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LTC3854EMSE 数据表(PDF) 20 Page - Linear Technology

部件名 LTC3854EMSE
功能描述  Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTC3854EMSE 数据表(HTML) 20 Page - Linear Technology

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LTC3854
0
3854fa
applicaTions inForMaTion
Design Example
Consider the design of a 1.2V, 15A buck regulator with a
VIN range of 4.5V to 28V using a DCR sensing scheme.
Inductor Selection
Assuming an inductor ripple of 40% of IOUT, L can be
calculated for the worst case of VIN = VIN(MAX).
LMIN =
1
∆IL • fSW
• VOUT 1−
VOUT
VIN(MAX)
LMIN =
1
0.40 • 15A • 400kHz
• 1.2V • 1
1.2V
20V


LMIN = 0.47µH
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
coefficientofresistance,whichisapproximately0.4%/°C.A
conservative value for TLMAX is 100°C which corresponds
to a delta of 0.3. To allow the converter to source 15A with
aninductortemperatureof100°Cwithouthittingmaximum
current limit we need a DCR at 25°C of:
DCR(25°C) =
0.8 • VSENSE(MAX)
IMAX +
∆IL
2


•(1
+ δ)
DCR(25°C) =
0.8 • 50mV
15A
+
15A • 0.4
2


•(1
+ 0.3)
DCR(25°C) = 1.7m
The 0.56µH inductor from the IHLP4040DZ-01 series has
a typical DCR of 1.7mΩ and a maximum of 1.8mΩ and
as ISAT of 49A. The saturation current is well above our
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
also help insure the minimum on-time requirement of
75ns is not violated).
tON(MIN) =
VOUT
VIN(MAX) • fSW
tON(MIN) =
1.2V
20V • 400kHz
tON(MIN) =150ns
To choose R1 for DCR sensing we use:
R1• C1
=
L
DCR
at 25°C
Choosing C1 = 100nF and using the maximum DCR value
at 25°C, we get:
R1
=
0.56µH
1.8m
Ω •100nF
R1
= 3.11k
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:
∆VOUT = ∆IL ESR+
1
8 • fSW •COUT


The second term is the AC capacitive impedance part of
the above equation and used alone will yield a minimum
COUT of:
COUT >
∆IL
8 • fSW • ∆VOUT
COUT >
0.4 • 15A
8 • 400kHz • 0.01• 1.2V
COUT >156µF


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