LT8685SRVPBF datasheet(17/24 Pages) AD

17 / 24 page

LT8685S

Rev. 0

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APPLICATIONS INFORMATION

The maximum duty cycle achievable at a given operating

frequency is calculated as:

DMAX = 1 – (tOFF(MIN) • fSW)

Combining these equations, the minimum VIN voltage

while regulating at full frequency is

VVINx(MIN)=

VOUTx

1– tOFF(MIN) • fSW

(

)

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

VVINx(MAX)=

VOUTx

tON(MIN) • fSW

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 the following equation:

ΔIL =

VOUT

fSW •L

⎛

⎝

⎜

⎞

⎠

⎟ 1–

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

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

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

nominal input voltage rating. A trade-off between compo-

nent size, efficiency and operating frequency can be seen

from this equation. 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.

The inductor value should be chosen to give a peak-to-

peak ripple current ΔIL of between 35% and 45% of the

rated channel output current at the nominal input voltage.

Note, the rated channel output current is 2.5A for chan-

nels 1 and 2, and 4A for channels 3 and 4. Channels 1

and 2 have a rating of 5A when combined, and channels

3 and 4 have a rating of 8A when combined. Rearranging

the equation above, select the inductor value according to:

VOUT

fSW • ΔIL

⎛

⎝

⎜

⎞

⎠

⎟ 1–

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

To avoid overheating and poor efficiency, an inductor

must be chosen with an RMS current rating that is greater

than the maximum expected output load of the applica-

tion. In addition, for best efficiency the inductor series

resistance should be as small as possible, and the core

material should be intended for the application switching

frequency.

The saturation current rating of the inductor must be

higher than the load plus half the ripple current. This

peak inductor current can be computed per the following

equation:

IL(PEAK) = IOUT(MAX) +

ΔIL

where IOUT(MAX) is the maximum output current for a

given application.

The optimum inductor for a given application may differ

from the one indicated by this design guide. Careful eval-

uation of the application circuit should be completed with

the chosen inductor to ensure adequate design margin.

Input Capacitor Selection

Buck, or step-down, converters draw current from the

input supply in pulses with very fast rise and fall times. An

input capacitor is required to reduce the resultant voltage

ripple at the input and minimize EMI. For this function, a

ceramic X7R or X5R bypass capacitor should be placed

between each buck regulator’s VIN pin and ground. To be

most effective, the input capacitor must have low imped-

ance at the switching frequency and an adequate ripple

current rating.

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