LTC3370 datasheet(13/26 Pages) AD

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LTC3307A

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

Low Supply Operation

The LTC3307A is designed to operate down to an input

supply voltage of 2.25V. One important consideration at

low input supply voltages is that the RDS(ON) of the inter-

nal power switches increases. Calculate the worst case

LTC3307A power dissipation and die junction temperature

at the lowest input voltages.

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 LTC3307A 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 low. To

keep the inductor current in control, a secondary limit is

imposed on the valley of the inductor current. If the induc-

tor current measured through the bottom power switch

remains greater than IVALLEYMAX at the end of the cycle,

the top power switch will be held off. Subsequent switch-

ing cycles will be 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 will quickly

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

ramp until regulation is reached.

Refer to the Block Diagram for reference.

Output Voltage and Feedback Network

The output voltage is programmed by a resistor divider

between the output and the FB pin. Choose the resistor

values according to:

RA = RB

VOUT

500mV

– 1

⎛

⎝

⎜

⎞

⎠

⎟

(1)

as shown in Figure 1:

Figure 1.

COUT

CFF

VOUT

3307A F01

BUCK

SWITCHING

REGULATOR

(OPTIONAL)

Feedback Resistor Network

Reference designators refer to the Block Diagram. Typical

values for RB range from 40kΩ to 400kΩ. 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 22pF 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

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