MP2918

—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER

MP2918 Rev. 1.02

www.MonolithicPower.com

23

5/31/2017

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plus additional rings on the switch node.

The MOSFET

’s power dissipations can be

calculated with Equation (13) and Equation (14):

















2

OUT

HS

OUT

ON-HS

IN

IN

OUT

r

f

SW

g-HS

SW

driver

V

P =I

R

+

V

1

V

I

t +t

f

2

+Q

f

V

(13)









 













2

OUT

LS

OUT

ON-LS

IN

g-LS

SW

driver

DROP

OUT

dead1

dead2

SW

V

P =I

R

1

+

V

Q

f

V

+

V

I

t

+t

f

(14)

dead time between the HS-FET turning off and

time between the LS-FET turning off and the

HS-FET turning on.

Ensure that the thermal caused by the power

loss on the MOSFETs does not exceed the

allowed maximum thermal of the selected

MOSFETs.

Schottky Selection

The diode between SW and PGND (shown as

D2 in Figure 12) is used to absorb spikes, store

charges during dead time, and protect the body

diode of the LS-FET. Considering the size and

power loss during the dead time, a 1 - 3A

Schottky diode is recommended.

BST Charge Diode and Resistor Selection

An external BST diode can enhance the

efficiency of the regulator when the duty cycle is

high. A power supply between 2.5V and 5V can

be used to power the external bootstrap diode.

supply in the circuit (see Figure 8).

L

BST

SW

External BST diode

IN4148

VCC1

VCC1

Figure 8: External Bootstrap Diode and Resistor

The recommended external BST diode is

IN4148, and the recommended BST capacitor

value is 0.1µF to 1μF.

can reduce the SW rising rate and voltage

spikes. This helps enhance EMI performance

higher resistance is better for SW spike

reduction but compromises efficiency. To make

a tradeoff between EMI and efficiency, a

≤20Ω

Compensation Components

The MP2918 employs current-mode control for

easy compensation and fast transient response.

COMP is the output of the internal error

amplifier and controls system stability and

transient response. A series resistor-capacitor

(R-C) combination sets a pole zero combination

to control the control sys

tem’s characteristics

(see Figure 9). The DC gain of the voltage

feedback loop can be calculated with Equation

(15):

OUT

FB

O

CS

LOAD

VDC

V

V

A

G

R

A









(15)

(3000V/V),

is

the

current

sense

transconductance

(A/V),

and

COMP

R5

C6

C7

Figure 9: Compensation Network