MP1528 – 9 LAMP, 36V PRECISION WHITE LED DRIVER

MP1528 Rev. 1.7

www.MonolithicPower.com

6

2/21/2006

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© 2006 MPS. All Rights Reserved.

TM

OPERATION

INTERNAL

REGULATOR

& ENABLE

CIRCUIT

UVLO &

THERMAL

SHUTDOWN

330ns

ONE-SHOT

TIMER

FEEDBACK

COMPARATOR

OPEN LOAD

SHUTDOWN

COMPARATOR

CURRENT LIMIT

COMPARATOR

CONTROL

LOGIC &

OUTPUT

DRIVER

--

+

1.2V

FB

MP1528_F01_BD01

GND

SW

0.6V

1.23V

29V

--

+

--

+

--

+

BIAS

EN

BRT

1

2

3

4

5

6

Figure 1—Functional Block Diagram

The MP1528 utilizes a constant off-time control

scheme. For maximum brightness, the BRT pin

is biased at or above 1.2V. Its voltage is routed

through the unity gain buffer whose output is

clamped at 1.2V. A resistor divider from the

buffer to the inverting input of the feedback

comparator divides the BRT voltage by three to

set the current sense threshold. If the FB pin

voltage is lower than this threshold the power

MOSFET is turned on. The MOSFET and

inductor currents increase until the current limit

comparator trips at 950mA, turning off the

MOSFET for at least 330ns. At the end of the

330ns, if the FB pin is still lower than the

current sense threshold, the MOSFET is turned

on again.

Analog Brightness Adjust

Analog

brightness

adjustments

are

accomplished by applying a voltage between

0.27V to 1.2V on the BRT pin.

PWM Brightness Adjust

PWM brightness adjustments are accomplished

by driving the BRT pin with a digital signal

whose low voltage is below 0.18V and high

voltage is greater than 1.2V. A voltage below

0.18V forces the power MOSFET to turn off. A

voltage greater than 1.2V will cause the FB pin

to regulate at 0.4V.

Open Load Shutdown

Open Load Shutdown is implemented by

connecting the BIAS pin to the top of the LED

string. In the fault condition where the

exceeds 40V, the MP1528 will cease operation

until the EN pin is cycled off and on. Connecting

the BIAS pin to the input will disable Open Load

Shutdown.