LTC3854 LINER [Linear Technology], LTC3854 Datasheet - Page 20

• Manufacturer Part Number: LTC3854
• Description: Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3854
applicaTions inForMaTion
Design Example
Consider the design of a 1.2V, 15A buck regulator with a
V
Inductor Selection
Assuming an inductor ripple of 40% of I
calculated for the worst case of V
Next, determine the DCR of the inductor. When provided,
use the manufacturer’s maximum value, usually given at
25°C. Increase this value to account for the temperature
coefficient of resistance, which is approximately 0.4%/°C. A
conservative value for TL
to a delta of 0.3. To allow the converter to source 15A with
an inductor temperature of 100°C without hitting maximum
current limit we need a DCR at 25°C of:
The 0.56µH inductor from the IHLP4040DZ-01 series has
a typical DCR of 1.7mΩ and a maximum of 1.8mΩ and
as I
operating current maximum.
The maximum inductor will be the DC value plus one half
the ripple current. Using this inductor gives an inductor
ripple current of 6A (keeping the ripple current high will
0
IN
L
L
L
DCR(25°C) =
DCR(25°C) =
DCR(25°C) = 1.7mΩ
SAT
range of 4.5V to 28V using a DCR sensing scheme.
MIN
MIN
MIN
of 49A. The saturation current is well above our
=
=
= 0.47µH
0.40 • 15A • 400kHz
∆I
L
1
• f
SW

 

 
I
15A +
MAX
0.8 • V
1
• V
OUT
+
MAX
15A • 0.4
SENSE(MAX)
∆I
0.8 • 50mV



2
L
1−
is 100°C which corresponds
2

 
• 1.2V • 1−
• (1+ δ)
V
IN(MAX)
IN
V
OUT

 
= V
• (1+ 0.3)
IN(MAX)

 



OUT
1.2V
20V
, L can be
.

 
also help insure the minimum on-time requirement of
75ns is not violated).
To choose R1 for DCR sensing we use:
Choosing C1 = 100nF and using the maximum DCR value
at 25°C, we get:
Choose 3.09k.
Output Capacitor Selection
The output voltage AC ripple due to capacitive impedance
and ESR in normal continuous mode operation can be
calculated from:
The second term is the AC capacitive impedance part of
the above equation and used alone will yield a minimum
C
OUT
R1• C1=
R1=
R1= 3.11k
C
C
C
t
t
t
∆V
ON(MIN)
ON(MIN)
ON(MIN)
OUT
OUT
OUT
of:
OUT
1.8mΩ • 100nF
>
>
> 156µF
= ∆I
8 • f
8 • 400kHz • 0.01• 1.2V
=
=
= 150ns
DCR
0.56µH
L
20V • 400kHz
V
L
SW
IN(MAX)

 
∆I
ESR +
at 25°C
0.4 • 15A
• ∆V
1.2V
V
L
OUT
OUT
• f
8 • f
SW
SW
1
• C
OUT

 
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