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LTC1775

Design Example

As a design example, take a supply with the following

specifications: VIN = 6V to 22V (15V nominal), VOUT = 5V,

IO(MAX) = 10A. The required RDS(ON) can immediately be

estimated:

DS ON

()

()( . )

Ω

240

1 3

0 018

A 0.019

Ω Siliconix SUD50N03-10 MOSFET (θJA =

°C/W) is close to this value.

For 40% ripple current at maximum VIN the inductor

should be:

kHz

≥









=µ

150

0 4 10

(

)( . )(

)

–.

Choosing a Magnetics 55380-A2 core with 8 turns of 15

gauge wire yields a 6

µH inductor. The resulting maximum

ripple current will be:

∆I

kHz

LMAX

()

()(

)

–.









150

Next, check that the minimum value of the current limit is

acceptable. Assume a junction temperature about 20

°C

above the 70

°C ambient with ρ90°C = 1.3.

LIMIT ≥

Ω









300

0019

)( . )

–.

Now double-check the assumed TJ:

kHz

TOP =

() ( )

Ω

()+

( )() ()( )(

)

1 3 0 019

1 7 22

170

150

056

0 21

077

TJ = 70°C + (0.77W)(30°C/W) = 93°C

Since

ρ(93°C) ≅ ρ(90°C), the solution is self-consistent.

Automotive Considerations: Plugging into the

Cigarette Lighter

As battery-powered devices go mobile, there is a natural

interest in plugging into the cigarette lighter in order to

conserve or even recharge battery packs during opera-

tion. But before you connect, be advised: you are plug-

ging into the supply from hell. The main power line in an

automobile is the source of a number of nasty potential

transients, including load dump, reverse and double

battery.

Load dump is the result of a loose battery cable. When the

cable breaks connection, the field collapse in the alternator

can cause a positive spike as high as 60V which takes

several hundred milliseconds to decay. Reverse battery is

just what it says, while double battery is a consequence of

tow truck operators finding that a 24V jump start cranks

cold engines faster than 12V.

The network shown in Figure 10 is the most straightfor-

ward approach to protect a DC/DC converter from the

ravages of an automotive power line. The series diode

prevents current from flowing during reverse battery,

while the transient suppressor clamps the input voltage

during load dump. Note that the transient suppressor

should not conduct during double-battery operation, but

must still clamp the input voltage below breakdown of the

converter. Although the LTC1775 has a maximum input

voltage of 36V, most applications will be limited to 30V by

the MOSFET V(BR)DSS.

Figure 10. Automotive Application Protection

VIN

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

12V

LTC1775

50A IPK

RATING

1775 F10

PGND

APPLICATIO S I FOR ATIO

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