lt3470its8-trpbf Linear Technology Corporation, lt3470its8-trpbf Datasheet - Page 11

• Manufacturer Part Number: lt3470its8-trpbf
• Description: Micropower Buck Regulator With Integrated Boost And Catch Diodes
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3470. Not all ceramic capacitors are
suitable. X5R and X7R types are stable over temperature
and applied voltage and give dependable service. Other
types, including Y5V and Z5U have very large temperature
and voltage coefficients of capacitance. In an application
circuit they may have only a small fraction of their nominal
capacitance resulting in much higher output voltage ripple
than expected.
Ceramic capacitors are piezoelectric. The LT3470’s switch-
ing frequency depends on the load current, and at light
loads the LT3470 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3470
operates at a lower current limit during Burst Mode
operation, the noise is typically very quiet to a casual ear.
If this audible noise is unacceptable, use a high perfor-
mance electrolytic capacitor at the output. The input
capacitor can be a parallel combination of a 2.2µF ceramic
capacitor and a low cost electrolytic capacitor.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3470. A ce-
ramic input capacitor combined with trace or cable induc-
tance forms a high quality (under damped) tank circuit. If
the LT3470 circuit is plugged into a live supply, the input
voltage can ring to twice its nominal value, possibly
exceeding the LT3470’s rating. This situation is easily
avoided; see the Hot-Plugging Safely section.
U
Table 3. Capacitor Vendors
Vendor
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden
U
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
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
Block Diagram) are used to generate a boost voltage that
is higher than the input voltage. In most cases a 0.22µF
capacitor will work well. Figure 2 shows two ways to
arrange the boost circuit. The BOOST pin must be more
than 2.5V above the SW pin for best efficiency. For outputs
of 3.3V and above, the standard circuit (Figure 2a) is best.
For outputs between 2.5V and 3V, use a 0.47µF. For lower
output voltages the boost diode can be tied to the input
Figure 2. Two Circuits for Generating the Boost Voltage
Part Series
Ceramic,
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
Ceramic
Ceramic,
Tantalum
Ceramic
V
V
IN
IN
Comments
EEF Series
T494, T495
POSCAP
TPS Series
BIAS
V
V
IN
IN
LT3470
LT3470
GND
GND
BOOST
BOOST
BIAS
(2a)
(2b)
SW
SW
V
MAX V
V
MAX V
BOOST
BOOST
BOOST
BOOST
– V SW ≅ V
– V SW ≅ V
C3
0.22µF
C3
0.22µF
≅ V
≅ 2V
IN
IN
OUT
IN
+ V
OUT
3470 F02
LT3470
V
V
OUT
OUT
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