LTC1629C datasheet(21/28 Pages) LINER

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LTC1629/LTC1629-PG

Checking Transient Response

The regulator loop response can be checked by looking at

the load transient response. Switching regulators take

several cycles to respond to a step in DC (resistive) load

current. When a load step occurs, VOUT shifts by an

amount equal to

∆ILOAD(ESR), where ESR is the effective

series resistance of COUT(∆ILOAD) also begins to charge or

discharge COUT generating the feedback error signal that

forces the regulator to adapt to the current change and

return VOUT to its steady-state value. During this recovery

time VOUT can be monitored for excessive overshoot or

ringing, which would indicate a stability problem.

The

availability of the ITH pin not only allows optimization of

control loop behavior but also provides a DC coupled and

AC filtered closed loop response test point. The DC step,

rise time, and settling 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 0.2 to 5 times their suggested values) to maximize

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 feedback factor 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 ITHpinwaveforms

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 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 (>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.

APPLICATIO S I FOR ATIO

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