LTC3621 datasheet(13/20 Pages) AD

13 / 20 page

LTC3621/LTC3621-2

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

Inductor Selection

Given the desired input and output voltages, the inductor

valueandoperatingfrequencydeterminetheripplecurrent:

∆ L =

VOUT

f •L

1–

VOUT

VIN(MAX)









Lower ripple current reduces power losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Highest efficiency operation is obtained at low

frequency with small ripple current. However, achieving

this requires a large inductor. There is a trade-off between

component size, efficiency and operating frequency.

A reasonable starting point is to choose a ripple current

that is about 40% of IOUT(MAX). To guarantee that ripple

current does not exceed a specified maximum, the induc-

tance should be chosen according to:

L =

VOUT

f •∆IL(MAX)

1–

VOUT

VIN(MAX)









Once the value for L is known, the type of inductor must

be selected. Actual core loss is independent of core size

for a fixed inductor value, but is very dependent on the

inductance selected. As the inductance or frequency in-

creases, core losses decrease. Unfortunately, increased

inductance requires more turns of wire and therefore

copper losses will increase. Copper losses also increase

as frequency increases.

Ferrite designs have very low core losses and are pre-

ferred at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard”, which means that

inductancecollapsesabruptlywhenthepeakdesigncurrent

is exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Different core materials and shapes will change the size/

currentandprice/currentrelationshipofaninductor.Toroid

or shielded pot cores in ferrite or permalloy materials are

small and don’t radiate much energy, but generally cost

more than powdered iron core inductors with similar

characteristics. The choice of which style inductor to use

mainly depends on the price versus size requirements

and any radiated field/EMI requirements. New designs

for surface mount inductors are available from Coilcraft,

Toko, Vishay, NEC/Tokin, TDK and Würth Electronik. Refer

to Table 1 for more details.

Checking Transient Response

Theregularloopresponsecanbecheckedbylookingatthe

load transient response. Switching regulators take several

cyclestorespondtoastepinloadcurrent.Whenaloadstep

occurs, VOUT immediately shifts by an amount equal to the

∆ILOAD • ESR, where ESR is the effective series resistance

of COUT. ∆ILOAD also begins to charge or discharge COUT

generating a feedback error signal used by the regulator to

return VOUT to its steady-state value. During this recovery

time, VOUT can be monitored for overshoot or ringing that

would indicate a stability problem.

The initial output voltage step may not be within the

bandwidth of the feedback loop, so the standard second

order overshoot/DC ratio cannot be used to determine

phase margin. In addition, a feedforward capacitor can

be added to improve the high frequency response, as

shown in Figure 1. Capacitor CFF provides phase lead by

creating a high frequency zero with R2, which improves

the phase margin.

The output voltage settling behavior is related to the sta-

bility of the closed-loop system and will demonstrate the

actual overall supply performance. LTpowerCAD™ and

LTSpice® can be used to check control loop and transient

performance.

Insomeapplications,amoreseveretransientcanbecaused

by switching in loads with large (>1µF) load capacitors.

The discharged load capacitors are effectively put in paral-

lel with COUT, causing a rapid drop in VOUT. No regulator

can deliver enough current to prevent this problem if the

switchconnectingtheloadhaslowresistanceandisdriven

quickly. The solution is to limit the turn-on speed of the

load switch driver. A Hot Swap™ controller is designed

specifically for this purpose and usually incorporates

current limiting, short-circuit protection and soft-starting.

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