MP2388 – 1A, 21V, 2MHZ , HIGH-EFFICIENCY, SYNCHRONOUS, STEP-DOWN CONVERTER

PRELIMINARY SPECIFICATIONS SUBJECT TO CHANGE

MP2388 Rev. 1.0

www.MonolithicPower.com

13

4/15/2016

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© 2016 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see Typical Application on page

1). The feedback resistor R1 also sets the

feedback

loop

bandwidth

with

the

internal

compensation

capacitor

(see

the

Typical

Application

on

page

1).

R2

can

then

be

calculated with Equation (1):

OUT

R1

R2

V

1

0.798V

=

−

(1)

The feedback network is shown in Figure 5.

Figure 5: Feedback Network

Table

1

lists

the

recommended

feedback

resistor values for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

R1 (kΩ)

R2 (kΩ)

1.05

191

604

1.2

191

383

1.8

102

82

2.5

102

47.5

3.3

75

24

5

100

19.1

Selecting the Inductor

A 0.47µH-to-4.7µH inductor with a DC current

rating at least 25% percent higher than the

maximum load current is recommended for most

applications. For the highest efficiency, the

inductor DC resistance should be less than

15mΩ. For most designs, the inductance value

can be derived from Equation (2):

OUT

IN

OUT

1

IN

L

OSC

V

(V

V

)

L

V

If

×−

=

×∆ ×

(2)

Where

∆I

Choose the inductor current to be approximately

30% of the maximum load current. The maximum

inductor peak current can be calculated with

Equation (3):

2

I

I

I

L

LOAD

)

MAX

(

L

∆

+

=

(3)

Under light-load conditions below 100mA, a

larger inductance is recommended for improved

efficiency.

Setting the AAM Voltage

The AAM voltage is used to set the transition

point from AAM to PWM. It should be chosen to

provide

the

best

combination

of

efficiency,

stability, ripple, and transient.

If the AAM voltage is set lower, then stability and

ripple improve, but efficiency during AAM mode

and transient degrade. Likewise, if the AAM

voltage is set higher, then the efficiency during

AAM and transient improve, but stability and

ripple degrade.

Adjust the AAM threshold by connecting a

resistor from AAM to ground (see Figure 6). An

internal

6.2µA

current

source

charges

the

external resistor.

AAM

3

Figure 6: AAM Network

Generally, R3 is can be calculated with Equation

(4):

A

R3x6.2

(4)

The optimized AAM is shown in Figure 7.