Datasheet

LT3976
18
3976f
For more information www.linear.com/3976
Catch Diode Selection
The catch diode (D1 from the Block Diagram) conducts
current only during the switch off time. Average forward
current in normal operation can be calculated from:
I
D(AVG)
= I
OUT
V
IN
– V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
where I
OUT
is the output load current. The current rating of
the diode should be selected to be greater than or equal to
the application’s output load current, so that the diode is
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 LT3976 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 LT3976 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
applicaTions inForMaTion
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.
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 charg-
ing the boost capacitor from the input. However, such
a voltage rail is not always available in all systems. For
output voltages 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 external 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.
When the output is above 16V, the OUT pin can not be
tied to the output or the OUT pin abs max will be violated.
It should instead be tied to GND (Figure 4e). This is to
prevent the dropout circuitry from interfering with switch-
ing behavior and to prevent the 100mA active pull-down
from drawing power. It is important to note that when
the output is above 16V and the OUT pin is grounded,
the dropout circuitry
is not connected, so the minimum
dropout
will be about 1.5V, rather than 500mV. If the
output is less than 3.2V and an external Schottky is used
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 V
R
(V) I
AVE
(A)
V
F
at 5A
TYP 25°C
(mV)
V
F
at
5A MAX
25°C
(mV)
I
R
at
V
R
= 20V
25°C
(µA)
On Semiconductor
MBRS540T3 40 5 450 500 120
Diodes Inc.
B540C 40 5 510 550 2
PDS540 40 5 480 520 4
PDS560 60 5 610 670 0.9
SBR8A45SP5 45 8 450 — 18
SBR8AU60P5 60 8 400 — 60