-
chain of LEDs:
R FB =
300 mV
I LEDMAX
channel FET. This leaves a current path between the
input and the LEDs through the boost inductor and
catch diode. The minimum forward voltage of the LED
array must exceed the maximum V BATT to ensure that
the LEDs remain off in shutdown. Typical shutdown tim-
ing characteristics are shown in the Typical Operating
Characteristics.
where I LEDMAX is the maximum LED current.
Adjusting LED Current
R FB sets the maximum LED current. This current can
be reduced proportional to the voltage at the ADJ pin
(see Normalized Output Current vs. ADJ Voltage in the
Typical Operating Characteristics section). Figure 3
6
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