MAX1698AEUB View Datasheet(PDF) - Maxim Integrated
Part Name
Description
Manufacturer
MAX1698AEUB Datasheet PDF : 8 Pages
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High-Efficiency Step-Up
Current Regulator for LEDs
VBATT = 3.3V
10µH
10µF
VCC = 3.3V
1µF
VCC
EXT
MAX1698
MAX1698A
SHDN
CS
REF
ADJ
FB
GND PGND
MBR0540
OPTIONAL
DZ
1µF
LEDs
RFB
15Ω
15Ω
15Ω
15Ω
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, RFB, 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.
Shutdown
In shutdown, the supply current is reduced below 1µA.
EXT goes low in shutdown, shutting off the external N-
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 VBATT to ensure that
the LEDs remain off in shutdown. Typical shutdown tim-
ing characteristics are shown in the Typical Operating
Characteristics.
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 RFB sets the maximum current in the primary
chain of LEDs:
RFB =
300mV
I LEDMAX
where ILEDMAX is the maximum LED current.
Adjusting LED Current
RFB 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
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