MP1470B – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP1470B Rev. 1.0

www.MonolithicPower.com

13

8/25/2015

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage. Also, the feedback resistor (R1) sets

the feedback loop bandwidth through the

internal compensation capacitor (see the

Typical Application circuits). Refer to Table 1 to

choose R1 and R2 using Equation (1):

OUT

R1

R2

V

1

0.8V

=

−

(1)

Use a T-type network (see Figure 4).

Figure 4: T-Type Network

Table 1 lists the recommended T-type resistor

values for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

R1 (kΩ)

R2 (kΩ)

Rt (kΩ)

1.05

10 (1%)

32.4 (1%) 300 (1%)

1.2

20.5 (1%) 41.2 (1%) 249 (1%)

1.8

40.2 (1%) 32.4 (1%) 120 (1%)

2.5

40.2 (1%) 19.1 (1%) 100 (1%)

3.3

40.2 (1%)

13 (1%)

75 (1%)

5

40.2 (1%) 7.68 (1%) 75 (1%)

Selecting the Inductor

Use a 1µH to 10µH inductor with a DC current

rating at least 25% higher than the maximum

load current for most applications. For highest

efficiency, select an inductor with a DC

resistance less than 15mΩ. For most designs,

derive the inductance value from Equation (2):

OUT

IN

OUT

1

IN

L

OSC

V

(VV

)

L

VI

f

×−

=

×Δ ×

(2)

an inductor current approximately 30% of the

maximum load current. The maximum inductor

peak current is calculated with Equation (3):

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

(3)

Under light-load conditions (below 100mA), use

a larger inductor to improve efficiency.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous,

and

therefore

requires

a

capacitor to both supply the AC current to the

step-down converter and maintain the DC input

voltage. For best performance, use low ESR

capacitors, such as ceramic capacitors with

X5R or X7R dielectrics, and small temperature

coefficients. A 22µF capacitor is sufficient for

most applications.

The input capacitor (C1) requires an adequate

ripple-current rating because it absorbs the

input switching. Estimate the RMS current in

the input capacitor with Equation (4):

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(4)

as shown in Equation (5):

2

I

I

LOAD

1

(5)

For simplification, choose an input capacitor

with an RMS current rating greater than half the

maximum load current.

The input capacitor can be electrolytic, tantalum,

or ceramic. Place a small, high-quality ceramic

capacitor (0.1μF) as close to the IC as possible

when using electrolytic or tantalum capacitors.

When using ceramic capacitors, make sure that

they have enough capacitance to provide

sufficient charge to prevent excessive input

voltage ripple. Estimate the input voltage ripple

caused by the capacitance with Equation (6):

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

(6)

Selecting the Output Capacitor

The output capacitor (C2) maintains the DC

output voltage. Use ceramic, tantalum, or low