TPS62746_15 Datasheet, PDF (12/25 Pages) Texas Instruments – TPS62746 300 mA High Efficiency Buck Converter with Ultra-low Quiescent Current and VIN Switch

TPS62746
SLVSD27A – JUNE 2015 – REVISED AUGUST 2015
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10.2.2 Detailed Design Procedure
The first step in the design procedure is the selection of the output filter components. To simplify this process,
Table 3 outlines possible inductor and capacitor value combinations.
Inductor Value
[µH] ⁽¹⁾
2.2
Table 3. Recommended LC Output Filter Combinations
Output Capacitor Value [µF]⁽²⁾
4.7µF
√
10µF
√ ⁽³⁾
22µF
√
47µF
√
(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 varies by +20% and –50%.
(3) Typical application configuration. Other check marks indicate alternative filter combinations.
100µF
10.2.2.1 Inductor Selection
The inductor value affects the 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 1.
Equation 2 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 2. This is
recommended because during a heavy load transient the inductor current rises 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.
Vout
1-
DI_L = Vout ´
Vin
L´ ¦
(1)
I_Lmax
=
I_outmax
+
DI_L
2
where
• f = Switching Frequency
• L = Inductor Value
• ΔI_L= Peak to Peak inductor ripple current
• I_Lmax = Maximum Inductor current
(2)
The table below shows a list of possible inductors.
Table 4. List of Possible Inductors⁽¹⁾
INDUCTANCE [µH]
2.2
2.2
2.2
2.2
DIMENSIONS
[mm³]
2.0 x 1.6 x 1.0
2.0 × 1.25 × 1.0
2.0 x 1.2 x 1.0
1.6 x 0.8 x 0.8
INDUCTOR TYPE
DFE201610C
MIPSZ2012D 2R2
744 797 752 22
MDT1608-
CH2R2M
Isat/DCR
1.4 A/170 mΩ
0.7 A/230 mΩ
0.7 A/200 mΩ
0.7 A/300 mΩ
(1) See Third-party Products Disclaimer
SUPPLIER
TOKO
FDK
Würth Elektronik
TOKO
Comment
Efficiency plot
Figure 12
10.2.2.2 Output Capacitor Selection
The DCS-Control™ scheme of the TPS62746 allows the use of tiny ceramic capacitors. Ceramic capacitors with
low ESR values have the lowest output voltage ripple and are recommended. The output capacitor requires
either an X7R or X5R dielectric. At light load currents, the converter operates in Power Save Mode and the
output voltage ripple is dependent on the output capacitor value. A larger output capacitors can be used reducing
the output voltage ripple. The leakage current of the output capacitor adds to the overall quiescent current.
12
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