LTC3407AB datasheet(13/22 Pages) LINER

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LTC3565

3565fb

drop of the ﬁrst cycle. Thus, a good place to start is with

the output capacitor value of approximately:

COUT ≈ 2.5

ΔIOUT

fO •VDROOP

More capacitance may be required depending on the duty

cycle and load step requirements.

In most applications, the input capacitor is merely required

to supply high frequency bypassing, since the impedance

to the supply is very low. A 10μF ceramic capacitor is

usually enough for these conditions.

Setting the Output Voltage

The LTC3565 develops a 0.6V reference voltage between

the feedback pin, VFB, and the signal ground as shown in

Figure 4. The output voltage is set by a resistive divider

according to the following formula:

VOUT ≈ 0.6V 1+

⎛

⎝

⎜

⎞

⎠

⎟

Keeping the current small (<5μA) in these resistors maxi-

mizes efﬁciency, but making them too small may allow

stray capacitance to cause noise problems and reduce the

phase margin of the error amp loop.

To improve the frequency response, a feed-forward capaci-

tor CF may also be used. Great care should be taken to

route the VFB line away from noise sources, such as the

inductor or the SW line.

Shutdown and Soft-Start

Pulling the RUN pin high allows an internal soft-start circuit

to slowly ramp the output voltage up until regulation.

Soft-start prevents surge currents from VIN by gradually

ramping the output voltage up during start-up. The output

will ramp from zero to full scale over a time period of ap-

proximately 0.9ms. This prevents the LTC3565 from having

to quickly charge the output capacitor and thus supplying

an excessive amount of instantaneous current.

The LTC3565 can start into a back-biased output in force

continuous operation. When the output is pre-biased at

either a higher or lower value than the regulated output

voltage, the LTC3565 will sink or source current as needed

to bring the output back into regulation. However, during

soft-start the regulator will always start in pulse skip

mode ignoring the mode selected with the SYNC/MODE

pin. This prevents the output from discharging to below

the regulation point when soft-starting.

Mode Selection and Frequency Synchronization

The SYNC/MODE pin is a multipurpose pin which provides

mode selection and frequency synchronization. Connect-

ing this pin to VIN enables Burst Mode operation, which

provides the best low current efﬁciency at the cost of a

higher output voltage ripple. When this pin is connected to

ground, pulse skipping operation is selected which provides

the lowest output voltage and current ripple at the cost

of low current efﬁciency. Applying a voltage that is half

the value of the input voltage results in forced continuous

mode, which creates a ﬁxed output ripple and is capable of

sinking up to 0.4A. Since the switching noise is constant

in this mode, it is also the easiest to ﬁlter out.

The LTC3565 can also be synchronized to an external

clock signal by the SYNC/MODE pin. The internal oscilla-

tor frequency should be set to ±20% of the external clock

frequency to ensure adequate slope compensation, since

slope compensation is derived from the internal oscillator.

During synchronization, the mode is set to pulse skipping

and the top switch turn on is synchronized to the falling

edge of the external clock.

APPLICATIONS INFORMATION

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