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SC417EVB 数据表(PDF) 21 Page - Semtech Corporation

部件名 SC417EVB
功能描述  10A Integrated FET Regulator with Programmable LDO
PDF  29 Pages
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制造商  SEMTECH [Semtech Corporation]
网页  http://www.semtech.com
标志 SEMTECH - Semtech Corporation

SC417EVB 数据表(HTML) 21 Page - Semtech Corporation

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SC417/SC427
21
Applications Information (continued)
The inductor value is typically selected to provide a ripple
current that is between 25% to 50% of the maximum load
current. This provides an optimal trade-off between cost,
efficiency, and transient performance.
During the DH on-time, voltage across the inductor is (V
IN
- V
OUT
). The equation for determining inductance is shown
next.
RIPPLE
ON
OUT
IN
I
T
)
V
V
(
L
Example
In this example, the inductor ripple current is set equal to
50% of the maximum load current. Thus ripple current
will be 50% x 10A or 5A. To find the minimum inductance
needed, use the V
IN
and T
ON
values that correspond to
V
INMAX
.
H
77
.
0
A
5
ns
318
)
05
.
1
2
.
13
(
L
A slightly larger value of 0.88μH is selected. This will
decrease the maximum I
RIPPLE
to 4.4A.
Note that the inductor must be rated for the maximum DC
load current plus 1/2 of the ripple current.
The ripple current under minimum V
IN
conditions is also
checked using the following equations.
ns
384
ns
10
V
V
R
pF
25
T
INMIN
OUT
TON
VINMIN
_
ON
L
T
)
V
V
(
I
ON
OUT
IN
RIPPLE
A
25
.
4
H
88
.
0
ns
384
)
05
.
1
8
.
10
(
I
VIN
_
RIPPLE
Capacitor Selection
The output capacitors are chosen based on required ESR
and capacitance. The maximum ESR requirement is con-
trolled by the output ripple requirement and the DC toler-
ance. The output voltage has a DC value that is equal to
the valley of the output ripple plus 1/2 of the peak-to-peak
ripple. Change in the output ripple voltage will lead to a
change in DC voltage at the output.
The design goal is that the output voltage regulation be
±4% under static conditions. The internal 500mV refer-
ence tolerance is 1%. Allowing 1% tolerance from the FB
resistor divider, this allows 2% tolerance due to V
OUT
ripple.
Since this 2% error comes from 1/2 of the ripple voltage,
the allowable ripple is 4%, or 42mV for a 1.05V output.
The maximum ripple current of 4.4A creates a ripple
voltage across the ESR. The maximum ESR value allowed
is shown by the following equations.
A
4
.
4
mV
42
I
V
ESR
RIPPLEMAX
RIPPLE
MAX
ESR
MAX = 9.5 mΩ
The output capacitance is usually chosen to meet tran-
sient requirements. A worst-case load release, from
maximum load to no load at the exact moment when
inductor current is at the peak, determines the required
capacitance. If the load release is instantaneous (load
changes from maximum to zero in < 1μs), the output
capacitor must absorb all the inductor’s stored energy.
This will cause a peak voltage on the capacitor according
to the following equation.
2
OUT
2
PEAK
2
RIPPLEMAX
OUT
MIN
V
V
I
2
1
I
L
COUT
Assuming a peak voltage V
PEAK
of 1.150 (100mV rise upon
load release), and a 10A load release, the required capaci-
tance is shown by the next equation.
2
2
2
MIN
05
.
1
15
.
1
4
.
4
2
1
10
H
88
.
0
COUT
COUT
MIN = 595μF
If the load release is relatively slow, the output capacitance
can be reduced. At heavy loads during normal switching,
when the FB pin is above the 500mV reference, the DL
output is high and the low-side MOSFET is on. During this
time, the voltage across the inductor is approximately
-V
OUT
. This causes a down-slope or falling di/dt in the



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