TM
MP1580 – 2A, 380KHz STEP-DOWN CONVERTER
OPERATION
The MP1580 is a current mode regulator; the
COMP pin voltage is proportional to the peak
inductor current. At the beginning of a cycle: the
upper transistor M1 is off; the lower transistor
M2 is on (refer to Figure 1); the COMP pin
voltage is higher than the current sense
amplifier output and the current comparator’s
output is low. The rising edge of the 380KHz
CLK signal sets the RS Flip-Flop. Its output
turns off M2 and turns on M1, thus connecting
the SW pin and inductor to the input supply.
The increasing inductor current is sensed and
amplified by the Current Sense Amplifier. Ramp
compensation is summed to Current Sense
Amplifier output and compared to the Error
Amplifier output by the Current Comparator.
When the Current Sense Amplifier plus Slope
Compensation signal exceeds the COMP pin
voltage, the RS Flip-Flop is reset and the
MP1580 reverts to its initial M1 off, M2 on,
state. If the Current Sense Amplifier plus Slope
Compensation signal does not exceed the
COMP voltage, then the falling edge of the CLK
resets the Flip-Flop.
The output of the Error Amplifier integrates the
voltage difference between the feedback and
the 1.222V bandgap reference. The polarity is
such that an FB pin voltage less than 1.222V
increases the COMP pin voltage. Since the
COMP pin voltage is proportional to the peak
inductor current, an increase in its voltage
increases the current delivered to the output.
The lower 10Ω switch ensures that the
bootstrap capacitor voltage is charged during
light load conditions. An external Schottky
Diode D1 carries the inductor current when M1
is off (see Figure 1).
2
8
IN
CURRENT
SENSE
AMPLIFIER
INTERNAL
REGULATORS
+
--
5V
OSCILLATOR
SYNC
SLOPE
COMP
1
3
BS
40/380kHz
CLK
+
--
+
S
R
Q
Q
SW
CURRENT
COMPARATOR
SHUTDOWN
COMPARATOR
--
0.7V
7
EN
LOCKOUT
COMPARATOR
--
+
1.8V
2.285V/
2.495V
+
--
4
GND
0.7V 1.222V
5
--
+
FREQUENCY
FOLDBACK
COMPARATOR
ERROR
AMPLIFIER
6
FB
COMP
MP1580_BD01
Figure 1—Functional Block Diagram
MP1580 Rev. 3.0
12/5/2005
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4
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