MP2012 – 1.5A SYNCHRONOUS STEP-DOWN CONVERTER
MP2012 Rev. 1.01 www.MonolithicPower.com 7
9/22/2011 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2011 MPS. All Rights Reserved.
OPERATION
The MP2012 is a fixed frequency 1.2MHz current
mode 1.5A step-down converter, optimized for
low voltage, Li-Ion battery powered applications
where high efficiency and small size are critical.
MP2012 integrates a high side PFET main switch
and a low side synchronous rectifier. It always
operates in continuous conduction mode,
simplifies the control scheme and eliminates the
random spectrum noise due to discontinuous
conduction mode.
The steady state duty cycle D for this mode can
be calculated as:
%100
V
V
%100fTD
IN
OUT
OSCON
×≈××=
Where T
ON
is the main switch on time and f
OSC
is
the oscillator frequency (1.2MHz typ.).
Current Mode PWM Control
Slope compensated current mode PWM control
provides stable switching and cycle-by-cycle
current limiting for superior load and line
response as well as protection of the internal
main switch and synchronous rectifier. The
MP2012 switches at a constant frequency
(1.2MHz) and modulates the inductor peak
current to regulate the output voltage. Specifically,
for each cycle the PWM controller forces the
inductor peak current to an internal reference
level derived from the feedback error voltage. At
normal operation, the main switch is turned on at
each rise edge of the internal oscillator, and
remains on for a certain period of time to ramp up
the inductor current. As soon as the inductor
current reaches the reference level, the main
switch is turned off and immediately the
synchronous rectifier will be turned on to provide
the inductor current. In forced PWM mode, the
synchronous rectifier will stay on until the next
oscillator cycle.
Dropout Operation
The MP2012 allows the main switch to remain on
for more than one switching cycle to increase the
duty cycle when the input voltage is dropping
close to the output voltage. When the duty cycle
reaches 100%, the main switch is held on
continuously to deliver current to the output up to
the PFET current limit. In this case, the output
voltage becomes the input voltage minus the
voltage drop across the main switch and the
inductor.
Maximum Load Current
The MP2012 can operate down to 2.5V input
voltage; however the maximum load current
decreases at lower input due to a large IR drop
on the main switch and synchronous rectifier.
The slope compensation signal reduces the peak
inductor current as a function of the duty cycle to
prevent sub-harmonic oscillations at duty cycles
greater than 50%. Conversely, the current limit
increases as the duty cycle decreases.
Short Circuit Protection
When short circuit or over current condition
happens, and FB is lower than about 0.3V, the
MP2012 enters fold back mode. The oscillator
frequency is reduced to prevent the inductor
current from increasing beyond the PFET current
limit. The PFET current limit is also reduced to
lower the short circuit current. The frequency and
current limit will return to the normal values once
the short circuit condition is removed and the
feedback voltage approaches 0.8V.
