LTC1439CG datasheet(22/32 Pages) LINER

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LTC1438/LTC1439

Design Example

As a design example, assume VIN = 12V(nominal), VIN =

22V(max), VOUT = 3.3V, IMAX = 3A and f = 250kHz, RSENSE

and COSC can immediately be calculated:

RSENSE = 100mV/3A = 0.033Ω

COSC = [1.37(10

4)/250] – 11

≈ 43pF

Refering to Figure 3, a 10

µH inductor falls within the

recommended range. To check the actual value of the

ripple current the following equation is used :

∆I

OUT









()( )

–

The highest value of the ripple current occurs at the

maximum input voltage:

∆I

kHz

L =









250

112

()

–

The power dissipation on the topside MOSFET can be

easily estimated. Using a Siliconix Si4412DY for example;

RDS(ON) = 0.042Ω, CRSS = 100pF. At maximum input

voltage with T(estimated) = 50

°C:

kHz

MAIN =

() +() °− °

()

[]()

() ( )(

)(

) =

0 005 50

0 042

2 5 22

100

250

122

185

Ω

The most stringent requirement for the synchronous

N-channel MOSFET is with VOUT = 0V (i.e. short circuit).

During a continuous short circuit, the worst-case dissipa-

tion rises to:

PSYNC = [ISC(AVG)]

2(1 +

δ)RDS(ON)

With the 0.033

Ω sense resistor ISC(AVG) = 4A will result,

increasing the Si4412DY dissipation to 950mW at a die

temperature of 105

°C.

CIN will require an RMS current rating of at least 1.5A at

temperature and COUT will require an ESR of 0.03Ω for low

output ripple. The output ripple in continuous mode will be

highest at the maximum input voltage. The output voltage

ripple due to ESR is approximately:

VORIPPLE = RESR(∆IL) = 0.03Ω(1.12A) = 34mVP-P

switch resistance is low and it is driven quickly. The only

solution is to limit the rise time of the switch drive so that

the load rise time is limited to approximately (25)(CLOAD).

Thus a 10

µF capacitor would require a 250µs rise time,

limiting the charging current to about 200mA.

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 operation.

But before you connect, be advised: you are plugging into

the supply from hell. The main battery line in an automo-

bile is the source of a number of nasty potential transients,

including load dump, reverse battery 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 12 is the most straightfor-

ward approach to protect a DC/DC converter from the

ravages of an automotive battery 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 LT1438/LT1439 has a maximum

input voltage of 36V, most applications will be limited to

30V by the MOSFET BVDSS.

APPLICATIONS INFORMATION

1438 F12

50A IPK RATING

LTC1438

LTC1439

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

VIN

12V

Figure 12. Automotive Application Protection

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