ADPL74101 datasheet(23/49 Pages) AD

23 / 49 page

Data Sheet

ADPL74101

analog.com

Rev. 0

23 of 49

APPLICATIONS INFORMATION

Use Cases

Figure 1 is a simplified ADPL74101 application circuit. External component selection is largely driven by the load

requirements and begins with selecting 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, inluding 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 the inductor value on ripple current and

low-current operation must also be considered. The inductor value directly affects 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:

∆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 higher inductance (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. 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, meaning 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.

ADPL74101 Datasheet(PDF) 23 Page - Analog Devices

ADPL74101 Datasheet(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

Datasheet Download

Link URL