LTC3541EDD-2(RevA) View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC3541EDD-2 Datasheet PDF : 20 Pages
| |||
LTC3541-2
APPLICATIO S I FOR ATIO
changes. Note that bypass capacitors used to decouple
individual components powered by the LTC3541-2 will
increase the effective output capacitor value. High ESR
tantalum and electrolytic capacitors may be used, but
a low ESR ceramic capacitor must be in parallel at the
output. There is no minimum ESR or maximum capacitor
size requirement.
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
temperature and applied voltage. The most common
dielectrics used are Z5U, Y5V, X5R and X7R. The Z5U
and Y5V dielectrics are good for providing high capaci-
tances in a small package, but exhibit large voltage and
temperature coefficients as shown in Figures 6 and 7.
When used with a 2V regulator, a 1µF Y5V capacitor can
lose as much as 75% of its initial capacitance over the
operating temperature range. The X5R and X7R dielectrics
result in more stable characteristics and are usually more
suitable for use as the output capacitor. The X7R type has
better stability across temperature, while the X5R is less
expensive and is available in higher values. In all cases,
the output capacitance should never drop below 1µF or
instability or degraded performance may occur.
20
BOTH cAPAcITORS ARE 1µF,
10V, 0603 cASE SIZE
0
X5R
–20
–40
Y5V
–60
–80
–100
0
2
4
6
8
10
Dc BIAS VOLTAGE (V)
35412 F06
Figure 6. Change in Capacitor vs Bias Voltage
20
0
X5R
–20
Y5V
–40
–60
–80
BOTH cAPAcITORS ARE 1µF,
10V, 0603 cASE SIZE
–100
–50 –25 0
25 50
TEMPERATURE (°c)
75
35412 F07
Figure 7. Change in Capacitor vs Temperature
EFFICIENCY CONSIDERATIONS
Generally, the efficiency of a regulator is equal to the out-
put power divided by the input power times 100%. It is
often useful to analyze individual loss terms to determine
which terms are limiting efficiency and what if any change
would yield the greatest improvement. Efficiency can be
expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual loss terms as a per-
centage of input power.
Although all dissipative elements in the circuit produce
losses, three main sources typically account for the major-
ity of the losses in the LTC3541-2 circuits: VIN quiescent
current, I2R losses, and loss across VLDO output device.
When operating with both the buck and VLDO regulator
active (ENBUCK and ENVLDO equal to logic high), VIN
quiescent current loss and loss across the VLDO output
device dominate the efficiency loss at low load currents,
whereas the I2R loss and loss across the VLDO output
device dominate the efficiency loss at medium to high load
currents. At low load currents with the part operating with
the linear regulator (ENBUCK equal to logic low, ENVLDO
equal to logic high), efficiency is typically dominated by
the loss across the linear regulator output device and VIN
quiescent current. In a typical efficiency plot, the efficiency
curve at very low load currents can be misleading since
the actual power lost is of little consequence.
35412fb
14
Share Link: 