LTC4418 datasheet(22/30 Pages) LINER

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LTC4418

Rev A

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APPLICATIONS INFORMATION

P-Channel MOSFET Selection

Using the list of suggested P-channel MOSFETs in Table 1

as a guideline, the FDS4465 (Max VDS = –20V, RDS(ON) =

8.5mΩ) is appropriate for this application. When selecting

external MOSFETs the following items are relevant:

1. MAX VDS

2. RDS(ON)

3. IDM

4. CRSS (Maximum)

5. VGS (At IINRUSH + ILOAD Current)

6. SOA

Inrush Current Component Selection

The UV2 threshold for V2 is larger than the OV/UV window

of V1. This means that there could be significant inrush

current during switchover from V1 to V2. There are two

inrush current issues to address:

1. Avoid damaging the P-channel MOSFETs by violating

their IDM specification

2. Prevent UV faults on channel 2 from V2 input droop

The maximum inrush current occurs when switching to

the V2 supply which has a maximum voltage of 14.03V.

The minimum voltage of VOUT is 4.5V if V1 is at its UV

threshold. For these conditions, the maximum inrush

current through the P-channel MOSFETs is:

MAX IINRUSH =

V2(MAX) − VOUT(MIN)

RSRC +ESRCOUT + 2 • RDS(ON)

14.03V − 4.5V

0.02Ω + 0.025Ω + 0.17Ω

= 154A

The 154A worst case inrush current far exceeds the 50A

IDM spec of the FDS4465. The worst case condition for

generating a UV fault on Channel 2 is if V2 is just above

UVHYS threshold of 10.2V. Given the input impedance of

20mΩ, the maximum tolerable inrush current is:

Tolerable IINRUSH =

UVHYS2

RSRC

250mV

0.02Ω

= 12.5A

Based on these calculations, the target inrush current for

the V2 PowerPath switches is 12A. Now the appropriate

values for output capacitance and inrush current limiting

components are determined.

The first step is to calculate the minimum required output

capacitance, COUT, to satisfy the desired output voltage

droop, 500mV. Assume the inrush current limiting com-

ponents, RS and CS, add 15µs to the switchover time.

COUT ≥

ILOAD • tG(SWITCHOVER) + 15µs

(

)

ΔVOUT(DROOP) − ESRCOUT • ILOAD

≥

1.25A • (2.7µs + 15µs)

500mV − 1.25A • 25mΩ

≥ 47µF

For aluminum electrolytic capacitors add at least 20%.

For margin, choose 82µF for COUT for this application.

The inrush current limiting components must now be

determined. Information from the FDS4465 data sheet

required is VGS at IINRUSH + ILOAD which is 1.25V at ap-

proximately 12A and the maximum value of CRSS which

is 4000pF. The minimum value of CS is 10 • CRSS or 40nF

so a 47nF CS value is selected. Next calculate RS using

the maximum specification for ∆VG(SINK):

RS ≥

(ΔVG(SINK) − VGS) • COUT

CS •IINRUSH(TARGET)

≥

(6V − 1.25V) • 82µF

47nF • 12A

≥ 690.6Ω

where the closest 1% value is 698Ω.

With the inrush current limiting components known, the

desired output capacitance is checked with the equation:

COUT ≥

ILOAD • tG(SWITCHOVER) + 0.79 • RS • CS

(

)

ΔVOUT(DROOP) − ESR • ILOAD

≥

1.25A • (2.7µs + 0.79 • 698Ω • 47nF)

500mV − 25mΩ

≥ 71.54µF

The selected 82µF output capacitance is therefore suit-

able. A typical value for CVS1 and CVS2 is 0.1µF. For the

situation where inrush current limiting components are

used, so CVS2 is chosen to be 0.47µF.

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