MP2918

—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER

MP2918 Rev. 1.02

www.MonolithicPower.com

18

5/31/2017

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

© 2017 MPS. All Rights Reserved.

OPERATION

The MP2918 is a high-performance, step-down,

synchronous, DC/DC controller IC with a wide

input voltage range. It implements current-mode

control and programmable switching frequency

control architecture to regulate the output

voltage with external N-channel MOSFETs.

The MP2918 senses the voltage at FB. The

generate an error voltage on COMP. This is

used as the threshold for the current-sense

comparator with a slope compensation ramp.

Under normal-load conditions, the controller

operates in full pulse-width modulation (PWM)

mode (see Figure 3). 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 condition.

Advanced Asynchronous Mode (AAM)

The MP2918 employs advanced asynchronous

mode (AAM) functionality to optimize efficiency

during light-load or no-load condition (see

Figure 3). AAM is enabled when CCM/AAM is

at a low level by connecting an appropriate

resistor to SGND to ensure that the AAM

Equation (1):

(1)

AAM is disabled when CCM/AAM is floating or

connected to VCC1. Calculate the CCM/AAM

(2)

If AAM is enabled, the MP2918 first enters non-

synchronous operation for as long as the

inductor current approaches zero at light load. If

the load decreases further to make the COMP

as the benchmark for the next clock cycle.

When the load increases and the DC value of

is discontinuous conduction mode (DCM) or

continuous conduction mode (CCM), which

have a constant switching frequency.

Forced CCM

Inductor

Current

t

t

t

Load

Decreased

AAM

Inductor

Current

t

t

t

Load

Decreased

Figure 3: Forced CCM and AAM

Floating Driver and Bootstrap Charging

The floating top gate driver is powered by an

refreshed when the high-side MOSFET (HS-

FET) turns off, typically. This floating driver has

its

own

under-voltage

lockout

(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 MP2918 enters constant-off-time

mode to ensure that the BST capacitor 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,

Connect a

≥1μF ceramic capacitor from VCC1

to PGND.

If VCC2 is left open or connected to a voltage

less than 4.7V, an internal 5V regulator supplies

than 4.7V, the internal regulator that supplies

power to VCC1 from VCC2 is triggered. If

VCC2 is between 4.7V and 5V, the 5V regulator

is in dropout, and VCC1 approximately equals

VCC2. Using the VCC2 power supply allows the

VCC1 power to be derived from a high-

efficiency external source, such as one of the

MP2918

’s switching regulator outputs.