LT3975
APPLICATIONS INFORMATION
robust for a wide input voltage range. A diode with even
higher current rating can be selected for the worst-case
scenarioofoverload,wherethemaxdiodecurrentcanthen
increase to the typical peak switch current. Short circuit is
not the worst-case condition due to current limit foldback.
Peakreversevoltageisequaltotheregulatorinputvoltage.
For inputs up to 40V, a 40V diode is adequate.
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
V at
I at
R
R
25°C
(µA)
F
V at 3A 3A MAX V = 20V
F
TYP 25°C
25°C
(mV)
PART NUMBER V (V)
I
(A)
AVE
(mV)
R
On Semiconductor
MBRA340T3
MBRS340T3
MBRD340
Diodes Inc.
B340A
40
40
40
3
410
410
450
450
500
600
10
10
4
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.
3
3
40
40
60
40
60
40
30
30
60
40
40
3
3
3
3
3
3
3
3
3
2
2
485
400
600
450
570
420
390
460
580
500
700
500
450
700
490
620
470
430
500
650
2
100
50
4
B340LA
B360A
PDS340
PDS360
0.45
40
100
12
1.7
4
SBR3U40P1
SBR3U30P1
SBR3M30P1
SBR3U60P1
DFLS240L
DFLS240
1
BOOST and OUT Pin Considerations
CapacitorC3andtheinternalboostSchottkydiode(seethe
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
pinconnectedtotheoutput(Figure4a)isbest. Below3.2V
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
anexternalSchottkywillhavemuchlowerforwardvoltage
drop than the internal boost diode.
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
Linear [ Linear ]
