MP1521 datasheet(5/8 Pages) MPS

5 / 8 page

MP1521

High Efficiency

White LED Driver

MP1521 Rev 2.7 10/06/04

www.monolithicpower.com

Monolithic Power Systems

Functional Description

Operation

The MP1521 utilizes a constant off-time

control scheme (Refer to the Block

Diagram). For maximum brightness, the

BRT pin is biased at or above 1.2V. Its

voltage is routed through the unity gain

buffer whose the output of is clamped at

1.2V. A 20K/10K divider from the Buffer to

the non-inverting input of the Feedback

comparator divides the BRT voltage by

three to set the current sense threshold. If

any of the feedback pins FB1, FB2, or FB3

is lower than this threshold the Power FET

is turned on. The Power MOSFET and

inductor current increases until the current

limit comparator trips at 0.5A turning off the

MOSFET for 500ns. At the end of 500ns if

any of the Feedback pins are still lower

than the current sense threshold. The

Power FET is again turned on. This cycle

repeats itself until all three of the feedback

pins exceed the threshold.

Analog Brightness Adjust

Analog Brightness Adjust is accomplished

by applying a voltage between 0.26V to

1.2V on the BRT pin.

The 1.23V REF

output can be used to bias a potentiometer

whose wiper drives the BRT pin.

PWM/ Brightness Adjust

PWM Brightness Adjust is accomplished

by driving the BRT pin with a square wave

whose low voltage is below 0.2V and

whose high voltage is greater than 1.2V. A

voltage below 0.2V forces the Power

MOSFET off. A voltage greater than 1.2V

will cause all three feedback pins to

regulate at or above 0.4V

Open Load Shut-Off

Open Load Shut-Off is implemented by

connecting the OLS pin to VOUT. In the

fault condition where the connection to the

LED’s is open, VOUT will rise up, as all the

feedback pins are absent of voltage. Once

VOUT exceeds 28V, the part will cease

operation until the EN pin is cycled off and

on.

To disable the open load shut-off

feature, connect the OLS pin to GND.

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