TM
MP1518 – FIXED FREQUENCY WHITE LED DRIVER
OPERATION
The MP1518 uses a constant frequency, peak
current mode boost regulator architecture to
regulate the series string of white LEDs. The
operation of the MP1518 can be understood by
referring to the block diagram of Figure 1.
At the start of each oscillator cycle the FET is
turned on through the control circuitry. To
prevent sub-harmonic oscillations at duty cycles
greater than 50 percent, a stabilizing ramp is
added to the output of the current sense
amplifier and the result is fed into the positive
input of the PWM comparator. When this
voltage equals the output voltage of the error
amplifier the power FET is turned off.
The voltage at the output of the error amplifier
is an amplified version of the difference
between the 104mV reference voltage and the
feedback voltage. In this way the peak current
level keeps the output in regulation.
If the feedback voltage starts to drop, the output
of the error amplifier increases. This results in
more current flowing through the power FET,
thus increasing the power delivered to the
output.
FB
-
-
+
+
AMPLIFIER
CONTROL
LOGIC
M1
SW
104mV
PWM
COMPARATOR
+
+
-
CURRENT
SENSE
AMPLIFIER
1.3MHz
OSC
GND
Figure 1—Functional Block Diagram
MP1518 Rev. 1.6
7/25/2006
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2006 MPS. All Rights Reserved.
4
MPS [ MONOLITHIC POWER SYSTEMS ]
