Datasheet

LTC3862
31
3862fc
For more information www.linear.com/LTC3862
Figure 25. Normalized Input Peak-to-Peak Ripple Current
in high duty cycle applications. Figure 25 illustrates the
normalized input ripple current, where:
I
V
Lf
NORM
IN
=
•
applicaTions inForMaTion
3. The minimum on-time for this application operating
in CCM is:
t
f
VVV
VV k
ON MIN
OFIN MAX
OF
()
()
•
–
=
+
+






=
11
300 HHz
VVV
VV
ns
•
.–
.
48 05 36
48 05
859
+
+






=
The maximum DC input current is:
I
I
D
A
A
IN MAX
OMAX
MAX
()
()
––.
.== =
1
5
10505
10 1
4. A ripple current of 40% is chosen so the peak current
in each inductor is:
I
n
I
D
IN PK
OMAX
MAX
()
()
•– •
–
•
.
=






=+
1
1
21
1
2
1
0
χ
44
2
5
10505
606






=•
–.
.
A
A
5. The inductor ripple current is:
∆I
n
I
D
A
A
L
OMAX
MAX
== =
χ
•
–
.
•
–.
.
()
1
04
2
5
10505
202
6. The inductor value is therefore:
L
V
If
D
V
AkHz
IN MIN
L
MAX
==
=
()
•
•
.•
•.
∆
24
202 300
0 505
220µH
7. For a current limit value 30% higher than the maximum
load current:
I
O(CL)
= 1.3 • I
O(MAX)
= 1.3 • 5A = 6.5A
A Design Example
Consider the LTC3862 application circuit is shown in Fig-
ure 26a. The output voltage is 48V and the input voltage
range is 5V to 36V. The maximum output current is 5A
when the input voltage is 24V to 36V
. Below 24V, current
limit will linearly reduce the maximum load to 1A at 5V
in (see Figure 26b).
1. The duty cycle range (where 5A is available at the
output) is:
D
VVV
VV
VVV
V
MAX
OFIN
OF
=
+
+






=
+
+
–
.–48 05 24
48 0
..
.%
.–
.
5
50 5
48 05 36
48 05
V
D
VVV
VV
MIN






=
=
+
+






= 25 8.%
2. The operating frequency is chosen to be 300kHz so
the period is 3.33μs. From Figure 6, the resistor from
the FREQ pin to ground is 45.3k.
DUTY CYCLE
0
∆I
IN
/I
NORM
1.00
0.90
0.80
0.60
0.70
0.50
0.40
0.30
0.20
0.10
0
0.8
3862 F25
0.2
0.4
0.6
1.0
1-PHASE
2-PHASE