MP2227 – 24V, 3A, 1.3MHz SYNCHRONOUS STEP-DOWN CONVERTER
MP2227 Rev. 1.1 www.MonolithicPower.com 9
11/21/2011 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2011 MPS. All Rights Reserved.
FUNCTIONAL DESCRIPTION
PWM Control
The MP2227 is a constant frequency peak-
current-mode control PWM switching regulator.
Refer to the functional block diagram. The high
side N-Channel DMOS power switch turns on at
the beginning of each clock cycle. The current
in the inductor increases until the PWM current
comparator trips to turn off the high side DMOS
switch. The peak inductor current at which the
current comparator shuts off the high side
power switch is controlled by the COMP voltage
at the output of feedback error amplifier. The
transconductance from the COMP voltage to
the output current is set at 11.25A/V.
This current-mode control greatly simplifies the
feedback compensation design by
approximating the switching converter as a
single-pole system. Only Type II compensation
network is needed, which is integrated into the
MP2227. The loop bandwidth is adjusted by
changing the upper resistor value of the resistor
divider at the FB pin. The internal compensation
in the MP2227 simplifies the compensation
design, minimizes external component counts,
and keeps the flexibility of external
compensation for optimal stability and transient
response.
Enable and Frequency Synchronization
(EN/SYNC PIN)
This is a dual function input pin. Forcing this pin
below 0.4V for longer than 4us shuts down the
part; forcing this pin above 1.6V for longer than
4µs turns on the part. Applying a 1MHz to
2MHz clock signal to this pin also synchronizes
the internal oscillator frequency to the external
clock. When the external clock is used, the part
turns on after detecting the first few clocks
regardless of duty cycles. If any ON or OFF
period of the clock is longer than 4µs, the signal
will be intercepted as an enable input and
disables the synchronization.
Soft-Start and Output Pre-Bias Startup
When the soft-start period starts, an internal
current source begins charging an internal soft-
start capacitor. During soft-start, the voltage on
the soft-start capacitor is connected to the non-
inverting input of the error amplifier. The soft-
start period lasts until the voltage on the soft-
start capacitor exceeds the reference voltage of
0.8V. At this point the reference voltage takes
over at the non-inverting error amplifier input.
The soft-start time is internally set at 120µs. If
the output of the MP2227 is pre-biased to a
certain voltage during startup, the IC will disable
the switching of both high-side and low-side
switches until the voltage on the internal soft-
start capacitor exceeds the sensed output
voltage at the FB pin.
Over Current Protection
The MP2227 offers cycle-to-cycle current
limiting for both high-side and low-side switches.
The high-side current limit is relatively constant
regardless of duty cycles. When the output is
shorted to ground, causing the output voltage to
drop below 70% of its nominal output, the IC is
shut down momentarily and begins discharging
the soft start capacitor. It will restart with a full
soft-start when the soft- start capacitor is fully
discharged. This hiccup process is repeated
until the fault is removed.
Bootstrap (BST PIN)
The gate driver for the high-side N-channel
DMOS power switch is supplied by a bootstrap
capacitor connected between the BS and SW
pins. When the low-side switch is on, the
capacitor is charged through an internal boost
diode. When the Low-side switch is off and the
high-side switch turns on, the voltage on the
bootstrap capacitor is boosted above the input
voltage and the internal bootstrap diode
prevents the capacitor from discharging.
No external bootstrap diode is required for
typical applications. For applications with low
input VCC voltage or where output voltage is
very close to input voltage, an external Schottky
diode may be connected from the VCC to BS
pins to charge the bootstrapped capacitor more
strongly for increased gate drive voltage. When
using the external bootstrap diode, a resistor at
the regulator output or a minimal load current
may be required as the bootstrapped capacitor
