MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER

MP2908A Rev. 1.01

www.MonolithicPower.com

13

1/27/2016

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2016 MPS. All Rights Reserved.

OPERATION

The MP2908A is a high-performance, step-

down, synchronous DC/DC controller IC with a

wide input voltage range. It implements current

mode and switching frequency programmable

control architecture to regulate the output

voltage with external N-channel MOSFETs.

The MP2908A senses the voltage at FB; the

difference between the FB voltage and an

internal 0.8V reference is amplified to generate

an error voltage on COMP. This is used as a

threshold for the current sense comparator with

a slope compensation ramp.

Under normal load conditions, the controller

operates in full PWM mode (see Figure 2). At

the beginning of each oscillator cycle, the top

gate driver is enabled. The 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 remains on until the next

clock cycle begins.

There is an optional power-save mode for light-

load or no-load conditions.

Advanced Asynchronous Mode (AAM)

The MP2908A employs AAM functionality to

optimize efficiency during light-load or no-load

conditions (see Figure 2). It is enabled when

CCM/AAM is at a low level by connecting an

is no less than 480mV. See Equation (1):

(1)

Equation (2). AAM is disabled when CCM/AAM

is floating or connected to VCC1.

(2)

If AAM is enabled, the MP2908A first enters

non-synchronous operation as long as the

inductor current approaches zero at light-load. If

the load decreases further to make the COMP

voltage drop below the CCM/AAM voltage

for the next clock cycle. When the load

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.

If the BST voltage is lower than the bootstrap

UVLO, the MP2908A enters constant-off-time

mode to ensure that the BST cap is high

enough to drive the HS-FET.

VCC1 Regulator and VCC2 Power Supply

Both the top and bottom MOSFET drivers and

most of the internal circuitries are powered by

the VCC1 regulator. An internal, low, dropout

linear regulator supplies VCC1 power from VIN.

Connect a ceramic capacitor 4.7μF or smaller

from VCC1 to GND.

If VCC2 is left open or connected to a voltage

less than 4.5V, an internal 5V regulator supplies

than 4.5V, the internal regulator that supplies

power to VCC1 from VCC2 is triggered.

If VCC2 is greater than 4.5V but less than 5V,

the 5V regulator is in dropout, and VCC1 is

approximately equal to VCC2. Using the VCC2

power supply allows the VCC1 power to be

derived from a high-efficiency external source,

such as one of the MP2908A’s switching

regulator outputs.