AN3115 datasheet(13/28 Pages) STMICROELECTRONICS

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AN3115

Component selection guidelines

Doc ID 16830 Rev 3

13/28

2.1

DC-DC converter inductor (L1)

The LNBH23L operates with a standard 22 µH inductor for the entire range of supply

voltages and load current. The Inductor saturation current rating (where inductance is

approximately 70 % of zero current inductance) must be greater than the switch peak

current (ISAT > IPEAK) calculated at:

–

maximum load (IOUTmax);

–

minimum input voltage (VINmin);

–

maximum DC-DC output voltage (VUPmax = VOUTmax + 0.75 V typ.)

In this condition the switch peak current is calculated using the formula in Equation 1:

Equation 1

where:

–

Eff is the efficiency of the DC-DC converter (93 % typ. at highest load)

–

L is the inductance (22 µH typ.)

–

F is the PWM frequency (220 kHz typ.)

Example:

Application conditions:

VOUTmax = 19.2 V (supposing EN=VSEL=1, LLC=0)

VINmin = 11 V

VUPmax = VOUTmax + VDROP = 19.2 V + 0.75 V = 19.95 V

IOUTmax = 500 mA

Eff = 90 % (worst-case in these conditions)

Based on Equation 1 and the preceding application conditions, IPEAK is:

Then, in this example, an inductor with saturation current > 1.52 A should be recommended.

Several inductors suitable for the LNBH23L are listed in Table 6, although there are many

other manufacturers and devices that can be used. Consult each manufacturer for more

detailed information and for their entire selection of related parts, since many different

shapes and sizes are available. Ferrite core inductors should be used to obtain the best

efficiency. Choose an inductor that can handle at least the IPEAK current without saturating,

and ensure that the inductor has a low DCR (copper wire resistance) to minimize power

losses and, consequently, to maximize the total efficiency.

⎟

⎠

⎞

⎜

⎝

⎛ −

ma x

mi n

ma x

ma x *

VU P

VI N

Ef f

IO U T

VU P

Ip e a k

220

Ipeak

⎟

⎠

⎞

⎜

⎝

⎛ −

∗

−

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