MP1521
High Efficiency
White LED Driver
Monolithic Power Systems
Functional Description
Operation
PWM/ Brightness Adjust
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.
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.
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.
MP1521 Rev 2.7 10/06/04
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