Datasheet

LT3763
15
3763fa
For more information www.linear.com/LT3763
applicaTions inForMaTion
Table 1. Sense Resistor Values
MAXIMUM OUTPUT
CURRENT (A) RESISTOR, R
S
(mΩ) POWER DISSIPATION (W)
1 50 0.05
5 10 0.25
10 5 0.50
25 2 1.25
Inductor Selection
Size the inductor so that the peak-to-peak ripple current
is approximately 30% of the output current.
The following equation sizes the inductor for best per
-
formance:
L =
V
IN
• V
O
– V
O
2
0.3• f
SW
•I
O
• V
IN
⎛
⎝
⎜
⎞
⎠
⎟
where V
O
is the output voltage, V
IN
is the input voltage,
I
O
is the maximum regulated current in the inductor and
f
SW
is the switching frequency.
The overcurrent comparator terminates switching when
the voltage between the SENSE
+
and SENSE
–
pins exceeds
85mV. The saturation current for the inductor should be
at least 20% higher than the maximum regulated current.
Recommended inductor manufacturers are listed in Table 2.
Figure 3. Power Dissipation in R
S
Table 2. Recommended Inductor Manufacturers
VENDOR WEBSITE
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Vishay www.vishay.com
Würth Electronics www.we-online.com
NEC-Tokin www.nec-tokin.com
Switching MOSFET Selection
The following parameters are critical in determining the
best switching MOSFETs for a given application: total gate
charge (Q
G
), on-resistance (R
DS(ON)
), gate to drain charge
(Q
GD
), gate-to-source charge (Q
GS
), gate resistance (R
G
),
breakdown voltages (maximum V
GS
and V
DS
) and drain
current (maximum I
D
). The following guidelines provide
information to make the selection process easier, and
Table 3 lists some recommended parts and manufacturers.
For both switching MOSFETs the rated drain current should
be greater than the maximum inductor current. Use the
following equation to calculate the peak inductor current:
I
MAX
= I
O
+
V
IN
• V
O
– V
O
2
2• f
SW
•L • V
IN
⎛
⎝
⎜
⎞
⎠
⎟
The rated drain current is temperature dependent, and
most data sheets include a table or graph of the rated
drain current versus temperature.
The rated V
DS
should be higher than the maximum input
voltage (including transients) for both MOSFETs. As for the
rated V
GS
, the signals driving the gates of the switching
MOSFETs have a maximum voltage of 5V with respect to the
source. However, during start-up and recovery conditions,
the gate-drive signals may be as low as 3V. Therefore, to
ensure that the LT3763 recovers properly, the maximum
threshold voltage should be less than 2V, and for a robust
design, ensure that the rated V
GS
is greater than 7V.
Power losses in the switching MOSFETs are related to the
on-resistance, R
DS(ON)
; gate resistance, R
G
; gate-to-drain
charge, Q
GD
and gate-to-source charge, Q
GS
. Power lost to
the on-resistance is an Ohmic loss, I
2
R
DS(ON)
, and usually
dominates for input voltages less than 15V. Power lost
while charging the gate capacitance dominates for voltages
R
S
(mΩ)
0
0
POWER DISSIPATION (W)
0.2
0.6
0.8
1.0
1.4
2
10
14
3763 F03
0.4
1.2
8
18
20
4
6
12 16