LTC7899 datasheet(24/50 Pages) AD

24 / 50 page

Data Sheet

LTC7899

analog.com

Rev. 0

24 of 50

APPLICATIONS INFORMATION

The typical application on the first page is a basic LTC7899 application circuit. External component selection is

largely driven by the load requirement and begins with the selection of the inductor, current sense components,

operating frequency, and light load operating mode. The remaining power stage components, consisting of the input

and output capacitors, and power MOSFETs can then be chosen. Next, feedback resistors are selected to set the

desired output voltage. Then, the remaining external components are selected, such as for soft-start, biasing, and

loop compensation.

Inductor Value Calculation

The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use

of smaller inductor and capacitor values. So, why would anyone ever choose to operate at lower frequencies with

larger components? The answer is efficiency. A higher frequency generally results in lower efficiency because of

MOSFET switching and gate charge losses. In addition to this basic trade-off, the effect of inductor value on the ripple

current and low current operation must also be considered. The inductor value has a direct effect on the ripple

current.

The maximum average inductor current in continuous conduction is equal to the maximum average output current

multiplied by a factor (1 + VOUT/|VIN-|) or IOUT(MAX)= IL(MAX)•|VIN-|/(VOUT+ |VIN-|), where VOUT is referenced to ground. Be aware

that the maximum output current from this regulator decreases with decreasing |VIN-|. The choice of IL(MAX) therefore

depends on the maximum load current for a regulated VOUT at the minimum normal operating |VIN-|. If the load current

for a given |VIN-| is exceeded, VOUT will decrease until the IOUT(MAX) = IL(MAX)•|VIN-|/(VOUT + |VIN-|) equation is satisfied.

Accepting larger values of ∆IL allows the use of low inductances, but results in higher output voltage ripple and

greater core losses. A reasonable starting point for setting ripple current is ∆IL = 0.3•IL(MAX). The maximum ∆IL occurs

at |VIN-| = VOUT given by Equation 1, as follows:

∆IL =

|VIN−|

(f)(L)

(

VOUT

VOUT+|VIN−|

)

(1)

The inductor value also has secondary effects. The transition to burst mode operation begins when the average

inductor current required results in a peak current below 25% of the current limit determined by RSENSE. Lower

inductor values (higher ΔIL) cause this transition to occur at lower load currents, which can cause a dip in efficiency

in the upper range of low current operation. In burst mode operation, lower inductance values cause the burst

frequency to decrease.

Inductor Core Selection

When the value for L is known, select the type of inductor. High efficiency regulators generally cannot afford the core

loss found in low cost powdered iron cores, forcing the use of more expensive ferrite or molypermalloy cores. The

actual core loss is independent of core size for a fixed inductor value, but it is dependent on the inductance value

selected. As inductance increases, core losses decrease. However, because increased inductance requires more

turns of wire, copper losses increase.

Ferrite designs have low core loss and are preferred for high switching frequencies. Therefore, design goals can

concentrate on copper loss and preventing saturation. Ferrite core material saturates hard, which means that

inductance collapses abruptly when the peak design current is exceeded. This collapse results in an abrupt increase

in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate. The inductor

saturation design margin should account for the tolerance and temperature effects of the saturation current.

LTC7899 Datasheet(PDF) 24 Page - Analog Devices

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