MP24833A – 55V, 3A, WHITE LED DRIVER
MP24833A Rev. 1.01 www.MonolithicPower.com 9
1/13/2017 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2017 MPS. All Rights Reserved.
OPERATION
The MP24833A is a current-mode regulator.
The error amplifier (EA) output voltage is
proportional to the peak inductor current.
At the beginning of a cycle, the MOSFET is off.
The EA output voltage is higher than the current
sense amplifier output, and the current
comparator’s output is low. The rising edge of
the CLK signal sets the RS flip-flop. The CLK
frequency is the operating frequency. The flip-
flop output turns on the MOSFET that connects
SW and the inductor to the input supply.
The current sense amplifier detects and
amplifies the rising inductor current. The PWM
comparator compares the output of the EA
against the sum of the ramp compensator and
the current sense amplifier. When the sum of
the current sense amplifier output and the slope
compensation signal exceeds the EA output
voltage, the RS flip-flop resets, and the
MOSFET turns off. The external Schottky
rectifier diode (D) conducts the inductor current.
If the sum of the current sense amplifier output
and the slope compensation signal does not
exceed the EA output throughout the cycle,
then the falling edge of the CLK resets the flip-
flop.
The output of the EA integrates the voltage
difference between the feedback and the 0.2V
reference. The EA output increases when the
FB voltage is less than 0.2V. Since the EA
output voltage is proportional to the peak
inductor current, the increase in the EA output
voltage also increases the current delivered to
the output.
Soft Start (SS)
When the MP24833A is enabled and VDD
exceeds the UVLO threshold, switching begins.
The soft start is not active when V
FB
- VSS is
lower than half of V
REF
, which is useful for
charging the output capacitor quickly. At the
same time, the current limit is folded to half.
Once V
FB
- VSS rises up to half of V
REF
, the soft
start begins and forces the internal reference
input of the EA to rise up from ⅔ of V
REF
slowly.
The current limit also recovers the normal value.
The soft-start function can make the duty cycle
extend slowly to limit current overshoot at start-
up.
Open-LED Protection
The OVP pin is used for open-LED protection
and monitors the output voltage through a
voltage divider. If the LED is open, there is no
voltage on FB. The duty cycle increases until
V
OVP
- VSS reaches the protection threshold set
by the external resistor divider. The top switch
turns off until V
OVP
- VSS decreases sufficiently.
Dimming Control
The MP24833A allows for both DC and PWM
dimming. When the voltage on EN/DIM
(reference to INGND) is less than 0.6V, the chip
turns off.
For analog dimming, the LED current is
dependent linearly on the EN/DIM voltage
range between 0.67V and 1.35V, from 0% to
100%. An EN/DIM voltage higher than 1.35V
generates the maximum LED current.
For PWM dimming, V
EN/DIM
- V
INGND
must
exceed 1.5V. Use a PWM frequency in the
range of 100Hz to 2kHz for good dimming
linearity. For combined analog and PWM
dimming, apply a PWM signal with an amplitude
from 0.67V to 1.35V to EN/DIM.
Output Short-Circuit Protection (SCP)
The MP24833A features output short-circuit
protection (SCP). When the output is shorted to
VSS, the voltage on OVP drops below 0.2V,
and FB senses no voltage (<0.1V), since no
current is going through the WLED. Under this
condition, the operating frequency folds back to
decrease power consumption.
In boost or buck-boost applications when there
is a possibility that the LED+ can short-circuit to
VSS, place a 100Ω resistor in series from
power GND to INGND of the IC to protect the
IC.
