LTC7897 datasheet(24/53 Pages) AD

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Data Sheet

LTC7897

analog.com

Rev. A

24 of 53

APPLICATIONS INFORMATION

Figure 1 is a basic LTC7897 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 MOSFET

switching and gate charge losses. In addition to this trade-off, the effect of the 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 (IL(MAX)) is equal to the maximum output current. The peak current is equal

to the average inductor current plus half of the inductor ripple current (ΔIL), which decreases with higher inductance

(L) or higher frequency (f) and increases with higher VIN, as shown in Equation 1, as follows:

∆IL =

(f)(L)

VOUT (1 −

VOUT

VIN

)

(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 occurs

at the maximum input voltage.

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 the 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.

Current Sense Selection

The LTC7897 can be configured to use either inductor DC resistance (DCR) sensing or low value resistor sensing. The

choice between the two current sensing schemes is a design trade-off between cost, power consumption, and

accuracy. DCR sensing is popular because it saves expensive current sensing resistors and is more power efficient,

particularly in high current applications. However, current sensing resistors provide the most accurate current limits

for the controller. The selection of other external components is driven by the load requirement and begins with the

selection of RSENSE (if RSENSE is used) and the inductor value.

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