Adjusting the LED Current
Adjusting the output current changes the brightness of
the LEDs. The LED current is set by the voltage at BRT
(V
BRT
) and the sense resistor (R1) at FB. The V
BRT
range for adjusting output current is 0 to 1.25V. Over
this range, the LED current is found from the following
equation:
BRT can be overdriven; however, applying a V
BRT
greater than 1.72V does not increase the output current
above the level at 1.72V. See the LED Current vs. BRT
Voltage graph in the Typical Operating Characteristics
section. To set the maximum LED current, calculate R1
when V
BRT
is at its maximum, as follows:
where V
BRT(MAX)
is 1.72V if BRT is connected to any
value greater than 1.72V, such as V
CC
. Otherwise,
V
BRT(MAX)
is the maximum applied BRT control volt-
age. Power dissipation in R1 is typically less than 5mW;
therefore, power dissipation in a standard chip resistor
is not a concern.
PWM Dimming Control
The BRT input is also used as a digital input allowing
LED brightness control with a logic-level PWM signal
applied directly to BRT. The frequency range is from
100Hz to 10kHz, and the duty cycle range is 0 to 100%.
A 0% duty cycle corresponds to the minimum current,
and a 100% duty cycle corresponds to full current. See
the LED Current vs. BRT Duty Cycle graph in the
Typical Operating Characteristics section. The BRT
resistor and SS capacitor form a lowpass filter, so PWM
dimming results in DC current to the LEDs without the
need for additional RC filters.
Capacitor Selection
A 0.47µF ceramic output capacitor (C2) is recommend-
ed for most applications. For circuits driving six or
fewer LEDs, use a 4.7µF ceramic input capacitor (C1).
For circuits driving more than six LEDs, use a 10µF
input capacitor (C1). For best stability over a wide tem-
perature range, use capacitors with an X5R, X7R, or
better dielectric.
Inductor Selection
The MAX1553 has a 480mA inductor current limit and
can drive up to six LEDs at 20mA or nine LEDs at
15mA. Inductor values from 4.7µH to 47µH work satis-
factorily. Larger values provide the best efficiency while
small inductor values allow the smallest inductor size. A
good choice for best efficiency is the TOKO D62 or
D62L series at 47µH. For smallest size, the Murata
LQH32C at 4.7µH works well.
The MAX1554 has a 970mA inductor current limit and
can drive up to 10 LEDs at 20mA. Inductor values from
4.7µH to 22µH work satisfactorily. A good choice for
high efficiency and small size when driving 9 or 10
LEDs is the TOKO D62 series at 22µH.
When large inductor values are used to optimize effi-
ciency, the MAX1553/MAX1554 operate with continuous
inductor current. With large inductor values (typically
greater than 10µH), stability, input, and output ripple
are improved by connecting a capacitor in parallel with
the LEDs (C4 in Figures 1, 2, and 3).
To prevent saturation, use an inductor with a current
rating that matches the device’s LX current limit.
However, if size is particularly important, it is some-
times acceptable to operate the inductor 10% into satu-
ration. For best efficiency, the inductor’s DC resistance
should also be as low as possible.
Diode Selection
The MAX1553/MAX1554s’ high switching frequency
demands a high-speed rectification diode (D1) for opti-
mum efficiency. A Schottky diode is recommended due
to its fast recovery time and low forward-voltage drop.
