LTC3370 datasheet(12/20 Pages) AD

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LTC3302

Rev. 0

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APPLICATIONS INFORMATION

Output Short-Circuit Protection and Recovery

The peak inductor current level, at which the current com-

parator shuts off the top power switch, is controlled by the

internal VC voltage. When the output current increases,

the error amplifier raises VC until the average inductor

current matches the load current. The LTC3302 clamps

the maximum VC voltage, thereby limiting the peak induc-

tor current.

When the output is shorted to ground, the inductor cur-

rent decays very slowly when the bottom power switch

is on because the voltage across the inductor is small.

To keep the inductor current under control, a secondary

limit is imposed on the valley of the inductor current.

If the inductor current measured through the bottom

power switch remains greater than IVALLEYMAX at the end

of the cycle, the top power switch is held off. Subsequent

switching cycles are skipped until the inductor current

falls below IVALLEYMAX.

Recovery from an output short-circuit may involve a soft-

start cycle if VFB falls more than approximately 100mV

below regulation. During such a recovery, VFB quickly

charges up by that ~100mV and then follow the soft-start

ramp until regulation is reached.

Refer to the Block Diagram for reference.

FB Resistor Network (LTC3302 Adjustable Version)

The output voltage for the LTC3302 adjustable version

is programmed by a resistor divider between the out-

put and the FB pin. Choose the resistor values according

to Equation 1.

RA = RB

VOUT

500mV

– 1

⎛

⎝⎜

⎞

⎠⎟

(1)

as shown in Figure 1:

Figure 1.

COUT

CFF

3302 F01

LTC3302

(OPTIONAL)

VOUT

Feedback Resistor Network for the

LTC3302 Adjustable Version

The adjustable version can have an output voltage from

0.5V to VIN.

Reference designators refer to the Block Diagram. Typical

values for RB range from 10k to 400k. 0.1% resistors

are recommended to maintain output voltage accuracy.

The buck regulator transient response may improve with

an optional phase lead capacitor CFF that helps cancel

the pole created by the feedback resistors and the input

capacitance of the FB pin. Experimentation with capaci-

tor values between 2pF and 50pF may improve transient

response. The values used in the typical application cir-

cuits are a good starting point.

Operating Frequency Selection and Trade-Offs

Selection of the operating frequency is a trade-off between

efficiency, component size, transient response, and input

voltage range.

The advantage of high frequency operation is that smaller

inductor and capacitor values may be used. Higher

switching frequencies allow for higher control loop

bandwidth and, therefore, faster transient response. The

disadvantages of higher switching frequencies are lower

efficiency, because of increased switching losses, and a

smaller input voltage range, because of minimum switch

on-time limitations.

OPERATION

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