TM
MP1543 — 1.5A, 500KHZ SYNCHRONOUS RECTIFIED STEP-UP CONVERTER
OPERATION
UVLO
REFERENCE VOLTAGE
BIAS CURRENT
EN
THERMAL SHUTDOWN
OUT
OSC
500KHz
CONTROL
LOGIC
SW
RAMP
+
RS
CURRENT
GND
SENSE
AMPLIFIER
PWM
COMPARATOR
FB
1.2V
ERROR
AMPLIFIER
MP1543_BD01
Figure 1—Functional Block Diagram
The MP1543 uses a 500KHz fixed-frequency,
current-mode regulation architecture to regulate
the output voltage. The MP1543 measures the
output voltage through an external resistive
voltage divider and compares that to the
internal 1.2V reference to generate the error
voltage. The current-mode regulator compares
the error voltage to the inductor current to
regulate the output voltage. The use of
current-mode regulation improves transient
response and control loop stability.
At the beginning of each cycle, the N-Channel
MOSFET switch is turned on, forcing the
inductor current to rise. The current at the
source of the switch is internally measured and
converted to a voltage by the current sense
amplifier. That voltage is compared to the error
voltage. When the inductor current rises
sufficiently, the PWM comparator turns off the
switch, forcing the inductor current to the output
capacitor through the internal P-Channel
MOSFET rectifier, which forces the inductor
current to decrease. The peak inductor current
is controlled by the error voltage, which in turn
is controlled by the output voltage. Thus the
output voltage controls the inductor current to
satisfy the load.
When the MP1543 is disabled (EN = Low), both
power switches are off. The body of the
P-Channel MOSFET connects to SW and there
is no current path from SW to OUT. Therefore,
the output voltage discharges to ground. When
the MP1543 is enabled (EN = High), the body of
the P-Channel MOSFET connects to OUT and
forms a forward diode from SW to OUT. Thus
the output voltage rises up toward the input
voltage. When output voltage crosses 1.6V the
MP1543 starts the controller and regulates the
output voltage to the target value.
Soft-Start
The MP1543 includes a soft-start timer that
limits the voltage at the error amplifier output
during startup to prevent excessive current at
the input. This prevents premature termination
of the source voltage at startup due to inrush
current. This also limits the inductor current at
startup, forcing the input current to rise slowly to
the amount required to regulate the output
voltage during soft-start.
MP1543 Rev. 1.0
1/11/2006
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