LT3501EFE datasheet(13/28 Pages) LINER

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LT3501

3501fb

and its saturation current should be about 30% higher. To

keep efﬁciency high, the series resistance (DCR) should

be less than 0.05

Ω.

For applications with a duty cycle of about 50%, the induc-

tor value should be chosen to obtain an inductor ripple

current less than 40% of peak switch current.

Of course, such a simple design guide will not always result

in the optimum inductor for your application. A larger value

provides a slightly higher maximum load current, and will

reduce the output voltage ripple. If your load is lower than

2.5A, then you can decrease the value of the inductor and

operate with higher ripple current. This allows you to use

a physically smaller inductor, or one with a lower DCR

resulting in higher efﬁciency.

The current in the inductor is a triangle wave with an

average value equal to the load current. The peak switch

current is equal to the output current plus half the peak-to-

peak inductor ripple current. The LT3501 limits its switch

current in order to protect itself and the system from

overload faults. Therefore, the maximum output current

that the LT3501 will deliver depends on the current limit,

the inductor value, switch frequency, and the input and

output voltages. The inductor is chosen based on output

current requirements, output voltage ripple requirements,

size restrictions and efﬁciency goals.

When the switch is off, the inductor sees the output volt-

age plus the catch diode drop. This gives the peak-to-peak

ripple current in the inductor:

∆I

DC V

OUT

= ()

()

1–

•

where f is the switching frequency of the LT3501 and L

is the value of the inductor. The peak inductor and switch

current is

SW PK

LPK

OUT

() ==

+ ∆

To maintain output regulation, this peak current must be

less than the LT3501’s switch current limit ILIM. ILIM is

3.5A over the entire duty cycle range. The maximum output

current is a function of the chosen inductor value:

OUT MAX

LIM

()

–. –

∆∆

If the inductor value is chosen so that the ripple current

is small, then the available output current will be near the

switch current limit.

One approach to choosing the inductor is to start with the

simple rule given above, look at the available inductors

and choose one to meet cost or space goals. Then use

these equations to check that the LT3501 will be able to

deliver the required output current. Note again that these

equations assume that the inductor current is continuous.

Discontinuous operation occurs when IOUT is less than

IL/2 as calculated above.

Figure 4 illustrates the inductance value needed for a 3.3V

output with a maximum load capability of 3A. Referring

to Figure 4, an inductor value between 3.3µH and 4.7µH

will be sufﬁcient for a 15V input voltage and a switch

frequency of 750kHz. There are several graphs in the

Typical Performance Characteristics section of this data

sheet that show inductor selection as a function of input

voltage and switch frequency for several popular output

voltages and output ripple currents. Also, low inductance

Figure 4. Inductor Values for 3A Maximum Load Current

vs Frequency and Input Voltage

INPUT VOLTAGE (V)

250

500

1000

1250

1500

3501 F04

750

913

L = 2.2

µH

L = 3.3

µH

L = 4.7

µH

L = 6.8

µH

VOUT = 3.3V

IRIPPLE = 1A

APPLICATIONS INFORMATION

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