Datasheet

LM21305
COMP
R
C
C
C1
'V
OUT
<
'i
Lp-p
x (
8f
S
C
OUT
1
+ ESR)
üV
OUT-C
=
8f
S
C
OUT
'i
Lp-p
www.ti.com Component Selection
An inductor with saturation current higher than the over-current protection limit is a safe choice. It is
desired to have small inductance in switching power supplies, because it usually means faster transient
response, smaller DCR, and smaller size for more compact design. But too low of an inductance will
generate too large of an inductor current ripple and it could falsely trigger over-current protection at
maximum load. It also generates more conduction loss, since the RMS current is slightly higher relative to
that with lower ripple current with the same DC load current. Larger inductor current ripple also implies
higher output voltage ripple with the same output capacitors. With peak current-mode control, it is
recommended not to have too small of an inductor current ripple so that the peak current comparator has
enough signal-to-noise ratio.
9.5 Output Capacitor
The LM21305 is designed to be used with a wide variety of LC output filters. It is generally desired 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 transient. The output voltage ripple is composed of two parts. One
is related to 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:
(5)
Since the two components in the ripple are not in phase, the actual peak-to-peak ripple is smaller than the
sum of the two peaks:
(6)
Output capacitance is usually limited by system transient performance specifications, pzrticularly if the
system requires tight voltage regulation in the presence of large current steps and fast slew rate. To
maintain a small overshoot or undershoot during a load transient, small ESR and large capacitance are
desired. But these also come with the penalty of higher cost and size. Clearly, the control loop should also
be fast to reduce the voltage droop.
One or more ceramic capacitors are recommended because they have very low ESR and remain
capacitive up to high frequencies. The dielectric should be X5R, X7R, or comparable material to maintain
proper tolerances. Other types of capacitors also can be used if large capacitance is needed, such as
tantalum, POSCAP and OSCON. Such capacitors have lower ESR zero frequency, 1/(2πESR *C), than
ceramic capacitors. The lower ESR zero frequency can affect the control loop if it is close to the crossover
frequency. If high switching frequency and high crossover frequency are desired, an all ceramic capacitor
design is sometimes more appropriate.
9.6 Compensation Circuit
The LM21305 is designed to achieve high performance in terms of the transient response, audio
susceptibility and output impedance, and will typically require only a single resistor R
c
and capacitor C
c1
for
compensation. However, depending on the power stage, it could require a second capacitor to create a
high frequency pole to cancel the output capacitor ESR.
Figure 1. LM21305 Compensation Network
9
SNVA432C–March 2010–Revised May 2013 AN-2042 LM21305 Evaluation Board
Submit Documentation Feedback
Copyright © 2010–2013, Texas Instruments Incorporated