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LTC1929

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 over-

all supply performance.

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

alter its delivery of current quickly enough to prevent this

sudden step change in output voltage if the load switch

resistance is low and it is driven quickly. If the ratio of

CLOAD to COUT is greater than1:50, the switch rise time

should be controlled so that the load rise time is limited to

approximately 25 • CLOAD. Thus a 10µF capacitor would

require a 250

µs 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-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 8 is the most straightforward

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 LT1929 has a maximum input voltage of 36V,

most applications will be limited to 30V by the MOSFET

BVDSS.

APPLICATIO S I FOR ATIO

Figure 8. Automotive Application Protection

VIN

1929 F08

12V

50A IPK RATING

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

LTC1929

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