MP2115 – 2A, 6V SYNCHRONOUS STEP-DOWN CONVERTER

MP2115 Rev. 1.0

www.MonolithicPower.com

8

2/28/2011

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© 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

%

100

f

T

D

IN

OUT

OSC

ON

×

≈

×

×

=

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

R5 (kΩ)

5

3.8

0.2

47.2

0.3

33.3

0.4

25.75

0.45

23.29

0.5

24.9