MP1494 – SYNCHRONOUS STEP-DOWN CONVERTER

MP1494 Rev. 1.04

www.MonolithicPower.com

11

12/26/2012

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© 2012 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets 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 Typical Application on page 1).

Choose R1 around 40kΩ. R2 is then given by:

OUT

R1

R2

V

1

0.807V

=

−

The T-type network—as shown in Figure 4—is

FB

8

RT

R2

R1

VOUT

Figure 4: T-Type Network

Table 1 lists the recommended T-type resistors

value for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

R1 (kΩ)

R2 (kΩ)

Rt (kΩ)

1.0

20.5(1%)

82(1%)

82(1%)

1.2

30.1(1%)

60.4(1%)

82(1%)

1.8

40.2(1%)

32.4(1%)

56(1%)

2.5

40.2(1%)

19.1(1%)

33(1%)

3.3

40.2(1%)

13(1%)

33(1%)

5

40.2(1%)

7.68(1%)

33(1%)

Selecting the Inductor

Use a1µH-to-10µH inductor with a DC current

rating of at least 25% percent higher than the

maximum load current for most applications. For

highest efficiency, use an inductor with a DC

resistance less than 15mΩ. For most designs,

the inductance value can be derived from the

following equation.

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×−

=

×Δ ×

Choose the inductor ripple current to be

approximately 30% of the maximum load current.

The maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Use a larger inductor for improved efficiency

under light-load conditions—below 100mA.

Setting the AAM Voltage

The AAM voltage sets the transition point from

AAM to CCM. Select a voltage to balance

efficiency, stability, ripple, and transient.

A low AAM voltage improves stability and ripple,

but degrades transient and efficiency during AAM.

Likewise, a high AAM voltage improves the

transient

and

efficiency

during

AAM,

but

degrades stability and ripple.

The AAM voltage comes from the tap of a

in Figure 5.

R3

AAM

VCC(5V)

R4

Figure 5: AAM Network

Generally, choose R4 to be around 10kΩ, then

R3 is:

⎟

⎠

⎞

⎜

⎝

⎛

−

=

1

AAM

VCC

R4

R3