MP1495 – SYNCHRONOUS STEP-DOWN CONVERTER

MP1495 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. 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, then R2 is:

OUT

R1

R2

V

1

0.807V

=

−

shown in Figure 4.

FB

8

RT

R2

R1

VOUT

Figure 4: T-Type Network

Table 1 lists the recommended T-type resistor

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

For most applications, use a 1µH-to-10µH

inductor with a DC current rating that is at least

25% percent higher than the maximum load

current. Select an inductor with a DC resistance

less than 15mΩ for highest efficiency. 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 an 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 light-load conditions

(below 100mA) for improved efficiency.

Setting the AAM Voltage

The AAM voltage sets the transition point from

AAM to CCM. Select a voltage that balances

efficiency, stability, ripple, and transient: A

relatively low AAM voltage improves stability and

ripple, but degrades transient and efficiency

during AAM mode; a relatively high AAM voltage

improves the transient and efficiency during AAM,

but degrades stability and ripple.

AAM voltage is set from the tap of a resistor

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