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LTC1709-8/LTC1709-9

estimated using the percentage of overshoot seen at this

pin. The bandwidth can also be estimated by examining

the rise time at the pin. The ITH external components

shown in the Figure 1 circuit will provide an adequate

starting point for most applications.

The ITH series RC-CC filter sets the dominant pole-zero

loop compensation. The values can be modified slightly

(from 0.2 to 5 times their suggested values) to optimize

transient response once the final PC layout is done and the

particular output capacitor type and value have been

determined. The output capacitors need to be decided

upon because the various types and values determine the

loop gain and phase. An output current pulse of 20% to

80% of full-load current having a rise time of <2

µs will

produce output voltage and ITH pin waveforms that will

give a sense of the overall loop stability without breaking

the feedback loop. The initial output voltage step resulting

from the step change in output current may not be within

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

response. The gain of the loop will be increased by

increasing 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 opera-

tion. But before you connect, be advised: you are plugging

into the supply from hell. The main battery 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 9 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 LT1709 has a maximum input

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

the MOSFET BVDSS.

APPLICATIO S I FOR ATIO

Figure 9. Automotive Application Protection

VIN

170989 F09

12V

50A IPK RATING

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

LTC1709

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