LT3493-3 LINER [Linear Technology], LT3493-3 Datasheet - Page 11

• Manufacturer Part Number: LT3493-3
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
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
may be required to get the full benefit (see the Compen-
sation section).
For small size, the output capacitor can be chosen accord-
ing to:
where C
this small results in an increased loop crossover fre-
quency and increased sensitivity to noise. A 22pF capaci-
tor connected between V
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 capacitor and will have a larger capacitance, be-
cause the capacitor must be large to achieve low ESR.
Table 2 lists several capacitor vendors.
Figure 4 shows the transient response of the LT3493-3
with several output capacitor choices. The output is 3.3V.
The load current is stepped from 250mA to 1A and back to
250mA, and the oscilloscope traces show the output
C
OUT
OUT
OUT
= 25/V
is in µF. However, using an output capacitor
is in µF. Use X5R or X7R types and keep in
OUT
U
Table 2. Capacitor Vendors
Vendor
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden
OUT
U
and the FB pin is required to
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
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
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 5 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 5a)
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 the boost diode can be tied to the
input (Figure 5b). The circuit in Figure 5a 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.
The minimum operating voltage of an LT3493-3 applica-
tion is limited by the undervoltage lockout (6.8V) and by
the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by the
boost circuit. If the input voltage is ramped slowly, or the
LT3493-3 is turned on with its SHDN pin when the output
Part Series
Ceramic,
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
Ceramic
Ceramic,
Tantalum
Ceramic
Comments
EEF Series
T494, T495
POSCAP
TPS Series
LT3493-3
11
3493-3f