MP24894 – STEP-DOWN LED-CURRENT CONTROLLER WITH WIDE 6V TO 60V INPUT VOLTAGE
MP24894 Rev 1.01 www.MonolithicPower.com 9
12/20/2012 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
OPERATION
Steady State
The MP24894 is a step-down LED-current
controller with hysteresis control that is easily
configured for a wide input that ranges from 6V to
60V input. The MP24894 uses a high-side
current-sense resistor to detect and regulate
LED current. The voltage across the current-
sense resistor is measured and regulated in the
200mV±30mV range.
The internal 1.18V reference voltage provides a
0.3V reference to enable the part. When
V
EN
>0.3V, the output of the comparator goes high
and enables the other blocks. The MP24894 also
provides a 5V pull-up voltage as current reference
voltage when EN/DIM pin is float.
The inductor current is sensed through the
high-side resistor, R
sense
. When the switch is on,
R2 (see Figure 1) is shorted and inductor current
upper-threshold is fixed by R1. When the switch is
off, inductor current lower-threshold is fixed by R1
and R2. The ratio of R1 and R2 determines the
current hysteresis.
System Soft Start
The voltage on the EN/DIM pin provides the
inductor current reference. An external capacitor
from the EN/DIM pin to ground provides a
soft-start delay. When V
IN
starts, internal voltage
source charges the capacitor from 0V to 5V to
fulfill soft-start function.
Dimming Control
The MP24894 allows the EN/DIM pin to control
both Analog and PWM dimming. Whenever the
voltage on DIM is less than 0.25V, the chip turns
off. For analog dimming, when the voltage on DIM
is from 0.3V to 2.7V, the LED current will change
from 20% to 100% of the maximum LED current.
If the voltage on EN pin is higher than 2.9V,
output LED current will equal the maximum LED
current. For PWM dimming, the signal amplitude
must exceed 3V. Choose a PWM frequency in
range of 100Hz to 20kHz for good dimming
linearity.
