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LTC1929

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 and f = 310kHz,

RSENSE1 and RSENSE2 can immediately be calculated:

RSENSE1 = RSENSE2 = 50mV/10A = 0.005Ω

If L1 = L2 = 2

µH the actual value of the ripple current for

each channel, the following equation is used:

∆I

OUT

=−









The highest value of the ripple current occurs at the

maximum input voltage:

∆I

kHz

L =

() µ

()

−









≈

310

195

The ripple current for each inductor is 20% at maximum

output current which is conservative.

Next verify the minimum on-time of 200ns is not violated.

The minimum on-time occurs at maximum VIN:

kHz

ON MIN

OUT

IN MAX

()













≈µ

5 5 310

Since the output voltage is below 2.4V the output resistive

divider will need to be sized to not only set the output

voltage but also to absorb the sense pin current for both

channels.

MIN

OUT

22 4

() =

−









−









Choosing 1% resistors; R1=13.2k and R2=16.5k 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°C at an elevated ambient

temperature:

kHz

MAIN =

() +() °− °

()

[]

() ( )( )

()=

0 005 110

0 013

1 7 5 5

300

310

0 65

Ω

The worst-case power disipated by the synchronous

MOSFET under normal operating conditions at elevated

ambient temperature and estimated 50

°C junction tem-

perature rise is:

SYNC =

−

() ( )

Ω

()

1 48 0 013

129

A short-circuit to ground will result in a folded back current

of:

SC =

Ω

()













0 005

200

5 5

528

The worst-case power disipated by the synchronous

MOSFET under short-circuit conditions at elevated ambi-

ent temperature and estimated 50

°C junction temperature

rise is:

SYNC =

−

() ( )

Ω

()

5 28

1 48 0 013

360

which is much less than normal, full-load conditions.

Incidentally, since the load no longer dissipates power in

the shorted condition, total system power dissipation is

decreased by over 99%.

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