LT8686SJVPBF datasheet(17/26 Pages) AD

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LT8686S

Rev. 0

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The value of R2 is best selected first as this establishes

how much current is in the resistor divider network cal-

culated as based on I = 0.8V/R2. The current should be

chosen such that it is not influenced by anticipated leak-

age or noise. R1 can then be calculated with Equation 4.

=R2•

VOUTx

0.8

–1

⎛

⎝⎜

⎞

⎠⎟

(4)

CFF can optionally be used to improve the transient

response and stability of the internally compensated feed-

back loops. The values shown in the Typical Applications

section will provide a good starting point for selecting CFF,

though careful evaluation of regulator stability should be

made to ensure adequate design margin.

For the LT8686S’s 42V input capable channels, the max-

imum allowed programmed output voltage is 14V.

When the 8V input capable channels are combined (chan-

nels 3 and 4), the maximum allowed programmed output

voltage is 4V.

OPERATING FREQUENCY AND INPUT VOLTAGE RANGE

Each buck regulator’s, minimum on-time, tON(MIN), and

minimum off-time, tOFF(MIN), impose limitations on the

achievable duty cycle range and operating frequency.

For buck regulators, the duty cycle is calculated with

Equation 5.

VOUTx

VINx

(5)

Further, the minimum duty cycle achievable at a given

operating frequency is calculated with Equation 6.

DMIN = tON(MIN) • fSW

(6)

where fSW is the programmed operating frequency.

The maximum duty cycle achievable at a given operating

frequency is calculated with Equation 7.

DMAX =1–(tOFF(MIN) • fSW)

(7)

Combining Equation 6 and Equation 7, the minimum

VIN voltage while regulating at full frequency is given by

Equation 8.

VINx(MIN) =

VOUTx

1–(tOFF(MIN) • fSW)

(8)

Below VVINx(MIN), the buck regulator will enter dropout,

and the top switch will stay on longer than a clock cycle.

While operating in dropout, the buck regulator’s output

voltage will be below the programmed value.

The maximum VIN voltage while regulating at full fre-

quency is given by Equation 9.

VINx(MAX) =

VOUTx

tON(MIN) • fSW

(9)

If the VVINX(MAX) given above is exceeded during regu-

lation, the buck regulator will skip switch-on cycles to

maintain regulation.

INDUCTOR SELECTION

For a given input and output voltage, the inductor value

and operating frequency determine the inductor ripple

current. More specifically, the inductor ripple current

decreases with higher inductor value or higher operating

frequency according to Equation 10.

IL =

VOUT

fSW •L

• 1–

VOUT

VIN

(10)

where ΔIL = inductor ripple current (A), fSW = switching

frequency (Hz), L = inductor value (H), and VIN is the nom-

inal input voltage rating. A trade-off between component

size, efficiency and operating frequency can be seen from

Equation 10. Accepting larger values of ΔIL allows the use

of lower value inductors but results in greater core loss

in the inductor, greater ESR loss in the output capacitor,

and larger output ripple.

APPLICATIONS INFORMATION

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