TM
MP1528 – 9 LAMP, 36V PRECISION WHITE LED DRIVER
OPERATION
INTERNAL
BIAS
EN
4
5
UVLO &
THERMAL
SHUTDOWN
REGULATOR
& ENABLE
CIRCUIT
3
SW
29V
CONTROL
+
--
LOGIC &
OUTPUT
DRIVER
1.23V
OPEN LOAD
SHUTDOWN
COMPARATOR
2
1
GND
330ns
ONE-SHOT
TIMER
+
--
CURRENT LIMIT
COMPARATOR
0.6V
FEEDBACK
COMPARATOR
FB
+
--
6
BRT
+
--
1.2V
MP1528_F01_BD01
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.
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
connection to the LED is open, VOUT will rise up,
as the FB pin has no voltage. Once VOUT
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.
Analog Brightness Adjust
Analog
brightness
adjustments
are
accomplished by applying a voltage between
0.27V to 1.2V on the BRT pin.
MP1528 Rev. 1.7
2/21/2006
www.MonolithicPower.com
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© 2006 MPS. All Rights Reserved.
6
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