MP24833 – 3A, 55V WHITE LED DRIVER

MP24833 Rev. 1.0

www.MonolithicPower.com

9

05/05/2010

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© 2013 MPS. All Rights Reserved.

OPERATION

The MP24833 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 (its frequency is the operating

frequency) sets the RS flip-flop. Its output turns

on the MOSFET that connects the SW pin and

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

0.198V reference. The EA output will increase

when the FB pin voltage less than 0.198V.

Since the EA output voltage is proportional to

the peak inductor current, The increase in EA

output voltage also increases the current

delivered to the output.

Open LED Protection

The OVP pin is used for open LED protection. It

monitors the output voltage through a voltage

divider. If the LED is open, there is no voltage

on the FB pin. The duty cycle will increase until

by the external resistor divider. The top switch

Dimming Control

The MP24833 allows both DC and PWM

dimming. When the voltage on EN (reference to

INGND) is less than 0.6V, the chip turns off. For

analog dimming, the LED current is linearly

dependent on the EN voltage range between

0.67V and 1.36V, from 0% to 100%. EN voltage

higher than 1.36V results in the maximum LED

frequency in the range of 100Hz to 2kHz for

good dimming linearity. For combined analog

and PWM dimming, apply a PWM signal with

amplitude from 0.67V to 1.36V to EN pin.

Output Short Circuit Protection

The MP24833 features output short-circuit

protection. When the output is shorted to VSS,

the voltage on OVP pin drops below 0.2V, and

the FB pin senses no voltage (<0.1V) as no

current goes through the WLED. Under this

condition, the operating frequency folds back to

decrease the power consumption.

In buck-boost applications when there is

possibility that the LED+ short circuit to VSS,

add a diode from VSS to INGND to protect the

IC, as shown in Figure 2 below.

BST

OVP

SW

VIN

EN/DIM

VDD

EN/DIM

INGND

VSS

L

1

4.5V<VIN<55V

5

OFF

ON

DC or PWM Input

6

2

FB

4

8

7

3

VSS

VSS

VSS

Figure 2: Buck-Boost Application where

LED+ Possibly Shorted to VSS