MP28115 – 4A, 1.5MHz SYNCHRONOUS STEP-DOWN CONVERTER
MP28115 Rev. 0.92 www.MonolithicPower.com 7
1/6/2016 MPS Proprietary Information. Patent Protected.
Unauthorized Photocopy and Duplication Prohibited.
© 2016 MPS. All Rights Reserved.
FUNCTIONAL DESCRIPTION
PWM Control
The MP28115 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 is 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 MP28115. The loop
bandwidth is adjusted by changing the upper
resistor value of the resistor divider at the FB pin.
The internal compensation in the MP28115
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 4μs 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 MP28115 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 MP28115 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.
Power Good Output (POK PIN)
The MP28115 includes an open-drain Power
Good output that indicates whether the regulator
output is within ±10% of its nominal output. When
the output voltage moves outside this range, the
POK output is pulled to ground. There is a 30µs
deglitch time when the POK output change its
state.
Bootstrap (BST PIN)
The gatedriver voltage 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 high-side switch is on and the
low-side switch turns off, the voltage on the
bootstrap capacitor is boosted above the input
voltage and the internal bootstrap diode prevents
the capacitor from discharging.
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