Datasheet

1 1
8
Co
Vripple
fsw
Iripple
> ×
×
2
OUT o
out
I L
Co
V Vout
D ×
>
×D
2
Iripple
ILpeak Iout= +
2
2
1 ( max )
12 max
Vo Vin Vo
ILrms Io
Vin Lo fsw
æ ö
× -
= + ×
ç ÷
× ×
è ø
Vin Vout Vout
Iripple
Lo Vin fsw
-
= ×
×
ind
Vin Vout Vout
Lo
Io K Vin fsw
-
= ×
× ×
TPS65251
SLVSAA4D –JUNE 2010–REVISED DECEMBER 2012 www.ti.com
Selecting the Switching Frequency
The first step is to decide on a switching frequency for the regulator. Typically, you will want to choose the
highest switching frequency possible since this will produce the smallest solution size. The high switching
frequency allows for lower valued inductors and smaller output capacitors compared to a power supply that
switches at a lower frequency. However, the highest switching frequency causes extra switching losses, which
hurt the converter’s performance. The converter is capable of running from 300 kHz to 2.2 MHz. Unless a small
solution size is an ultimate goal, a moderate switching frequency of 500 kHz is selected to achieve both a small
solution size and a high efficiency operation. Using Figure 21, R1 is determined to be 383 kΩ
Output Inductor Selection
To calculate the value of the output inductor, use Equation 8. KIND is a coefficient that represents the amount of
inductor ripple current relative to the maximum output current. In general, KIND is normally from 0.1 to 0.3 for the
majority of applications.
For this design example, use KIND = 0.2 and the inductor value is calculated to be 3.6 µH. For this design, a
nearest standard value was chosen: 4.7 µH. For the output filter inductor, it is important that the RMS current
and saturation current ratings not be exceeded. The RMS and peak inductor current can be found from
Equation 9 and Equation 10.
(8)
(9)
(10)
(11)
Output Capacitor
There are two primary considerations for selecting the value of the output capacitor. The output capacitors are
selected to meet load transient and output ripple’s requirements.
Equation 12 gives the minimum output capacitance to meet the transient specification. For this example,
L
O
= 4.7 µH, ΔI
OUT
= 1.5 A – 0.75 A = 0.75 A and ΔV
OUT
= 120 mV. Using these numbers gives a minimum
capacitance of 18 µF. A standard 22-µF ceramic capacitor is chose in the design.
(12)
Equation 13 calculates the minimum output capacitance needed to meet the output voltage ripple specification.
Where fsw is the switching frequency, V
RIPPLE
is the maximum allowable output voltage ripple, and I
RIPPLE
is the
inductor ripple current. In this case, the maximum output voltage ripple is 30 mV. From Equation 9, the output
current ripple is 0.46 A. From Equation 13, the minimum output capacitance meeting the output voltage ripple
requirement is 1.74 µF.
(13)
Additional capacitance de-rating for aging, temperature and DC bias should influence this minimum value. For
this example, one 22-µF, 6.3-V X7R ceramic capacitor with 3 mΩ of ESR will be used.
24 Submit Documentation Feedback Copyright © 2010–2012, Texas Instruments Incorporated
Product Folder Links: TPS65251