MP4473GL datasheet(16/21 Pages) MPS

16 / 21 page

MP4473–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4473 Rev. 1.0

www.MonolithicPower.com

12/15/2014

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

APPLICATION INFORMATION

Setting the Output Voltage

To set VOUT, connect a resistor divider from the

output voltage to the FB pin.

Without an external ramp, the feedback resistors

(R1 and R2) set the output voltage. To determine

the values for the resistors, first choose R2

(typically 5kΩ-40kΩ). Then R1 is calculated as

follows:

OUT

REF

−

=×

(10)

When using a low-ESR ceramic capacitor on the

output, add an external voltage ramp to the FB

pin through R4 and C4. The ramp voltage (VRAMP)

affects output voltage. Calculate VRAMP using

equation 19. Choose R2 between 5kΩ and 40kΩ.

Determine R1 as follows :

−

⎟

⎠

⎞

⎜

⎝

⎛

−

)

(

RAMP

REF

OUT

RAMP

REF

(11)

Using equation 11 to calculate the output voltage

is complicated. As VRAMP changes (due to

changes in VOUT and VIN), VFB also varies. To

improve output-voltage accuracy and simplify the

R2 calculation from equation 11, add a DC-

blocking capacitor (CDC). Figure 8 shows a

simplified

circuit

with

external

ramp

compensation and a DC-blocking capacitor.

(Equation 10 can then estimate R1.)

Select a CDC value between 1µF and 4.7μF to

improve DC-blocking performance.

Cdc

Figure 8: Simplified Circuit with External Ramp

Compensation and DC-Blocking Capacitor

Input Capacitor

The input current to the step-down converter is

discontinuous, requiring a capacitor to supply

the AC current while maintaining the DC-input

voltage. Ceramic capacitors are recommended

for high performance. 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

fairly

stable

over

temperature fluctuations.

The capacitors must have a ripple-current rating

greater than the converter’s maximum input-

ripple current. The input-ripple current can be

estimated as follows:

OUT

CIN

OUT

)

=×

× −

(12)

The worst condition occurs at VIN = 2VOUT:

where,

OUT

CIN

(13)

To simplify calculations, 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

V(1

)

Δ=

× −

(14)

The worst condition occurs at VIN = 2VOUT:

where,

OUT

Δ= ×

(15)

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