B340LA View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
B340LA
Linear Technology
'B340LA' PDF : 24 Pages View PDF
LT3975
APPLICATIONS INFORMATION
robust for a wide input voltage range. A diode with even
higher current rating can be selected for the worst-case
scenario of overload, where the max diode current can then
increase to the typical peak switch current. Short circuit is
not the worst-case condition due to current limit foldback.
Peak reverse voltage is equal to the regulator input voltage.
For inputs up to 40V, a 40V diode is adequate.
An additional consideration is reverse leakage current.
When the catch diode is reversed biased, any leakage
current will appear as load current. When operating under
light load conditions, the low supply current consumed
by the LT3975 will be optimized by using a catch diode
with minimum reverse leakage current. Low leakage
Schottky diodes often have larger forward voltage drops
at a given current, so a trade-off can exist between low
load and high load efficiency. Often Schottky diodes with
larger reverse bias ratings will have less leakage at a given
output voltage than a diode with a smaller reverse bias
rating. Therefore, superior leakage performance can be
achieved at the expense of diode size. Table 4 lists several
Schottky diodes and their manufacturers.
BOOST and OUT Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see the
Block Diagram) are used to generate a boost voltage that
is higher than the input voltage. In most cases a 0.47μF
capacitor will work well. The BOOST pin must be more
than 1.8V above the SW pin for best efficiency and more
than 2.6V above the SW pin to allow the LT3975 to skip
off times to achieve very high duty cycles. For outputs
between 3.2V and 16V, the standard circuit with the OUT
pin connected to the output (Figure 4a) is best. Below 3.2V
the internal Schottky diode may not be able to sufficiently
charge the boost capacitor. Above 16V, the OUT pin abs
max is violated. For outputs between 2.5V and 3.2V, an
external Schottky diode to the output is sufficient because
an external Schottky will have much lower forward voltage
drop than the internal boost diode.
Table 4. Schottky Diodes. The Reverse Current Values Listed
Are Estimates Based Off of Typical Curves for Reverse Current
vs Reverse Voltage at 25°C
PART NUMBER VR (V)
IAVE (A)
VF at 3A
TYP 25°C
(mV)
VF at
3A MAX
25°C
(mV)
IR at
VR = 20V
25°C
(µA)
On Semiconductor
MBRA340T3
40
3
410
450
10
MBRS340T3
40
3
410
500
10
MBRD340
40
3
450
600
4
Diodes Inc.
B340A
40
3
485
500
2
B340LA
40
3
400
450
100
B360A
60
3
600
700
50
PDS340
40
3
450
490
4
PDS360
60
3
570
620
0.45
SBR3U40P1
40
3
420
470
40
SBR3U30P1
30
3
390
430
100
SBR3M30P1
30
3
460
500
12
SBR3U60P1
60
3
580
650
1.7
DFLS240L
40
2
500
4
DFLS240
40
2
700
1
For output voltages less than 2.5V, there are two options.
An external Schottky diode can charge the boost capaci-
tor from the input (Figure 4c) or from an external voltage
source (Figure 4d). Using an external voltage source is the
better option because it is more efficient than charging the
boost capacitor from the input. However, such a voltage
rail is not always available in all systems. For output volt-
ages greater than 16V, an external Schottky diode from
an external voltage source should be used to charge the
boost capacitor (Figure 4e). In applications using an ex-
ternal voltage source, the supply should be between 3.1V
and 16V. When using the input, the input voltage may not
exceed 27V. In all cases, the maximum voltage rating of
the BOOST pin must not be exceeded.
3975f
17
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