MP24830 — 4.5V–90V, PROGRAMABLE FREQUENCY WHITE LED DRIVER

MP24830 Rev. 1.0

www.MonolithicPower.com

9

10/30/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

The MP24830 is a current-mode regulator. The

error

amplifier

(EA)

output

voltage

is

proportional to the peak inductor current.

At the beginning of a cycle, M1 is off. The EA

output voltage exceeds the current sense

amplifier output, and the current comparator’s

output is low. The rising edge of the CLK signal

(its frequency equals the switching frequency)

triggers the RS flip-flop. The driver turns on the

external MOSFET, thus connecting the SW pin

and inductor to the input supply.

The current-sense amplifier (CSA) senses the

increasing

inductor

current.

The

PWM

comparator compares the sum of the ramp

generator and the CSA output against the

output of the error amplifier. When the sum of

the CSA output and the ramp generator signal

exceeds the EA output voltage, the RS flip-flop

resets and driver turns off the external

MOSFET. The external Schottky rectifier diode

(D1) conducts the inductor current.

If the sum of the CSA output and the ramp

compensation signal does not exceed the EA

output for a whole 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

output voltage. Since the EA output voltage is

proportional to the peak inductor current,

increasing its voltage also increases the current

delivered to the output.

LED Open Protection

If the LED is open, there is no voltage on the

FB pin. The duty cycle increases until OVP-

VSS reaches the shutdown threshold set by the

external resistor divider. The top switch remains

off until the voltage OVP-VSS drops below 1.2V.

LED Short Protection

If the FB voltage exceeds 600mV, the latches

off immediately and DIMO goes low. If the FB

voltage exceeds 300mV for 450µs, the IC

latches off and DIMO is pulled low. The EN

needs to reset to restart the IC.

Dimming Control

The MP24830 allows both DC and PWM

dimming on the DIM pin. For analog dimming, a

voltage range from 0.6V to 1.95V linearly sets

the LED current from 0% to 100% of the

maximum LED current. DIM voltages exceeding

2V results in the maximum LED current. For

PWM dimming, use a square signal with an

For good dimming linearity, select a PWM

frequency in range of 100Hz to 2kHz. For a

higher dimming frequency or dimming ratio, use

the DIMO pin to control an external dimming

MOSFET. For combined analog and PWM

dimming, apply a PWM signal with amplitude of

0.6V to 1.95V on the DIM pin.

Output Short-Circuit Protection

The MP24830 integrates output short-circuit

protection (SCP) to foldback the operating

frequency and decrease power consumption

when the output is shorted to VSS. Such shorts

cause the voltage on the OVP pin to drop below

0.4V, and the FB pin senses no voltage (<0.1V)

as no current goes through the WLED.

In buck-boost applications, when there is a

possibility that LED+ short-circuits to VSS, add

a diode from VSS to INGND to protect the IC,

as shown in below in Figure 2.

VDD

OVP

BST

SW

DR

CS

FB

DIMO

RSET

COMP

MP24830

VSS

INGND

EN

DIM

OFF ON

VIN

Step-up/down White LED Driver Applicatoin

DC or PWM Input

Figure 2: Buck-Boost Application with Possible

LED+ Short to VSS