LTC1735CS datasheet(25/32 Pages) LINER

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LTC1735

1735fc

APPLICATIO S I FOR ATIO

VIN = 12V

VOUT = 1.5V

1.5V

100mV/DIV

15A

10A/DIV

OUTPUT

VOLTAGE

LOAD

CURRENT

µs/DIV

1735 F09

Figure 9. Normal Transient Response (Without R1, R4)

VIN = 12V

VOUT = 1.5V

1.582V

1.5V

1.418V

100mV/DIV

15A

10A/DIV

µs/DIV

1735 F10

Figure 10. Transient Response with Active Voltage Positioning

OUTPUT

VOLTAGE

LOAD

CURRENT

Figure 11. Plugging into the Cigarette Lighter

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 power line in an

automobile 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 11 is the most straight

forward 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 LTC1735 has a maximum input

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

the MOSFET BVDSS.

VIN

50A IPK RATING

1735 F11

LTC1735

12V

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

Design Example

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

VIN = 22V(max), VOUT = 1.8V, IMAX = 5A and f = 300kHz.

RSENSE and COSC can immediately be calculated:

RSENSE = 50mV/5A = 0.01Ω

COSC = 1.61(107)/(300kHz) – 11pF = 43pF

Assume a 3.3

µH inductor and 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 =









300

3 3

)

–

The maximum ripple current is 33% of maximum output

current, which is about right.

FIGURE 8 CIRCUIT

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