MP2205A – 16V, 2.5A, 1.3MHz SYNCHRONOUS STEP-DOWN CONVERTER
FUNCTIONAL DESCRIPTION
PWM Control
The MP2205A 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 MP2205A. The loop bandwidth is
adjusted by changing the upper resistor value of the
resistor divider at the FB pin. The internal
compensation in the MP2205A 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 MP2205A 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.
MP2205A Rev. 0.9
9/1/2010
Over Current Protection
The MP2205A 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
The MP2205A 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 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 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.
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 always see the supply
voltage even when the part is disabled.
Input UVLO
Both VCC and IN pins have input UVLO detection.
Until both VCC and IN voltage exceed under voltage
lockout threshold, the parts remain in shutdown
condition. There are also under voltage lockout
hysesteres at both VCC and IN pins.
Vcc Power Supply
Vcc is the power supply of both the internal control
circuit and the gate drivers.
Generally, the Vcc power supply could be provided
directly by a proper power rail or generated from
other Vcc generation circuits. For instance, figure 4
shows a typical Vcc generation circuit for VOUT=5V
application.
It is noteworthy that the voltage applied on the Vcc
pin should never be higher than 6V.
9
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2010 MPS. All Rights Reserved.
MPS [ MONOLITHIC POWER SYSTEMS ]
