lt3470a Linear Technology Corporation, lt3470a Datasheet - Page 12

• Manufacturer Part Number: lt3470a
• Description: Micropower Buck Regulator With Integrated Boost And Catch Diodes
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3470A
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3470A. 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 coeffi cients 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 LT3470A’s
switching frequency depends on the load current, and at
light loads the LT3470A can excite the ceramic capacitor
at audio frequencies, generating audible noise. Since the
LT3470A operates at a lower current limit during Burst
Mode operation, the noise is typically very quiet to a ca-
sual ear. If this audible noise is unacceptable, use a high
performance 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 fi nal precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3470A. A ceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT3470A circuit is plugged into a live supply, the input volt-
age can ring to twice its nominal value, possibly exceeding
the LT3470A’s rating. This situation is easily avoided; see
the Hot-Plugging Safely section.
12
Table 2. Capacitor Vendors
Vendor
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden
Phone
[phone]
[phone]
[phone]
[phone]
[phone]
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 ar-
range the boost circuit. The BOOST pin must be more than
2.5V above the SW pin for best effi ciency. 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
Part Series
Ceramic,
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
Ceramic
Ceramic,
Tantalum
Ceramic
Figure 2. Two Circuits for Generating the Boost Voltage
V
V
IN
IN
Comments
EEF Series
T494, T495
POSCAP
TPS Series
BIAS
V
V
IN
IN
LT3470A
LT3470A
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
≅ 2•V
IN
OUT
IN
IN
+ V
OUT
3470a F02
V
V
OUT
OUT
3470af