LTC1709 datasheet(22/28 Pages) LINER

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LTC1709

the bandwidth of the feedback loop, so this signal cannot

be used to determine phase margin. This is why it is better

to look at the Ith pin signal which is in the feedback loop

and is the filtered and compensated control loop re-

sponse. The gain of the loop will be increased by increas-

ing RC and the bandwidth of the loop will be increased by

decreasing CC. If RC is increased by the same factor that

CC is decreased, the zero frequency will be kept the same,

thereby keeping the phase the same in the most critical

frequency range of the feedback loop. The output voltage

settling behavior is related to the stability of the closed-

loop system and will demonstrate the actual overall supply

performance.

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-bat-

tery.

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 9 is the most straightforward

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 LT1709 has a maximum input voltage of 36V,

most applications will be limited to 30V by the MOSFET

BVDSS.

Design Example

As a design example, assume VIN=5V(nominal),VIN = 5.5V

(max), VOUT = 1.8V, IMAX = 20A, TA = 70°C and f = 300kHz.

The inductance value is chosen first based on a 30% ripple

current assumption. The highest value of ripple current

occurs at the maximum input voltage. Tie the PLLFLTR pin

to the INTVCC pin for 300kHz operation. The minimum

inductance for 30% ripple current is:

kHz

OUT

≥

∆

()

−









≥ ()( )( ) −









≥µ

300

135

A 1.5

µH inductor will produce 27% ripple current. The

peak inductor current will be the maximum DC value plus

one half the ripple current, or 11.4A. The minimum on-

time occurs at maximum VIN:

kHz

ON MIN

OUT

() == ()(

)

=µ

5 5

300

The RSENSE resistors value can be calculated by using the

maximum current sense voltage specification with some

accomodation for tolerances:

SENSE =≈

Ω

11 4

0 004

APPLICATIO S I FOR ATIO

Figure 9. Automotive Application Protection

VIN

1709 F09

12V

50A IPK RATING

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

LTC1709

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