LT8650SPJVPBF datasheet(19/34 Pages) AD

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LT8650SP

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

Each channel has a secondary bottom switch current limit.

Afterthetopswitchhasturnedoff,thebottomswitchcarries

the inductor current. If for any reason the inductor current

is too high, the bottom switch will remain on, delaying the

top switch turning on until the inductor current returns to

a safe level. This level is specified as the Bottom NMOS

Current Limit, and is independent of duty cycle. Maximum

output current in the application circuit is limited to this

valley current plus one half of the inductor ripple current.

In most cases current limit is enforced by the top switch.

Thebottomswitchlimitcontrolstheinductorcurrentwhen

the minimum on-time condition is violated (high input

voltage, high frequency or saturated inductor).

The bottom switch current limit is designed to be equal to

thepeakcurrentlimittoavoidanycontributiontomaximum

rated current of the LT8650SP.

For more information about maximum output current and

discontinuous operation, see the Analog Devices, Inc.

Application Note 44.

Table 2. Inductor Manufacturers

VENDOR

URL

Coilcraft

www.coilcraft.com

Sumida

www.sumida.com

Wurth Elektronik

www.we-online.com

Vishay

www.vishay.com

Input Capacitor

Bypass the input of the LT8650SP circuit with a ceramic

capacitor of X7R or X5R type placed as close as possible

totheVINandGNDpins.Y5Vtypeshavepoorperformance

over temperature and applied voltage, and should not be

used. A 4.7μF to 10μF ceramic capacitor is adequate to

bypass the LT8650SP and will easily handle the ripple

current. Note that larger input capacitance is required

when a lower switching frequency is used. If the input

power source has high impedance, or there is significant

inductance due to long wires or cables, additional bulk

capacitance may be necessary. This can be provided with

a low performance electrolytic capacitor.

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

at the LT8650SP and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

Typically a 0.1µF capacitor in a small 0402 case size is

placed as close as possible to the LT8650SP and a larger

bulk ceramic is added for more capacitance (see the PCB

Layoutsection).Asecondprecautionregardingtheceramic

input capacitor concerns the maximum input voltage rat-

ing of the LT8650SP. A ceramic input capacitor combined

with trace or cable inductance forms a high quality (under

damped) tank circuit. If the LT8650SP circuit is plugged

into a live supply, the input voltage can ring to twice its

nominal value, possibly exceeding the LT8650SP’s volt-

age rating. This situation is easily avoided (see the Analog

Devices, Inc. Application Note 88).

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by

the LT8650SP to produce the DC output. In this role it

determines the output ripple, thus low impedance at the

switching frequency is important. The second function

is to store energy in order to satisfy transient loads and

stabilize the LT8650SP’s control loop. Ceramic capaci-

tors have very low equivalent series resistance (ESR) and

provide the best ripple performance. For good starting

values, see the Typical Applications section.

Use X5R or X7R types. This choice will provide low output

rippleandgoodtransientresponse.Transientperformance

can be improved with a higher value output capacitor and

the addition of a feed forward capacitor placed between

VOUT and FB. Increasing the output capacitance will also

decrease the output voltage ripple. A lower value of output

capacitor can be used to save space and cost but transient

performancewillsufferandmaycauseloopinstability.See

the Typical Applications in this data sheet for suggested

capacitor values.

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