Datasheet

11
LT3503
3503f
APPLICATIO S I FOR ATIO
WUU
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Ceramic capacitors have very low equivalent series resis-
tance (ESR) and provide the best ripple performance. A
good value is:
C
OUT
= 24/V
OUT
where C
OUT
is in µF. Use X5R or X7R types and keep in
mind that a ceramic capacitor biased with V
OUT
will have
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
may be required to get the full benefit (see the Compen-
sation section). Using a small output capacitor results in
an increased loop crossover frequency and increased
sensitivity to noise. A 22pF capacitor connected between
V
OUT
and the FB pin is required to filter noise at the FB pin
and ensure stability.
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 capaci
tor and will have a larger capacitance, be-
cause the capacitor must be large to achieve low ESR.
Table 2 lists several capacitor vendors.
Figure 5 shows the transient response of the LT3503 with
a few output capacitor choices. The output is 3.3V. The
load current is stepped from 0.5A to 1.1A and back to 0.5A,
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 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.
BOOST Pin Considerations
Capacitor C3 and diode D2 are used to generate a boost
voltage that is higher than the input voltage. In most cases
a 0.1µF capacitor and fast switching diode (such as the
1N4148 or 1N914) will work well. Figure 6 shows two
ways to arrange the boost circuit. The BOOST pin must be
at least 2.3V above the SW pin for best efficiency. For
outputs of 3.3V and above, the standard circuit (Figure 6a)
is best. For outputs between 3V and 3.3V, use a 0.22µF
capacitor. For outputs between 2.5V and 3V, use a 0.47µF
capacitor and a small Schottky diode (such as the BAT-54).
For lower output voltages tie a Schottky diode to the input
(Figure 6b). The circuit in Figure 6a is more efficient
because the BOOST pin current comes from a lower
voltage source. You must also be sure that the maximum
voltage rating of the BOOST pin is not exceeded.
Table 2. Capacitor Vendors
Vendor Phone URL Part Series Comments
Panasonic [phone] www.panasonic.com Ceramic,
Polymer, EEF Series
Tantalum
Kemet [phone] www.kemet.com Ceramic,
Tantalum T494, T495
Sanyo [phone] www.sanyovideo.com Ceramic,
Polymer, POSCAP
Tantalum
Murata [phone] www.murata.com Ceramic
AVX www.avxcorp.com Ceramic,
Tantalum TPS Series
Taiyo Yuden [phone] www.taiyo-yuden.com Ceramic