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LTC3728L-1

3728l1fc

APPLICATIONS INFORMATION

at this test point truly reﬂects the closed loop response.

Assuming a predominantly second order system, phase

margin and/or damping factor can be 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 0.5 to 2 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 selected

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 1μs to 10μs will

produce output voltage and ITH pin waveforms that will

give a sense of the overall loop stability without break-

ing the feedback loop. Placing a power MOSFET directly

across the output capacitor and driving the gate with an

appropriate signal generator is a practical way to produce

a realistic load step condition. 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 in-

creased 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 shift 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.

A second, more severe transient is caused by switching

in loads with large (>1μF) 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 than 1:50, 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 250μs 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 opera-

tion. But before you connect, be advised: you are plug-

ging 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 alterna-

tor 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 LTC3728L-1 has a maximum input voltage

of 30V, most applications will also be limited to 30V by

the MOSFET BVDSS.

Figure 9. Automotive Application Protection

VIN

3728L1 F09

LTC3728L-1

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

50A IPK RATING

12V

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