MP2136 – 2.5V TO 6V INPUT, 2MHz, 1A SYNCHRONOUS STEP-DOWN CONVERTER
FUNCTIONAL DESCRIPTION
MP2136 is a 2MHz constant frequency, current
mode, PWM step-down converter. The device
integrates both a main switch and a synchronous
rectifier, which provides high efficiency and
eliminates an external Schottky diode. It is ideal
for powering portable equipment that runs from a
single Li-lon or Li-Polymer battery.
The MP2136 can achieve 100% duty cycle. The
duty cycle D of a step-down converter is defined
as:
Dropout Operation
The MP2136 allows the main switch to remain on
for more than one switching cycle and increases
the duty cycle while 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. The output voltage then is
the input voltage minus the voltage drop across
the main switch and the inductor.
Short Circuit Protection
The MP2136 has short circuit protection. When
the output is shorted to ground, 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 reaches 0.6V.
Maximum Load current
The MP2136 can operate down to 2.5V input
voltage, however the maximum load current
decreases at lower input due to 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.
V
D
=
T
ON
×
f
OSC
×
100%
≈
OUT
×
100%
V
IN
Where,
Ton: Main Switch ON time
Fosc: Switching frequency (2MHz)
Vout: Output voltage
Vin: Input voltage
Peak Current Mode PWM Control
Slope compensated current mode PWM control
provides stable switching and cycle-by-cycle
current limit for superior load and line response
and protection of the internal main switch and
synchronous rectifier. The MP2136 switches at a
constant frequency (2MHz) and regulates the
output voltage. During each cycle the PWM
comparator modulates the power transferred to
the load by changing the inductor peak current
based on the feedback error voltage.
During normal operation, the main switch is
turned on for a certain time to ramp the inductor
current at each rising edge of the internal
oscillator, and switched off when the peak
inductor current is above the error voltage.
When the main switch is off, the synchronous
rectifier will be turned on immediately and stay on
until the next cycle starts.
MP2136 Rev. 0.9
8/23/2010
www.MonolithicPower.com
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