LT3502 LINER [Linear Technology], LT3502 Datasheet - Page 13

• Manufacturer Part Number: LT3502
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
For small size, the output capacitor can be chosen
according to:
where C
this small results in an increased loop crossover frequency
and increased sensitivity to noise.
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 specifi ed 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 4 shows the transient response of the LT3502A with
several output capacitor choices. The output is 3.3V. The
load current is stepped from 150mA to 400mA and back to
150mA, and the oscilloscope traces show the output voltage.
The upper photo shows the recommended value. The sec-
ond 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 capacitance.
Table 2
VENDOR
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden
C
OUT
OUT
=
V
25
OUT
is in μF. However, using an output capacitor
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 Pin Considerations
Capacitor C3 and the internal boost diode are used to
generate a boost voltage that is higher than the input
voltage. In most cases a 0.1μF capacitor will work well.
Figure 5 shows two ways to arrange the boost circuit. The
BOOST pin must be at least 2.2V above the SW pin for
best effi ciency. For outputs of 3V and above, the standard
circuit (Figure 5a) is best. For outputs less than 3V and
above 2.5V, place a discrete Schottky diode (such as the
BAT54) in parallel with the internal diode to reduce V
following equations can be used to calculate and minimize
boost capacitance in μF:
V
the forward drop of the catch diode (D1).
For lower output voltages the BD pin can be tied to an
external voltage source with adequate local bypassing
(Figure 5b). The above equations still apply for calculating
the optimal boost capacitor for the chosen BD voltage.
The absence of BD voltage during startup will increase
minimum voltage to start and reduce effi ciency. You must
also be sure that the maximum voltage rating of BOOST
pin is not exceeded.
D
0.012/(V
0.030/(V
is the forward drop of the boost diode, and V
PART SERIES
Ceramic
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic
Polymer,
Tantalum
Ceramic
Ceramic,
Tantalum
Ceramic
BD
BD
+ V
+ V
CATCH
CATCH
– V
– V
LT3502/LT3502A
D
D
– 2.2) for the LT3502
– 2.2) for the LT3502A
COMMENTS
EEF Series
T494,T495
POSCAP
TPS Series
CATCH
13
D
. The
3502fc
is