LT3501 LINER [Linear Technology], LT3501 Datasheet - Page 14

• Manufacturer Part Number: LT3501
• Description: Monolithic Dual Tracking 3A Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
may result in discontinuous mode operation, which is
okay, but further reduces maximum load current. For
details of maximum output current and discontinuous
mode operation, see Linear Technology Application Note
44. Finally, for duty cycles greater than 50% (V
> 0.5), there is a minimum inductance required to avoid
subharmonic oscillations. See Application Note 19 for
more information.
Input Capacitor Selection
Bypass the inputs of the LT3501 circuit with a 4.7µF or
higher ceramic capacitor of X7R or X5R type. A lower
value or a less expensive Y5V type can be used if there
is additional bypassing provided by bulk electrolytic or
tantalum capacitors. The following paragraphs describe
the input capacitor considerations in more detail.
Step-down regulators draw current from the input supply in
pulses with very fast rise and fall times. The input capaci-
tor is required to reduce the resulting voltage ripple at the
LT3501 and to force this very high frequency switching
current into a tight local loop, minimizing EMI. The input
capacitor must have low impedance at the switching fre-
quency to do this effectively, and it must have an adequate
ripple current rating. With two switchers operating at the
same frequency but with different phases and duty cycles,
calculating the input capacitor RMS current is not simple.
However, a conservative value is the RMS input current for
the channel that is delivering most power (V
This is given by:
and is largest when V
the second, lower power channel draws input current,
the input capacitor’s RMS current actually decreases as
the out-of-phase current cancels the current drawn by the
higher power channel. Considering that the maximum load
current from a single channel is ~3A, RMS ripple current
will always be less than 1.5A.
The frequency, V
rent requirement of the LT3501 along with the input supply
source impedance, determine the energy storage require-
LT3501
14
I
CIN RMS
(
)
=
I
OUT
IN
to V
V
IN
OUT
OUT
= 2V
V
ratio, and maximum load cur-
•
IN
(
V
OUT
IN
–
(50% duty cycle). As
V
OUT
)
<
I
OUT
OUT
2
OUT
• I
OUT
/V
IN
).
ments of the input capacitor. Determine the worst-case
condition for input ripple current and then size the input
capacitor such that it reduces input voltage ripple to an
acceptable level. Typical values for input capacitors run
from 10µF at low frequencies to 2.2µF at higher frequen-
cies. The combination of small size and low impedance
(low equivalent series resistance or ESR) of ceramic
capacitors make them the preferred choice. The low ESR
results in very low voltage ripple and the capacitors can
handle plenty of ripple current. They are also comparatively
robust and can be used in this application at their rated
voltage. X5R and X7R types are stable over temperature
and applied voltage, and give dependable service. Other
types (Y5V and Z5U) have very large temperature and
voltage coeffi cients of capacitance, so they may have
only a small fraction of their nominal capacitance in your
application. While they will still handle the RMS ripple
current, the input voltage ripple may become fairly large,
and the ripple current may end up fl owing from your input
supply or from other bypass capacitors in your system,
as opposed to being fully sourced from the local input
capacitor. An alternative to a high value ceramic capacitor
is a lower value along with a larger electrolytic capacitor,
for example a 1µF ceramic capacitor in parallel with a low
ESR tantalum capacitor. For the electrolytic capacitor, a
value larger than 10µF will be required to meet the ESR and
ripple current requirements. Because the input capacitor
is likely to see high surge currents when the input source
is applied, tantalum capacitors should be surge rated. The
manufacturer may also recommend operation below the
rated voltage of the capacitor. Be sure to place the 1µF
ceramic as close as possible to the V
the IC for optimal noise immunity.
When the LT3501’s input supplies are operated at different
input voltages, an input capacitor sized for that channel
should be placed as close as possible to the respective
V
A fi nal caution regarding the use of ceramic capacitors
at the input. A ceramic input capacitor can combine with
stray inductance to form a resonant tank circuit. If power
is applied quickly (for example by plugging the circuit
into a live power source) this tank can ring, doubling the
input voltage and damaging the LT3501. The solution is to
IN
pins.
IN
and GND pins on
3501fb