TM
MP1423 – 3A, 23V, 385KHz STEP-DOWN CONVERTER
PIN FUNCTIONS (continued)
Pin # Name Description
Compensation Node. COMP is used to compensate the regulation control loop. Connect a
COMP series RC network from COMP to GND to compensate the regulation control loop. In some
cases, an additional capacitor from COMP to GND is required. See Compensation
6
7
8
Enable Input. EN is a digital input that turns the regulator on or off. Drive EN high to turn on the
regulator, low to turn it off. For automatic startup, leave EN unconnected.
EN
Soft Start Control Input. SS controls the soft start period. Connect a capacitor from SS to GND
SS
to set the soft-start period. A 0.1µF capacitor sets the soft-start period to 10ms. To disable the
soft-start feature, leave SS unconnected.
OPERATION
The MP1423 is a current-mode step-down
regulator. It regulates input voltages from 6V to
23V down to an output voltage as low as
1.222V, and is able to supply up to 3A of load
current.
The voltage at COMP is compared to the switch
current measured internally to control the output
voltage.
The converter uses an internal N-Channel
MOSFET switch to step-down the input voltage
to the regulated output voltage. Since the
MOSFET requires a gate voltage greater than
the input voltage, a boost capacitor connected
between SW and BS drives the gate. The
capacitor is internally charged while SW is low.
The MP1423 uses current-mode control to
regulate the output voltage. The output voltage
is measured at FB through a resistive voltage
divider and amplified through the internal error
amplifier. The output current of the
transconductance error amplifier is presented at
COMP where a network compensates the
regulation control system.
An internal 10ꢀ switch from SW to GND is used
to insure that SW is pulled to GND when SW is
low to fully charge the BS.capacitor.
2
IN
CURRENT
SENSE
AMPLIFIER
INTERNAL
REGULATORS
+
--
5V
OSCILLATOR
SLOPE
COMP
1
3
BS
40/385KHz
CLK
+
--
+
S
R
Q
Q
SW
CURRENT
COMPARATOR
SHUTDOWN
COMPARATOR
--
0.7V
7
EN
LOCKOUT
COMPARATOR
--
+
1.8V
2.37V/
2.71V
+
--
4
GND
MP1423_BD01
4
0.7V 1.222V
5
--
+
FREQUENCY
FOLDBACK
COMPARATOR
ERROR
AMPLIFIER
6
8
FB
COMP
SS
Figure 1—Functional Block Diagram
MP1423 Rev. 1.1
1/6/2006
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2006 MPS. All Rights Reserved.
MPS [ MONOLITHIC POWER SYSTEMS ]
