MP2155 –HIGH EFFICIENCY SINGLE INDUCTOR BUCK-BOOST CONVERTER WITH 2.2A SWITCHES

MP2155 Rev. 1.0

www.MonolithicPower.com

13

1/26/2014

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© 2014 MPS. All Rights Reserved.

OPERATION

The MP2155 is a high efficiency, dual mode

buck-boost converter that provides output voltage

above, equal to or below the input voltage. When

the MODE pin is held high, the MP2155 operates

in constant-frequency PWM mode with peak

current mode control. As shown in Figure 1, the

output voltage is sensed via the FB pin through

an external resistor-divider from the output to

ground. The voltage difference between FB pin

and the internal reference is amplified by error

ramp (the sensed SWA’s current with slope

compensation) through Buck comparator, a PWM

control signal for PWM buck mode is outputted.

Buck

through

level

shift.

Similarly,

compares with the same ramp signal through

Boost comparator and generates the PWM

control signal for PWM boost mode. The switch

topology for the buck-boost converter is shown in

Figure 2.

SWA

SWB

SWC

SW1

SW2

SWD

Vin

Vout

MP2155

Figure 2—Buck-Boost Switch Topology

Buck Region (Vin > Vout)

When the input voltage is significantly greater

than output voltage, which means the converter

can deliver energy to load within the maximum

duty cycle of SWA, so the converter operates in

lower than compensation ramp because Buck

can deliver enough energy to load, thus switch D

turns on constantly and switch C remains off.

ramp normally and generates PWM signal,

therefore, switches A and B are pulse-width-

modulated to produce the required duty cycle to

support the output voltage.

Buck-Boost Region (Vin ≈ Vout)

When Vin is close to Vout, due to duty cycle limit

of SWA the converter isn’t able to provide wanted

energy to load. In this case SWA will be turned

on over all the period, that is, there is no BD

operation(SWB and SWD being turned on

simultaneously). Now a new period begins. Since

there is no BD in last period, an offset voltage is

added to the ramp signal to make the ramp

to some level, so that the ramp signal can

intersect it to produce the PWM driving signal for

Boost operation. After SWC is turned off the

ramp signal continues rising (the actual inductor

current may rise or fall depending on the

difference between Vin and Vout), when the

operation then is generated. Now the buck’s duty

cycle is within its limit, so there is BD operation in

current period, which means next period the

offset voltage will be removed. This is the so-

called buck-boost region. With heavy load due to

voltage drop on switches the actual input range

for this region may be a little wide.

Boost Region (Vin < Vout)

When the input voltage is significantly lower than

output voltage, the converter operates in boost

than compensation ramp even with the offset

voltage always added, thus switch A turns on

continuously

and

switch

B

remains

off.

ramp normally and generates PWM signal,

therefore, switches C and D are pulse-width-

modulated to produce the required duty cycle to

support the output regulation voltage.

PSM

When Mode Pin is pulled down below the low

level threshold, the MP2155 will automatically

enter PSM if load is light. When working in PSM,

a train of SW pulses are initiated by a Boost

operation, and ended with BD operation. During

this process, SWD will be turned off if inductor

current is below about 100mA. In actual