SC461 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC461' PDF : 31 Pages View PDF
SC461
Applications Information (continued)
TON
V OUT
VINMAXu f SW
TON = 310 nsec at 26.4VIN, 1.8VOUT, 220kHz
Substituting for RTON results in the following solution.
RTON = 156kΩ, use RTON = 154kΩ
Inductor Selection
In order to determine the inductance, the ripple current
must first be defined. Low inductor values result in smaller
size but create higher ripple current which can reduce
efficiency. Higher inductor values will reduce the ripple
current/voltage and for a given DC resistance are more
efficient. However, larger inductance translates directly
into larger packages and higher cost. Cost, size, output
ripple, and efficiency are all used in the selection process.
/ u QV ȝ+
$
A slightly smaller value of 1.5µH is selected.
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 VIN conditions is also
checked using the following equations.
721B9,10,1
S) u 5721 u 9287 QV QV
9,10,1
IRIPPLE
(VIN VOUT ) u TON
L
,5,33/(B9,1 0,1
u QV
ȝ+
$
The ripple current will also set the boundary for power-
save operation. The switching will typically enter power-
save mode when the load current decreases to 1/2 of the
ripple current. For example, if ripple current is 4A then
Power-save operation will typically start for loads less than
2A. If ripple current is set at 40% of maximum load current,
then power-save will start for loads less than 20% of
maximum current.
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 inductor value is typically selected to provide a ripple
current that is between 25% to 60% 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
(VIN - VOUT). The following equation for determining induc-
tance is shown.
L (VIN VOUT ) u TON
IRIPPLE
In this example the inductor ripple current is set approxi-
mately equal to 50% of the maximum load current. Thus
ripple current target will be 50% x 10A or 5A.
The design goal is for the output voltage regulation to be
±4% under static conditions. The internal 600mV refer-
ence tolerance is 1%. Allowing 1% tolerance from the FB
resistor divider, this allows 2% tolerance due to VOUT ripple.
Since this 2% error comes from 1/2 of the ripple voltage,
the allowable ripple is 4%, or 72mV for a 1.8V output.
The maximum ripple current of 5A creates a ripple voltage
across the ESR. The maximum ESR value allowed is shown
by the following equations.
(650$;
95,33/(
,5,33/(0$;
P9
$
ESRMAX = 14.4 mΩ
To find the minimum inductance needed, use the VIN and The output capacitance is chosen to meet transient
TON values that correspond to VINMAX.
requirements. A worst-case load release, from maximum
22
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