Datasheet

L
Lmax outmax
ΔI
I = I +
2
( )
IN OUT
ON
V V
I = T
L
-
D ´
L
TPS62125
SLVSAQ5C –MARCH 2012–REVISED DECEMBER 2013 www.ti.com
OUTPUT FILTER DESIGN (INDUCTOR AND OUTPUT CAPACITOR)
The external components have to fulfill the needs of the application, but also the stability criteria of the devices
control loop. The TPS62125 is optimized to work within a range of L and C combinations. The LC output filter
inductance and capacitance have to be considered together, creating a double pole, responsible for the corner
frequency of the converter. Table 3 can be used to simplify the output filter component selection.
Table 3. Recommended LC Output Filter Combinations
Output Capacitor Value [µF]
(2)
Inductor Value
[µH]
(1)
10µF 2 x 10µF 22µF 47µF
V
OUT
1.2V - 1.8V
15 √ √ √ √
22 √
(3)
√ √ √
VOUT 1.8V - 3.3V
15 √
(3)
√ √ √
22 √
(3)
√ √ √
VOUT 3.3V - 5V
10 √ √ √
15 √
(3)
√
(3)
√
22
VOUT 5V - 10V
10 √
(3)
√
(3)
√
15 √ √ √
22 √ √ √
(1) Inductor tolerance and current de-rating is anticipated. The effective inductance can vary by 20% and -30%.
(2) Capacitance tolerance and bias voltage de-rating is anticipated. The effective capacitance can vary by 20% and -50%.
(3) This LC combination is the standard value and recommended for most applications.
More detailed information on further LC combinations can be found in application note SLVA515.
INDUCTOR SELECTION
The inductor value affects its peak-to-peak ripple current, the PWM-to-PFM transition point, the output voltage
ripple and the efficiency. The selected inductor has to be rated for its DC resistance and saturation current. The
inductor ripple current (ΔI
L
) decreases with higher inductance and increases with higher V
IN
or V
OUT
and can be
estimated according to Equation 13.
Equation 14 calculates the maximum inductor current under static load conditions. The saturation current of the
inductor should be rated higher than the maximum inductor current as calculated with Equation 14. This is
recommended because during heavy load transient the inductor current will rise above the calculated value. A
more conservative way is to select the inductor saturation current according to the high-side MOSFET switch
current limit I
LIMF
.
(13)
(14)
With:
T
ON
= see equation (3)
L = Inductance
ΔI
L
= Peak to Peak inductor ripple current
I
Lmax
= Maximum Inductor current
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