MP28164 – SINGLE-INDUCTOR, BUCK-BOOST CONVERTER WITH 4.2A SWITCHES

MP28164 Rev. 1.0

www.MonolithicPower.com

14

3/28/2016

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OPERATION

The MP28164 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 (see Figure 1). The voltage

difference between FB and the internal reference

is amplified by the error amplifier to generate a

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.

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.

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

compares 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 load due to

SWA’s maximum duty cycle, so the current ramp

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

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

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

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 MP28164’s UVLO circuit

monitors the VCC voltage. During start-up, VIN

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

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

another power supply, the MP28164 can work

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

MP28164 cannot supply high power to the output.

If VIN drops to 0.69V, the MP28164 stops

working.