Datasheet

üv
out-
ESR
üv
out-C
üi
Lp-
p
I
L
time
time
time
üV
OUT
<
'i
Lp-p
x (
8f
S
C
OUT
1
+ ESR)
üV
OUT-C
=
8f
S
C
OUT
'i
Lp-p
LM21305
SNVS639F –DECEMBER 2009–REVISED MARCH 2013 www.ti.com
Once the inductance is determined, the type of inductor must be selected. Ferrite designs have very low core
losses and are preferred at high switching frequencies, so design goals can concentrate on copper loss and
preventing saturation. Ferrite core material saturates hard, which means that inductance collapses abruptly when
the saturation current is exceeded. The ‘hard’ saturation results in an abrupt increase in inductor ripple current
and consequent output voltage ripple. Do not allow the core to saturate!
OUTPUT CAPACITOR
The device is designed to be used with a wide variety of LC filters. It is generally desirable to use as little output
capacitance as possible to keep cost and size down. The output capacitor(s), C
OUT
, should be chosen with care
since it directly affects the steady state output voltage ripple, loop stability and the voltage over/undershoot
during a load current transient.
The output voltage ripple is essentially composed of two parts. One is caused by the inductor current ripple going
through the equivalent series resistance (ESR) of the output capacitors: ΔV
OUT-ESR
= Δi
Lp-p
* ESR.
The other is caused by the inductor current ripple charging and discharging the output capacitors:
(15)
Figure 27 shows an illustration of the two ripple components. Since the two ripple components are not in phase,
the actual peak-to-peak ripple is smaller than the sum of the two peaks:
(16)
Figure 27. Two Components of V
OUT
Ripple
Output capacitance is usually limited by system transient performance specifications if the system requires tight
voltage regulation with presence of large current steps and fast slew rates. When a fast large load transient
occurs, output capacitors provide the required charge before the inductor current can slew to the appropriate
level. The initial output voltage step is equal to the load current step multiplied by the ESR. V
OUT
continues to
droop until the control loop response increases or decreases the inductor current to supply the load. To maintain
a small over- or undershoot during a transient, small ESR and large capacitance are desired. But these also
come with the penalty of higher cost and size. Thus, the motivation is to seek a fast control loop response to
reduce the output voltage deviation.
18 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated
Product Folder Links: LM21305