LTC3621/LTC3621-2
11
3621fc
For more information www.linear.com/LTC3621
applicaTions inForMaTion
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple current:
I∆
L
=
V
OUT
f •L
1–
V
OUT
V
IN(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 I
OUT(MAX)
. To guarantee that ripple
current does not exceed a specified maximum, the induc-
tance should be chosen according to:
L =
V
OUT
f •∆I
L(MAX)
1–
V
OUT
V
IN(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
in
ductance collapses abruptly when the peak design current
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/
current and price/current relationship of an inductor. 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
The regular loop response can be checked by looking at the
load transient response. Switching regulators take several
cycles to respond to a step in load current. When a load step
occurs, V
OUT
immediately shifts by an amount equal to the
∆I
LOAD
• ESR, where ESR is the effective series resistance
of C
OUT
. ∆I
LOAD
also begins to charge or discharge C
OUT
generating a feedback error signal used by the regulator to
return V
OUT
to its steady-state value. During this recovery
time, V
OUT
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 C
FF
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. L
TpowerCAD™ and
LTSpice
®
can be used to check control loop and transient
performance.
In some applications, a more severe transient can be caused
by switching in loads with large (>1µF) load capacitors.
The discharged load capacitors are effectively put in paral
-
lel with C
OUT
, causing a rapid drop in V
OUT
. No regulator
can deliver enough current to prevent this problem if the
switch connecting the load has low resistance and is driven
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.
