Data Sheet

LT83203/LT83205

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Rev. B

26 of 44

In addition, the saturation current rating (typically labeled ISAT) of the inductor must be higher than the load current

plus ½ of the inductor ripple current. See Equation 5.

IL(PEAK) = ILOAD(MAX) +

1

2

∆IL

(5)

where ΔIL is the inductor ripple current as calculated in Equation 7 and ILOAD(MAX) is the maximum output load for a

given application.

As a quick example, an application requiring 3A output should use an inductor with an RMS rating of greater than 3A

and an ISAT of greater than 4A. During long-duration overload or short-circuit conditions, the inductor RMS rating

requirement is greater to avoid overheating the inductor. To keep the efficiency high, the series resistance (DCR)

should be less than 32mΩ, and the core material should be intended for high-frequency applications.

The LT83203/LT83205 limits the peak switch current to protect the switches and the system from overload faults.

The top switch current limit (IPEAK-LIMIT) is 6A (LT83203)/10A (LT83205) at low duty cycles and decreases linearly to 4.5A

(LT83203)/7.5A (LT83205) at duty cycle = 80%.

The inductor value must be sufficient to supply the desired maximum output current (IOUT(MAX)), which is a function of

the top switch current limit (IPEAK-LIMIT) and the ripple current (see Equation 6).

IOUT(MAX) = IPEAK−LIMIT −

∆IL

2

(6)

The peak-to-peak ripple current in the inductor can be calculated using Equation 7.

∆IL =

VOUT

L × fSW

× (1 −

VOUT

VIN(MAX)

)

(7)

where fSW is the switching frequency of the LT83203/LT83205, and L is the value of the inductor. Therefore, the

maximum output current that the LT83203/LT83205 delivers depends on the switch current limit, the inductor value,

and the input and output voltages. The inductor value may have to be increased if the inductor ripple current does

not allow sufficient maximum output current (IOUT(MAX)) given the switching frequency and maximum input voltage

used in the desired application.

The optimum inductor for a given application may differ from the one indicated by this design guide. A larger-value

inductor provides a higher maximum load current and reduces the output voltage ripple. For applications requiring

smaller load currents, the value of the inductor may be lower, and the LT83203/LT83205 may operate with a higher

ripple current. This allows the use of a physically smaller inductor or one with a lower DCR, resulting in higher

efficiency. Be aware that low inductance may result in discontinuous operation in pulse-skipping mode, which

further reduces the maximum load current.

For more information about maximum output current and discontinuous operation, refer to the Application Note 44:

LT1074/LT1076 Design Manual.

For duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillation.

For more information, refer to the Application Note 19: LT1070 Design Manual. Equation 8 calculates that minimum

inductance.

LMIN =

VIN(2 × DC−1)

1.1 × fSW

(8)

where DC is the duty cycle ratio (VOUT/VIN), and fSW is the switching frequency.