TPS62065-Q1 Datasheet, PDF (14/29 Pages) Texas Instruments – TPS6206x-Q1 3-MHz 2-A Step-Down Converter in 2 SON Package

TPS62065-Q1, TPS62067-Q1
SLVSCM3 – JANUARY 2015
www.ti.com
Typical Application (continued)
To minimize the current through the feedback divider network, R₂ should be within the range of 120 kΩ to 360
kΩ. The sum of R₁ and R₂ should not exceed ~1 MΩ, to keep the network robust against noise. An external feed-
forward capacitor C_ff is required for optimum regulation performance. Lower resistor values can be used. R₁ and
C_ff places a zero in the loop. The right value for C_ff can be calculated as:
1
f_z = _{2 ´ p ´ R1 ´ Cff} = 25 kHz
(3)
1
C_ff = _{2 ´ p ´ R1 ´ 25 kHz}
(4)
11.2.2.2 Output Filter Design (inductor And Output Capacitor)
The internal compensation network of TPS62065-Q1 and TPS62067-Q1 is optimized for a LC output filter with a
corner frequency of:
( ) ^{fC =}
1
= 50 kHz
2 ´ p ´ 1 µH ´ 10 µF
(5)
The part operates with nominal inductors of 1 µH to 1.2 µH and with 10-µF to 22-µF small X5R and X7R ceramic
capacitors. Please refer to the lists of inductors and capacitors. The part is optimized for a 1-µH inductor and 10-
µF output capacitor.
11.2.2.2.1 Inductor Selection
The inductor value has a direct effect on the ripple current. 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_I or V_O.
Equation 6 calculates the maximum inductor current in PWM mode under static load conditions. The saturation
current of the inductor should be rated higher than the maximum inductor current as calculated with Equation 7.
This is recommended because during heavy load transient the inductor current rises above the calculated value.
1 - ^VOUT
DI_L = V_OUT ´
V_IN
L´f
where
• ΔI_L = Peak-to-peak inductor ripple current
• L = Inductor value
• f = Switching frequency (3-MHz typical)
(6)
I_{L max}
= I_{OUT max}
+ ^DIL
2
where
• I_Lmax = Maximum inductor current
(7)
A more conservative approach is to select the inductor current rating just for the switch current limit I_LIMF of the
converter.
The total losses of the coil have a strong impact on the efficiency of the DC/DC conversion and consist of both
the losses in the DC resistance R_(DC) and the following frequency-dependent components:
• The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies)
• Additional losses in the conductor from the skin effect (current displacement at high frequencies)
• Magnetic field losses of the neighboring windings (proximity effect)
14
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