LT1934 LINER [Linear Technology], LT1934 Datasheet - Page 9

• Manufacturer Part Number: LT1934
• Description: Micropower Step-Down Switching Regulators in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
capacitor is subject to large surge currents if the LT1934
circuit is connected to a low impedance supply, and that
some electrolytic capacitors (in particular tantalum) must
be specified for such use.
Output Capacitor and Output Ripple
The output capacitor filters the inductor’s ripple current
and stores energy to satisfy the load current when the
LT1934 is quiescent. In order to keep output voltage ripple
low, the impedance of the capacitor must be low at the
LT1934’s switching frequency. The capacitor’s equivalent
series resistance (ESR) determines this impedance. Choose
one with low ESR intended for use in switching regulators.
The contribution to ripple voltage due to the ESR is
approximately I
for the LT1934 and less than ~500m for the LT1934-1.
The value of the output capacitor must be large enough to
accept the energy stored in the inductor without a large
change in output voltage. Setting this voltage step equal to
1% of the output voltage, the output capacitor must be:
For example, an LT1934 producing 3.3V with L = 47 H
requires 33 F. This value can be relaxed if small circuit size
is more important than low output ripple.
Sanyo’s POSCAP series in B-case and C-case sizes pro-
vides very good performance in a small package for the
LT1934. Similar performance in traditional tantalum ca-
pacitors requires a larger package (C- or D-case). The
C
OUT
> 50 • L • (I
LIM
• ESR. ESR should be less than ~150m
U
Table 3. Capacitor Vendors
Vendor
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden
LIM
/V
OUT
U
)
2
Phone
[phone]
[phone]
[phone]
[phone]
[phone]
W
U
URL
www.panasonic.com
www.kemet.com
www.sanyovideo.com
www.murata.com
www.avxcorp.com
www.taiyo-yuden.com
LT1934-1, with its lower switch current, can use a B-case
tantalum capacitor.
With a high quality capacitor filtering the ripple current
from the inductor, the output voltage ripple is determined
by the hysteresis and delay in the LT1934’s feedback
comparator. This ripple can be reduced further by adding
a small (typically 10pF) phase lead capacitor between the
output and the feedback pin.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT1934.
Not all ceramic capacitors are suitable. X5R and X7R types
are stable over temperature and applied voltage and give
dependable service. Other types (Y5V and Z5U) have very
large temperature and voltage coefficients of capacitance.
In the application circuit they may have only a small
fraction of their nominal capacitance and voltage ripple
may be much larger than expected.
Ceramic capacitors are piezoelectric. The LT1934’s switch-
ing frequency depends on the load current, and at light
loads the LT1934 can excite the ceramic capacitor at audio
frequencies, generating audible noise. If this is unaccept-
able, use a high performance electrolytic capacitor at the
output. The input capacitor can be a parallel combination
of a 2.2 F ceramic capacitor and a low cost electrolytic
capacitor. The level of noise produced by the LT1934-1
Part Series
Ceramic,
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
Ceramic
Ceramic,
Tantalum
Ceramic
Comments
EEF Series
T494, T495
POSCAP
TPS Series
LT1934/LT1934-1
9
1934f