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LTC1436A

LTC1436-PLL-A/LTC1437A

APPLICATIONS INFORMATION

A second, more severe transient is caused by switching in

loads with large (>1

µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

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

deliver enough current to prevent this problem if the load

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 14 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 LTC1436A/LTC1437A have a maxi-

mum input voltage of 36V, most applications will be

limited to 30V by the MOSFET BVDSS.

Design Example

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

22V (max), VOUT = 1.6V, IMAX = 3A and f = 250kHz, RSENSE

and COSC can immediately be calculated:

CpF

SENSE

OSC













100

0 033

137 10

250

11 43

)

–

Ω

Refering to Figure 3, a 4.7

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

4 7

The lowest duty cycle also occurs at maximum input

voltage. The on-time during this condition should be

checked to make sure it doesn’t violate the LTC1436A/

LTC1437A’s minimum on-time and cause cycle skipping

to occur. The required on-time at VIN(MAX) is:

kHz

ON MIN

OUT

IN MAX

()

= ()() = ()(

)

250

291

The

∆IL was previously calculated to be 1.3A, which is

43% of IMAX. From Figure 12, the LTC1436A/LTC1437A’s

minimum on-time at 43% ripple is about 235ns. There-

fore, the minimum on-time is sufficient and no cycle

skipping will occur.

Figure 14. Automotive Application Protection

1436 F14

50A IPK RATING

LTC1436A

LTC1437A

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

VIN

12V

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