LT1941 View Datasheet(PDF) - Linear Technology
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Manufacturer
LT1941 Datasheet PDF : 24 Pages
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LT1941
APPLICATIONS INFORMATION
You can estimate output ripple with the following
equations:
VRIPPLE = ΔIL/(8 • f • COUT) for ceramic capacitors
and
VRIPPLE = ΔIL • ESR for electrolytic capacitors (tantalum
and aluminum)
where ΔIL is the peak-to-peak ripple current in the inductor.
The RMS content of this ripple is very low so the RMS
current rating of the output capacitor is usually not of
concern. It can be estimated with the formula:
IC(RMS) = ΔIL/√12
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor transfers to the output, the resulting
voltage step should be small compared to the regulation
voltage. For a 5% overshoot, this requirement indicates:
COUT > 10 • L • (ILIM/VOUT)2
The low ESR and small size of ceramic capacitors make
them the preferred type for LT1941 applications. Not all
ceramic capacitors are the same, however. Many of the
higher value capacitors use poor dielectrics with high
temperature and voltage coefficients. In particular, Y5V
and Z5U types lose a large fraction of their capacitance
with applied voltage and at temperature extremes.
Because loop stability and transient response depend on
the value of COUT, this loss may be unacceptable. Use
X7R and X5R types.
Electrolytic capacitors are also an option. The ESRs of
most aluminum electrolytic capacitors are too large to
deliver low output ripple. Tantalum, as well as newer,
lower-ESR organic electrolytic capacitors intended for
power supply use are suitable. Chose a capacitor with a
low enough ESR for the required output ripple. Because
the volume of the capacitor determines its ESR, both the
size and the value will be larger than a ceramic capacitor
that would give similar ripple performance. One benefit
is that the larger capacitance may give better transient
response for large changes in load current. Table 2 lists
several capacitor vendors.
Table 2. Low ESR Surface Mount Capacitors
VENDOR
TYPE
Taiyo-Yuden
Ceramic
AVX
Ceramic
Tantalum
Kemet
Tantalum
Tantalum Organic
Aluminum Organic
Sanyo
Tantalum or Aluminum Organic
Panasonic
Aluminum Organic
TDK
Ceramic
SERIES
TPS
T491,T494,T495
T520
A700
POSCAP
SP CAP
Diode Selection
The catch diode (D1 from Figure 2) conducts current only
during switch off time. Average forward current in normal
operation can be calculated from:
ID(AVG) = IOUT (VIN – VOUT)/VIN
The only reason to consider a diode with a larger current
rating than necessary for nominal operation is for the
worst-case condition of shorted output. The diode current
will then increase to the typical peak switch current.
Peak reverse voltage is equal to the regulator input voltage.
Use a diode with a reverse voltage rating greater than the
input voltage. Table 3 lists several Schottky diodes and
their manufacturers.
Table 3. Schottky Diodes
PART
VR
NUMBER
(V)
On Semiconductor
MBRM120E
20
MBRM140
40
Diodes Inc.
B120
20
B130
30
B220
20
B230
30
International Rectifier
10BQ030
30
20BQ030
30
IAVE VF AT 1A VF AT 2A
(A)
(mV)
(mV)
1
530
595
1
550
1
500
1
500
2
500
2
500
1
420
470
2
470
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