LTC7892 datasheet(24/50 Pages) AD

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Data Sheet

LTC7892

analog.com

Rev. 0

24 of 50

APPLICATIONS INFORMATION

Figure 50 is a basic LTC7892 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 FETs 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. A higher frequency generally results in lower efficiency because of FET

switching and gate charge losses. In addition to this trade-off, the effect of inductor value on 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 (IL(MAX)) in continuous conduction mode is equal to the maximum average

output current (IOUT(MAX)) multiplied by a factor of VOUT/VIN, or IL(MAX) = IOUT(MAX) • VOUT/VIN. Be aware that the maximum

output current 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. 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 IL(MAX) = IOUT(MAX) • VOUT/VIN equation is satisfied.

The inductor ripple current (ΔIL) for a boost converter is given by Equation 1:

∆IL =

f • L

VIN (1 −

VIN

VOUT

)

(1)

Accepting larger values of ΔIL 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 ΔIL = 0.3 • IL(MAX). The maximum ΔIL for a

boost converter occurs at VIN = 1/2 VOUT.

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) will cause this to occur at lower load currents, which can cause a dip in efficiency in the

upper range of low-current operation. In the Burst Mode operation, lower inductance values will cause the burst

frequency to decrease.

Inductor Core Selection

Once the value for L is known, the type of inductor must be selected. 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. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent

on the inductance value selected. As inductance increases, core losses decrease. Unfortunately, increased

inductance requires more turns of wire, and therefore, copper losses will increase.

Ferrite designs have a very 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.

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