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LTC3856

3856f

applicaTions inForMaTion

A second, more severe transient is caused by switching

in loads with large (>1µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

with COUT, causing a rapid drop in VOUT. No regulator can

alter its delivery of current quickly enough to prevent this

sudden step change in output voltage if the load switch

resistance is low and it is driven quickly. If the ratio of

CLOAD to COUT is greater than 1:50, the switch rise time

should be controlled so that the load rise time is limited

to approximately 25 • CLOAD. Thus a 10µF capacitor would

require a 250µs rise time, limiting the charging current

to about 200mA.

Design Example (Using Two Phases)

As a design example, assume:

VIN = 5V (nominal)

VIN = 5.5V (max),

VOUT = 1.8V,

IMAX = 20A

TA = 70°C

f = 300kHz

The inductance value is chosen first based on a 30% ripple

current assumption. The highest value of ripple current

occurs at the maximum input voltage. Use a 71.5k resistor

from FREQ to ground to set the switching frequency at

about 300kHz. The minimum inductance for 30% ripple

current is:

f I

kHz

OUT

≥

∆

( )

−









≥ (

)(

1 8

300

))( )

−









≥

1 8

5 5

1 35

A 2µH inductor will produce 20% ripple current. The peak

inductor current will be the maximum DC value plus one

half the ripple current, or 11A. The minimum on-time oc-

curs at maximum VIN:

V f

kHz

µs

ON MIN

OUT

( ) =

= ( )(

)

1 8

5 5

300

1 1

With the ILIM pin tied to ground, the RSENSE resistors

value can be calculated by using the minimum current

sense voltage specification with some accommodation

for tolerances:

SENSE =

≈

Ω

0 002

Choosing 1% resistors: R1 = 10k and R2 = 20k yields an

output voltage of 1.80V.

The power dissipation on the topside MOSFET can be

easily estimated. Using a Siliconix Si4420DY for example;

RDS(ON) = 0.013Ω, CRSS = 300pF. At maximum input

voltage with TJ(estimated)=110°Catanelevatedambient

temperature:

MAIN =

( ) +(

) ° − °

(

)





1 8

5 5

1 0 005 110



( ) 





 (

)(

)

−

• .

0 013

5 5

300

Ω

•

2 6

300

0 606 0 022 0 628

kHz







 (

)

The worst-case power dissipated by the synchronous

MOSFET under normal operating conditions at elevated

ambient temperature and estimated 50°C junction

temperature rise is:

SYNC =

−

( ) ( )

Ω

(

)

5 5

1 8

5 5

1 25 0 013

1 0

. 99W

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