LTC3854_15 Datasheet, PDF (20/28 Pages) Linear Technology – Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller

LTC3854
APPLICATIONS INFORMATION
Design Example
Consider the design of a 1.2V, 15A buck regulator with a
V_IN range of 4.5V to 28V using a DCR sensing scheme.
Inductor Selection
Assuming an inductor ripple of 40% of I_OUT, L can be
calculated for the worst case of V_IN = V_IN(MAX).
L_MIN
=
∆I_L
1
• f_SW

• V_{OUT } 1−

V_OUT
V_IN(MAX
)



L_MIN
=
0.40
•
1
15A
•
400kHz
•
1.2V
•


1−
1.2V
20V


L_MIN = 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
coefficient of resistance, which is approximately 0.4%/°C. A
conservative value for TL_MAX is 100°C which corresponds
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:
DCR(25°C) = ^{0.8 • VSENSE(MAX)}
_IMAX
+
∆I_L
2


•
(1+
δ)
DCR(25°C) =
0.8 • 50mV


15A
+
15A •
2
0.4 

• (1+
0.3)
DCR(25°C) = 1.7mΩ
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_SAT 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).
t_ON(MIN)
=
V_OUT
V_IN(MAX) •
f_SW
t_ON(MIN)
=
1.2V
20V • 400kHz
t_ON(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
1.8mΩ • 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:
∆V_OUT
=
∆I_L


ESR
+
8
•
1
f_SW • C_OUT


The second term is the AC capacitive impedance part of
the above equation and used alone will yield a minimum
C_OUT of:
C_OUT
>
8
•
∆I_L
f_SW • ∆V_OUT
C_OUT
>
8
•
0.4 • 15A
400kHz • 0.01•
1.2V
C_OUT > 156µF
3854fb
20