MAX20047AFPASLASHV+ datasheet(23/29 Pages) MAXIM

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Inductor Selection

Three key inductor parameters must be specified for

operation with the device: inductance value (L), inductor

saturation current (ISAT), and DC resistance (RDCR). The

device is designed to operate with the ratio of inductor

peak-to-peak AC current to DC average current (LIR)

between 15% and 30% (typ). The switching frequency,

input voltage, and output voltage then determine the

inductor value as follows:

SUPSW

OUT_

OUT

MIN1

SUPSW

MAX

) V

33%

−×

where VSUPSW and VOUT are typical values (so that

efficiency is optimum for typical conditions). The switching

frequency is set by RFOSC.

RCS = 300mΩ

IMAX = 6A

fSW = The operating frequency is set by RFOSC.

The next equation ensures that the inductor current

downslope is less than the internal slope compensation.

For this to be the case, the following equation needs to

be satisfied:

−≥

where m2 = The inductor current downslope:

OUT

-m = Slope compensation:

1.333

µs













Solving for L and adding a 1.5 multiplier to account for

tolerances in the system:

MIN2

OUT

= V

1.5

To satisfy both LMIN1 and LMIN2, LMIN must be set to the

larger of the two:

LMIN = Max (LMIN1,LMIN2)

The maximum nominal inductor value recommended is

1.6 times the chosen value from the above formula.

LMAX = 1.6 x LMIN

Select a nominal inductor value based on the following

formula:

LMIN < LNOM < LMAX

The best choice of inductor is usually the first standard

inductor value greater than LMIN.

Input Capacitor

The input filter capacitor reduces peak currents drawn

from the power source and reduces noise and voltage

ripple on the input caused by the circuit’s switching.

Place a 4.7µF (or larger) ceramic capacitor directly from

SUPSW to PGND positioned as close as possible to the

pins. A bulk capacitor with higher ESR (such as an electro-

lytic capacitor) is normally required as well to lower the Q of

the front-end circuit and provide the remaining capacitance

needed to minimize input-voltage ripple.

The input capacitor RMS current requirement (IRMS) is

defined by the following equation:

×−

OUT

SUPSW

OUT

RMS

LOAD(MAX)

SUPSW

)

IRMS has a maximum value when the input voltage

equals twice the output voltage:

VSUPSW = 2 x VOUT

Therefore:

= LOAD(MAX)

RMS

SUPSW

Choose an input capacitor that exhibits less than +10°C

self-heating temperature rise at the RMS input current for

optimal long-term reliability.

The input-voltage ripple is comprised of ΔVQ (caused

by the capacitor discharge) and ΔVESR (caused by the

ESR of the capacitor). Use low-ESR ceramic capacitors

with high ripple-current capability at the input. Assume

the contribution from the ESR and capacitor discharge

equal to 50%. Calculate the input capacitance and ESR

required for a specified input voltage ripple using the

following equations:

ESR

OUT

ESR =

I+

∆

where:

(

)

SUPSW

OUT

SUPSW

I =

−

∆

MAX20047

Dual 3A USB DCP Charger

www.maximintegrated.com

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