MP2115 – 2A, 6V SYNCHRONOUS STEP-DOWN CONVERTER
MP2115 Rev. 1.0 www.MonolithicPower.com 8
2/28/2011 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2011 MPS. All Rights Reserved.
OPERATION
The MP2115 is a constant frequency current
mode PWM step-down converter. The MP2115
is optimized for low voltage, USB port and Li-
Ion battery powered applications where high
efficiency and small size are critical. The
MP2115 uses an external resistor divider to set
the output voltage from 0.6V to 6V. The device
integrates both a main switch and a
synchronous rectifier, which provides high
efficiency and eliminates an external Schottky
diode. The MP2115 can achieve 100% duty
cycle. The duty cycle D of a step-down
converter is defined as:
%100
V
V
%100fTD
IN
OUT
OSCON
×≈××=
Where T
ON
is the main switch on time, and f
OSC
is the oscillator frequency.
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.
This protects the internal main switch and
synchronous rectifier. The MP2115 switch is
programmable from 0.7MHz to 2MHz. 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, then switched off when
the peak inductor current rises above the error
voltage. When the main switch is off, the
synchronous rectifier will turn on immediately
and stay on until the next cycle starts.
Dropout Operation
The MP2115 allows the main switch to remain
on for more than one switching cycle and
increases the duty cycle when the input voltage
drops close to the output voltage. When the
duty cycle reaches 100%, the main switch
continuously delivers current to the output up to
the PFET current limit. The output voltage is the
input voltage minus the voltage drop across the
main switch and the inductor.
Short Circuit Protection
The MP2115 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 MP2115 can operate down to a 2.8V 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.
Programmable Input Current Limit
The MP2115 has an input current limit
protection function. It will insure that the input
current doesn't exceed the maximum, when the
input is supplied by the USB. Once the input
current triggers the set current limit level, the
output voltage will shut down and latch off until
the input is reset.
The input current limit resistor R5 value can be
found from Table 1.
Table 1—Resistor Selection vs. Input
Current Setting
V
IN
(V) V
OUT
(V) I
OUT
(A) R5 (k)
5 3.8
0.2 47.2
0.3 33.3
0.4 25.75
0.45 23.29
0.5 24.9
