TM
MP1540 – 1.3MHz, 18V STEP-UP CONVERTER
PIN FUNCTIONS
Pin #
Name Pin Function
Power Switch Output. SW is the drain of the internal MOSFET switch. Connect the power
inductor and output rectifier to SW. SW can swing between GND and 20V.
1
SW
2
3
GND
FB
Ground.
Feedback Input. FB voltage is 1.25V. Connect a resistor divider to FB.
Regulator On/Off Control Input. A high input at EN turns on the converter, and a low input
turns it off. When not used, connect EN to the input source for automatic startup. The EN
pin cannot be left floating.
4
5
EN
IN
Input Supply Pin. Must be locally bypassed.
OPERATION
The MP1540 uses a fixed frequency, peak
current mode boost regulator architecture to
regulate voltage at the feedback pin. The
operation of the MP1540 can be understood by
referring to the block diagram of Figure 1.
The voltage at the output of the error amplifier is
an amplified version of the difference between
the 1.25V bandgap 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 to flow through the power MOSFET, thus
increasing the power delivered to the output.
At the start of each oscillator cycle the MOSFET
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 negative
input of the PWM comparator. When this voltage
equals the output voltage of the error amplifier
the power MOSFET is turned off.
The MP1540 has internal soft start to limit the
amount of input current at startup and to also
limit the amount of overshoot on the output.
The current limit is increased by a fourth every
40µs giving a total soft start time of 120µs.
R
C
C
C
SW
1
-
3
FB
+
-
CONTROL
M1
+
LOGIC
ERROR
AMPLIFIER
PWM
COMPARATOR
1.25V
+
-
+
CURRENT
SENSE
AMPLIFIER
2
1.3MHz
OSC
GND
MP1540_F01_BD01
Figure 1—Functional Block Diagram
MP1540 Rev. 1.1
2/14/2006
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