Datasheet

LT3763
17
3763fa
For more information www.linear.com/LT3763
Table 3. Recommended Switching FETs
V
IN
(V)
V
OUT
(V)
I
OUT
(A) TOP FET BOTTOM FET MANUFACTURER
60 4 20 RJK0853DPB RJK0853DPB Renesas
www.renesas.com
24 4 5 RJK0368DP
A
RJK0332DPB
48 10 to 35 10 RJK0851DPB RJK0851DPB
12 2 to 4 10 FDMS8680 FDMS8672AS Fairchild
www.fairchildsemi.com
26 4 20 Si7884BDP SiR470DP Vishay
www.vishay.com
24 4 40 PSMN4R0-30YL RJK0346DPA
NXP/Philips
www.nxp.com
36 12 10 BSC100N06LS3 BSC100N06LS3 www.infineon.com
Input Capacitor Selection
The input capacitor should be sized at least 2µF for every
1A of output current and placed very close to the high side
MOSFET. The loop created by the input capacitor, high side
MOSFET, low side MOSFET should be minimized. It should
have a ripple current rating equal to half of the maximum
output current. Additionally, a small 4.7µF ceramic capaci
-
tor should be placed between V
IN
and ground as close as
possible to the V
IN
pin and the exposed pad of the package
for optimal noise immunity.
It is recommended that several low ESR (equivalent se
-
ries resistance) ceramic capacitors be used as the input
capacitance, although other capacitors with higher density
may be required to reduce board area. Only X5R or X7R
capacitors maintain their capacitance over a wide range
of operating voltages and temperatures.
Output Capacitor Selection
The output capacitors need to have very low ESR to reduce
output ripple. A minimum of 20µF/A of load current should
be used in most designs. The capacitors also need to be
surge rated to the maximum output current. To achieve
the lowest possible ESR, several low ESR ceramic capaci
-
tors should be used in parallel. Many lower output voltage
applications benefit from the use of high density POSCAP
capacitors, which are easily destroyed when exposed to
over
voltage conditions. T
o prevent this, select POSCAP
capacitors that have a voltage rating that is at least 50%
higher than the regulated voltage.
Note that when dimming, the output voltage increases at the
applicaTions inForMaTion
end of every pulse as the decreasing inductor current flows
into the output capacitor. Use of a small output capacitor
may trigger overvoltage protection through the FB pin.
C
BOOST
Capacitor Selection
The C
BOOST
capacitor must be sized no bigger than 220nF
and more than 50nF to ensure proper operation of the
LT3763. Use 220nF for high current switching MOSFETs
with high gate charge.
INTV
CC
Capacitor Selection
The bypass capacitor for the INTV
CC
pin should be larger
than 22µF to ensure stability, and it should be connected
as close as possible to the exposed pad underneath
the package. It is recommended that the ESR be lower
than 50mΩ to reduce noise within the LT3763. For driv
-
ing MOSFETs with gate charges larger than 44nC, use
0.5µF/nC of total gate charge.
Soft-Start
Unlike conventional voltage regulators, the L
T3763 utilizes
the soft-start function to control the regulated inductor
current instead of the output voltage. The charging current
is 11µA and reduces the set current as long as the SS pin
voltage is lower than CTRL1 and CTRL2.
Output Current Regulation
To adjust the regulated load current, an analog voltage is
applied to the CTRL1 pin. Figure 5 shows the regulated
voltage across the sense resistor for control voltages up to
V
CTRL1
(V)
0
0
V
SENSE
+
– V
SENSE
–
(mV)
10
20
30
40
50
60
0.5 1.0 1.5
3763 F05
2.0
Figure 5. Sense Voltage vs CTRL Voltage