MP4470/4470A–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4470/4470A Rev. 1.1

www.MonolithicPower.com

13

8/26/2013

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

A resistor divider from the output voltage to the

Without

an

external

ramp

employed,

the

voltage. To determine the values for the resistors,

OUT

REF

REF

VV

R1

R2

V

−

=×

(10)

When using a low-ESR ceramic capacitor on the

output, add an external voltage ramp to the FB

1

4

2

1

1

2

1

2

1

−

⎟

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎜

⎝

⎛

−

−

−

×

+

=

R

)

V

V

V

(

R

V

V

R

RAMP

REF

OUT

RAMP

REF

(11)

Using equation 11 to calculate the output voltage

varies. To improve the output voltage accuracy

shows a simplified circuit with external ramp

compensation and a DC-blocking capacitor.

improve DC-blocking performance.

Cdc

Figure 9: Simplified Circuit with External Ramp

Compensation and DC Blocking Capacitor

Input Capacitor

The input current to the step-down converter is

discontinuous,

and

Therefore

requires

a

capacitor to supply the AC current to the step-

down converter while maintaining the DC input

voltage. Ceramic capacitors are recommended

for best performance. Be sure to place the input

capacitors as close to the IN pin as possible.

The

capacitance

varies

significantly

with

temperature. Capacitors with X5R and X7R

ceramic dielectrics are are fairly stable over

temperature fluctuations.

The capacitors must also have a ripple-current

rating greater than the converter’s maximum

input-ripple current. The input ripple current can

be estimated as follows:

OUT

OUT

CIN

OUT

IN

IN

VV

II

(1

)

VV

=×

× −

(12)

where:

OUT

CIN

I

I

2

=

(13)

For simplification, choose an input capacitor

whose RMS current rating is greater than half of

the maximum load current.The input capacitance

value determines the input voltage ripple of the

converter. If there is an input-voltage-ripple

requirement in the system design, choose an

input capacitor that meets the specification

The input voltage ripple can be estimated as

follows:

OUT

OUT

OUT

IN

SW

IN

IN

IN

IV

V

V(1

)

FC

V

V

Δ=

×

× −

×

(14)

The worst-case condition occurs at VIN = 2VOUT,

where:

OUT

IN

SW

IN

I

1

V

4F

C

Δ= ×

×

(15)