Datasheet
LTC3864
21
3864f
APPLICATIONS INFORMATION
The FREQ pin is tied to signal ground in order to program
the switching frequency to 350kHz. The on-time required
at 55V to generate a 5V output can be calculated as:
t
ON
=
V
OUT
V
IN
• f
=
5V
55V • 350kHz
≈260ns
This on-time is larger than LTC3864’s minimum on-time
with sufficient margin to prevent cycle skipping.
Next, set the inductor value to give 60% worst-case ripple
at maximum V
IN
= 55V.
L =
5V
350kHz •(0.6• 2A)
1–
5V
55V
≈10.8µH
Select 10µH, which is a standard value.
The resulting maximum ripple current is:
∆I
L
=
5V
350kHz • 10µH
1–
5V
55V
≈1.3A
Figure 8. Design Example (5V, 2A 350kHz Step-Down Converter)
Next, set the R
SENSE
resistor value to ensure that the
converter can deliver a maximum output current of 2.0A
with sufficient margin to account for component varia-
tions and worst-case operating conditions. Using a 30%
margin factor:
R
SENSE
≅
95mV
1.3• 2A +
1.3A
2
≈27.5mΩ
Use a more readily available 25mΩ sense resistor.
The current limit is:
I
LIMIT
≅
95mV
25mΩ
–
1.3A
2
≈3.15A
Next choose a P-channel MOSFET with the appropri-
ate BV
DSS
and I
D
rating. In this example, a good choice
is the Fairchild FDMC5614P (BV
DSS
= 60V, I
D
= 5.7A,
R
DS(ON)
= 105mΩ, ρ
100°C
= 1.5, C
MILLER
= 100pF,
θ
JA
= 60°C/W). The expected power dissipation and the
Efficiency
C
SS
0.1µF
R
RUN
100k
R
PGD
100k
MP
D1
SW
L1
10µH
CAP
C
CAP
0.1µF
C
IN2
4.7µF
C
IN1
12µF
63V
PGND
LTC3864
3864 F08a
SS
ITH
FREQ
SGND
RUN
V
IN
MODE/PLLN
SENSE
GATE
PGOOD
VFB
R
SENSE
25mΩ
R
ITH
9.53k
R
FB2
422k
47µF
×2
V
OUT
*
5V
2A
R
FB1
80.6k
V
IN
*
5.2V TO 55V
C
VIN
0.1µF
C
ITH1
3.3nF
C
ITH2
100pF
+
C
IN1
: NICHICON UPJ1J120MDD
D1: DIODES INC SBR3U100LP
L1: TOKO 1217AS-H-100M
MP: FAIRCHILD FDMC5614P
*V
OUT
FOLLOWS V
IN
WHEN 3.5V ≤ V
IN
≤ 5.2V
SEE DROPOUT BEHAVIOR IN TYPICAL PERFORMANCE CHARACTERISTICS
C
FF
47pF
LOAD CURRENT (A)
0.01
50
EFFICIENCY (%)
80
70
60
90
100
0.1 1
3864 F08b
PULSE-SKIPPING
Burst Mode
OPERATION
V
IN
= 12V
V
OUT
= 5V