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LTC3732

3732f

APPLICATIO S I FOR ATIO

loop feedback factor gain and phase. An output current

pulse of 20% to 80% of full load current having a rise time

of <2

µswillproduceoutputvoltageandITHpinwaveforms

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 CLOAD is greater

than 2% of COUT , the switch rise time should be controlled

so that the load rise time is limited to approximately

1000 • RSENSE • CLOAD. Thus a 250µF capacitor and a 2mΩ

RSENSE resistor would require a 500µs rise time, limiting

the charging current to about 1A.

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 10 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 IC has a maximum input voltage

of 32V on the SW pins, most applications will be limited to

30V by the MOSFET BVDSS.

LTC3732

VCC

VBAT

12V

3732 F10

Figure 10. Automotive Application Protection

Design Example

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

20V(max), VOUT = 1.3V, IMAX = 45A and f = 400kHz. The

inductance value is chosen first based upon a 30% ripple

current assumption. The highest value of ripple current in

each output stage occurs at the maximum input voltage.

kHz

OUT

∆

()

−









= ()( )( ) −









≥

400

068

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