LT8697 datasheet(22/40 Pages) AD

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LT8698S/LT8698S-1

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

where VIN(MAX) is the maximum input voltage without

skipped cycles, VOUT/ISP is the output voltage at OUT/ISP,

and tON(MIN) is the minimum top switch on-time of about

30ns. This equation shows that a slower switching frequency

is necessary to accommodate a high VIN/VOUT/ISP ratio.

For transient operation, VIN may go as high as the abso-

lute maximum rating of 42V regardless of the RT value;

however the LT8698S will reduce switching frequency

as necessary to maintain control of inductor current to

assure safe operation.

The LT8698S is capable of a maximum duty cycle of

greater than 99%, and the VIN to VBUS/ISN dropout is lim-

ited by the RDS(ON) of the top switch. Operating in this

region results in lower switching frequency.

The highest switching frequency (fSW(MAX)) for a given

application before min off-time limited can be calculated

using Equation 6.

fSW(MAX) =

tOFF(MIN)

–

VOUT/ISP

tOFF(MIN)•VIN(MIN)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟•1000

(6)

where VIN(MIN) is the minimum input voltage without

skipped cycles, VOUT/ISP is the output voltage at OUT/ISP,

and tOFF(MIN) is the minimum switch off-time of 40ns.

Note that higher switching frequency will increase the

minimum input voltage for full frequency operation.

Inductor Selection and Maximum Output Current

The LT8698S is designed to minimize solution size by

allowing the inductor to be chosen based on the output

load requirements of the application. During overload

or short-circuit conditions the LT8698S safely tolerates

operation with a saturated inductor through the use of a

high speed peak-current mode architecture.

Based on the desired switching frequency, pick an induc-

tor for the LT8698S according to Table 3.

Table 3 lists nominal values for inductors. Inductors are

typically de-rated versus current, versus temperature and

have a tolerance spec. This table assumes the inductance

in the application will not be 40% less than the nominal

value at worst case operating conditions and tolerance.

For robust operation, use an inductor with value equal to

or greater than the value on Table 3.

Table 3. Inductor Value by Switching Frequency

SWITCHING FREQUENCY

(MHz)

MINIMUM NOMINAL INDUCTOR VALUE

(µH)

2.4 – 3.0

1.5

1.9 – 2.4

2.2

1.3 – 1.9

3.3

0.89 – 1.3

4.7

0.61 – 0.89

6.8

0.51 – 0.61

8.2

0.30 – 0.51

Table 4 lists inductor vendors and associated inductor

product series names that are recommended for use with

the LT8698S.

Table 4. Inductor Manufacturers

VENDOR

SERIES

WEBSITE

TDK

SLF, VLC, VLF

www.tdk.com

Sumida

CRH, CDR, CDMC

www.sumida.com

Coilcraft

XAL, XFL, MSS

www.coilcraft.com

NIC

NPIM, NPIS

www.niccomp.com

Würth

TPC, SPC, PD, PDF, PD3

www.we-online.com

Input Capacitor

Bypass the input of the LT8698S circuit with a ceramic

capacitor with the appropriate temperature and voltage

rating, placed as close as possible to the VIN and PGND

pins. Y5V types have poor performance over temperature

and applied voltage, and should not be used. A 2.2μF

to 4.7μF ceramic capacitor is adequate to bypass the

LT8698S 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 supply in

pulses with very fast rise and fall times. The input capaci-

tor is required to reduce the resulting voltage ripple at the

LT8698S and to force this very high frequency switching

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