MP9928—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER

MP9928 Rev. 1.0

www.MonolithicPower.com

16

5/20/2016

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OPERATION

Overview

The MP9928 is a high-performance, step down,

synchronous DC/DC converter controller IC

with a wide input range. It implements current

mode,

switching

frequency

programmable

control architecture to regulate the output

voltage

with

external

N-channel

MOSFET

switches.

The MP9928 senses the voltage at FB pin. The

difference between the voltage on this pin and

an internal 0.8V reference is amplified to

generate an error voltage on COMP pin which

is used as a threshold for the current sense

comparator with a slope compensation ramp.

Under normal load condition, the controller

operates in full PWM mode. At the beginning of

each oscillator cycle, the top gate driver is

enabled. Top gate turns on for a period

determined by the duty cycle. When the top

gate turns off, the bottom gate turns on after a

dead time and stays on until the beginning of

the next clock cycle.

There is an optional power save mode for light

load or no load conditions, and see details in

the following section.

AAM Mode

MP9928 employs AAM mode functionality to

optimize the efficiency during light-load or no-

load conditions. This AAM mode can be

optional enabled when CCM/AAM pin is at a

low level by connecting an appropriate resistor

480mV.

μA) x R

AAM

(kΩ)

shown below.

μA) = 600 (mV) / R

FREQ

(kΩ)

given operating frequency, and its value is

shown in ‘Programmable Switching Frequency’

section.

AAM is disabled when CCM/AAM pin is floating

or connected to VCC1.

If AAM is enabled, the MP9928 will firstly enter

non-synchronous operation as long as the

inductor current approaches zero at light-load. If

the load is further decreased or even no load

that make COMP voltage below the Voltage of

enters AAM mode. In AAM mode, the internal

taken as benchmark of the next clock. When

is DCM or CCM which has a constant switching

frequency.

AAM Mode

(AAM=Low)

Inductor

Current

t

t

t

Load

Decreased

PWM Mode

(AAM=High)

Inductor

Current

t

t

t

Load

Decreased

Figure 2: AAM and PWM

Floating Driver and Bootstrap Charging

The floating top gate driver is powered by an

normally

refreshed

when

the

high-side

MOSFET (HS-FET) turns off. This floating

driver has its own UVLO protection. This

UVLO’s

rising

threshold

is

3.05V

with

a

hysteresis of 170mV.

VCC1 Regulator and VCC2 Power Supply

Both high-side and low-side MOSFET drivers

and most of the internal circuitries are powered

from the VCC1 regulator. An internal low

dropout linear regulator supplies VCC1 power

from VIN, usually a 1

μF to 4.7μF ceramic

capacitor is recommended from VCC1 to GND.

If VCC2 pin is left open or connected to a

voltage

<4.45V,

an

internal

5V

regulator

supplies VCC1 power from VIN. If VCC2

is >4.7V, the 5V regulator is disabled and

another 5V regulator is triggered that supplies

VCC1 power from VCC2. If 4.5V<VCC2<5V,

the 5V regulator is in dropout and VCC1 is

approximately equal to VCC2. When VCC2 is

greater than 5V (max. is 12V), VCC1 is