LT1933 Linear Technology, LT1933 Datasheet - Page 8

• Manufacturer Part Number: LT1933
• Description: 600mA 500kHz Step-Down Switching Regulator in SOT-23
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1933
voltage rating. This situation is easily avoided; see the Hot
Plugging Safely section.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT1933 to produce the DC output. In this role it
determines the output ripple, and low impedance at the
switching frequency is important. The second function is
to store energy in order to satisfy transient loads and
stabilize the LT1933’s control loop.
Ceramic capacitors have very low equivalent series resis-
tance (ESR) and provide the best ripple performance. A
good value is
where C
mind that a ceramic capacitor biased with V
less than its nominal capacitance. This choice will provide
low output ripple and good transient response. Transient
performance can be improved with a high value capacitor,
but a phase lead capacitor across the feedback resistor R1
8
C
OUT
OUT
= 60/V
is in µF. Use X5R or X7R types, and keep in
OUT
Table 2. Capacitor Vendors
Vendor
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden
U
U
Phone
[phone]
[phone]
[phone]
[phone]
[phone]
W
OUT
URL
www.panasonic.com
www.kemet.com
www.sanyovideo.com
www.murata.com
www.avxcorp.com
www.taiyo-yuden.com
U
will have
may be required to get the full benefit (see the Compensa-
tion section).
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose one
that is intended for use in switching regulators. The ESR
should be specified by the supplier, and should be 0.1Ω or
less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR. Table 2 lists
several capacitor vendors.
Figure 1 shows the transient response of the LT1933 with
several output capacitor choices. The output is 3.3V. The
load current is stepped from 100mA to 400mA and back
to 100mA, and the oscilloscope traces show the output
voltage. The upper photo shows the recommended value.
The second photo shows the improved response (less
voltage drop) resulting from a larger output capacitor and
a phase lead capacitor. The last photo shows the response
to a high performance electrolytic capacitor. Transient
performance is improved due to the large output capaci-
tance, but output ripple (as shown by the broad trace) has
increased because of the higher ESR of this capacitor.
Part Series
Ceramic,
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
Ceramic
Ceramic,
Tantalum
Ceramic
Comments
EEF Series
T494, T495
POSCAP
TPS Series
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1933f