SC401B Datasheet, PDF (23/32 Pages) Semtech Corporation – 15A Integrated FET Regulator with Programmable LDO

SC401B
Applications Information (continued)
There are two values of load current to evaluate — con-
tinuous load current and peak load current. Continuous
load current relates to thermal stresses which drive the
selection of the inductor and input capacitors. Peak load
current determines instantaneous component stresses
and filtering requirements such as inductor saturation,
output capacitors, and design of the current limit circuit.
The following values are used in this design.
• V_IN = 12V + 10%
• V_OUT = 1.5V + 4%
• VDD = 5V
• f_SW = 300kHz
• Load = 15A maximum
Frequency Selection
Selection of the switching frequency requires making a
trade-off between the size and cost of the external filter
components (inductor and output capacitor) and the
power conversion efficiency.
The desired switching frequency is 300kHz which results
from using components selected for optimum size and
cost.
A resistor (R_TON) is used to program the on-time (indirectly
setting the frequency) using the following equation.
R_TON
k
25pF u f_SW
T_ON
V _OUT
V_INMAXu f _SW
Use the value associated with maximum V_IN for T_ON.
T_ON = 379 ns at 13.2V_IN, 1.5V_OUT, 300kHz
Substituting for R_TON results in the following solution.
R_TON = 133.3kΩ, use R_TON = 130kΩ
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.
The ripple current will also set the boundary for PSAVE
operation. The switching will typically enter PSAVE mode
when the load current decreases to 1/2 of the ripple
current. For example, if ripple current is 4A then PSAVE
operation will typically start for loads less than 2A. If ripple
current is set at 40% of maximum load current, then PSAVE
will start for loads less than 20% of maximum current.
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 on-time, voltage across the inductor is
(V_IN - V_OUT). The equation for determining inductance is
shown next.
L ^{(VIN  VOUT ) u TON}
I_RIPPLE
Example
In this example, the inductor ripple current is set equal to
30% of the maximum load current. Therefore ripple
current will be 30% x 15A or 4.5A. To find the minimum
inductance needed, use the V_IN and T_ON values that corre-
spond to V_INMAX.
L ^{(13.2  1.5) u 379ns} 0.99PH
4.5A
A slightly larger value of 1µH is selected. This will decrease
the maximum I_RIPPLE to 4.43A.
Note that the inductor must be rated for the maximum DC
load current plus 1/2 of the ripple current.
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