MPQ28164 – SINGLE-INDUCTOR, BUCK-BOOST CONVERTER W/ 4.2A SWITCHES

MPQ28164 Rev. 1.0

www.MonolithicPower.com

13

11/13/2018

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OPERATION

The MPQ28164 is a high-efficiency, dual-mode,

buck-boost converter that provides an output

voltage above, equal to, or below the input

voltage. The output voltage is sensed via FB

through an external resistor divider from the

output

to

ground.

The

voltage

difference

between FB and the internal reference is

amplified by the error amplifier to generate a

control signal (VC-Buck). By comparing VC-Buck

with the internal current ramp signal (the

sensed SWA’s current with slope compensation)

through the buck comparator, a pulse-width

modulation (PWM) control signal for the buck

leg (SWA, SWB) is generated.

Another control signal (VC-Boost) is derived from

VC-Buck through the level shift. Similarly, VC-Boost

is compared with the same ramp signal through

the boost comparator and generates a PWM

control signal for the boost leg (SWC, SWD).

The

switch

topology

for

the

buck-boost

converter is shown in Figure 2.

SWA

SWB

SWC

SW1

SW2

SWD

VIN

VOUT

MPQ28164

Figure 2: Buck-Boost Switch Topology

Buck Region (VIN > VOUT)

When the input voltage is significantly higher

than the output voltage, the converter can

deliver

energy

to

the

load

within

SWA’s

maximum duty cycle by switching SWA and

SWB. The converter operates in buck mode. In

this condition, SWD remains on and SWC

remains off. VC-Buck is compared with the current

ramp signal normally and generates a PWM

output. Therefore, SWA/SWB are pulse-width

modulated to produce the required duty cycle

and eventually support the output voltage.

Buck-Boost Region (VIN

≈ VOUT)

When VIN is close to VOUT, the converter is

unable to provide enough energy to the load

due to SWA’s maximum duty cycle, so the

current ramp signal cannot trigger VC-Buck in the

first period, and SWA remains on with 100%

duty cycle. If SWB is not turned on in the first

period, boost begins working in the secondary

period (SWC switches in the secondary period),

and an offset voltage is added to the current

ramp signal to allow it to reach VC-Buck. SWC

turns

off

when

the

current

ramp

signal

intersects with VC-Boost in the secondary period,

and SWD conducts the inductor current when

SWC is off. This is called boost operation.

SWA turns off when the current ramp signal

intersects with VC-Buck in the secondary period,

and SWB turns on to conduct the inductor

current after SWA turns off. This is called buck

operation.

If SWB turns on in the secondary period, the

boost operation (SWC on) is disabled in the

following cycle. If SWA continues to conduct

with 100% duty in the secondary cycle, the

boost operation is also enabled in the following

duty cycle. SWA/SWB and SWC/SWD switch

during this condition simultaneously. This is

called buck-boost mode.

Boost Region (VIN < VOUT)

When the input voltage is significantly lower

than the output voltage, the control voltage (VC-

Buck)

is always higher than the current ramp

signal. The offset voltage is added to the

current signal, so SWB cannot turn on in all

cycles. The boost operation (SWC on) is

enabled in every cycle based on the logic, so

only SWC and SWD switch. This is called boost

mode. In this condition, SWC/SWD are pulse-

width modulated to produce the required duty

cycle

and

eventually

support

the

output

regulation voltage.

Under-Voltage Lockout (UVLO)

Under-voltage

lockout

(UVLO)

is

used

to

protect

the

device

from

operating

at

an

insufficient supply voltage. The MPQ28164

’s

UVLO circuit monitors the VCC voltage. During

start-up, VIN must rise higher than VIN-UVLO-R to

support enough VCC voltage and enable the IC.

After the IC is enabled, VCC is powered by VIN

or VOUT (depending on which is higher), so the

IC can work, even if VIN drops to 1.2V, unless

VCC drops to the VCC-UVLO-F threshold.

During start-up, if VCC has a bias voltage from

another power supply, the MPQ28164 can work

with 1.2V of input power. If VIN is much lower

than 1.2V, the SWA RDS(ON) is high, and the