TM
MP2359 – 1.2A, 24V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6
OPERATION
The MP2359 is a current mode buck regulator.
That is, the EA output voltage is proportional to
the peak inductor current.
If the sum of the Current Sense Amplifier output
and the Slope Compensation signal does not
exceed the EA output for a whole cycle, then
the falling edge of the CLK resets the Flip-Flop.
At the beginning of a cycle, M1 is off. The EA
output voltage is higher than the current sense
amplifier output, and the current comparator’s
output is low. The rising edge of the 1.4MHz
CLK signal sets the RS Flip-Flop. Its output
turns on M1 thus connecting the SW pin and
inductor to the input supply.
The output of the Error Amplifier integrates the
voltage difference between the feedback and
the 0.81V bandgap reference. The polarity is
such that a FB pin voltage lower than 0.81V
increases the EA output voltage. Since the EA
output voltage is proportional to the peak
inductor current, an increase in its voltage also
increases current delivered to the output.
The increasing inductor current is sensed and
amplified by the Current Sense Amplifier. Ramp
compensation is summed to the Current Sense
Amplifier output and compared to the Error
Amplifier output by the PWM Comparator.
When the sum of the Current Sense Amplifier
output and the Slope Compensation signal
exceeds the EA output voltage, the RS Flip-
Flop is reset and M1 is turned off. The external
Schottky rectifier diode (D1) conducts the
inductor current.
IN
5
CURRENT SENSE
AMPLIFIER
- -
+
x20
RAMP
GENERATOR
D
REGULATOR
BST
SW
1
6
OSCILLATOR
1.4MHz/460KHz
EN
4
REGULATOR
REFERENCE
M1
Q
DRIVER
S
+
--
R
R
CURRENT
LIMIT
COMPARATOR
1pF
27pF
+
+
EA
--
--
FB
3
2
PWM
ERROR
AMPLIFIER
COMPARATOR
GND
Figure 1—Functional Block Diagram
MP2359 Rev. 1.1
6/23/2006
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5
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