TPS54116-Q1 Datasheet, PDF (26/41 Pages) Texas Instruments – 2.95-V to 6-V Input, 4-A Step-Down Converter and 1-A Source/Sink DDR Termination Regulator

TPS54116-Q1
SLVSCO3A – AUGUST 2016 – REVISED AUGUST 2016
www.ti.com
The maximum switching frequency for a given application is limited by the minimum on-time of the converter and
is estimated with Equation 8. For this application with the maximum minimum on-time of 125 ns at no load and
5.25 V maximum input voltage the maximum switching frequency is 2.28 MHz. A switching frequency of 2.1 MHz
is selected to stay above the AM band. Equation 9 calculates R14 to be 26.8 kΩ. A standard 1% 26.7 kΩ value
was chosen in the design.
f_SW max
1 _u V_OUT
tonmin V_IN max
(8)
R_T k:
72540
f_SW kHz ^1.033
(9)
8.2.2.2 Output Inductor Selection
To calculate the value of the output inductor, use Equation 10. K_IND is a ratio that represents the amount of
inductor ripple current relative to the maximum output current. The inductor ripple current is filtered by the output
capacitor. Therefore, choosing high inductor ripple currents impacts the selection of the output capacitor since
the output capacitor must have a ripple current rating equal to or greater than the inductor ripple current.
Additionally the inductor current ripple is used as part of the PWM control system. Choosing small inductor ripple
currents can degrade the transient response performance or introduce jitter in the duty cycle. In general, the
inductor ripple value is at the discretion of the designer; however, K_IND is normally from 0.1 to 0.3 for the majority
of applications giving a peak to peak ripple current range of 0.4 A to 1.2 A. It is recommended to always keep the
peak to peak ripple current above 0.4 A because with a current mode control the inductor current ramp is used in
the PWM control system.
For this design example, K_IND = 0.3 is used and the inductor value is calculated to be 0.43 μH. The next standard
value 0.68 µH is selected. It is important that the RMS current and saturation current ratings of the inductor not
be exceeded. The RMS and peak inductor current can be found from Equation 12 and Equation 13. For this
design, the RMS inductor current is 4.0 A and the peak inductor current is 4.4 A. The chosen inductor is a WE
744373240068. It has a saturation current rating of 10.0 A (20% inductance loss) and a RMS current rating of 5.5
A (40 °C. temperature rise). The series resistance is 16.0 mΩ typical.
The current flowing through the inductor is the inductor ripple current plus the output current. During power up,
faults or transient load conditions, the inductor current can increase above the calculated peak inductor current
level calculated above. In transient conditions, the inductor current can increase up to the switch current limit of
the device. For this reason, the most conservative approach is to specify an inductor with a saturation current
rating equal to or greater than the switch current limit rather than the steady-state peak inductor current.
Additionally if a hard short on the output occurs in a fault condition the peak inductor current can exceed the
current limit and may reach up to 10 A. The peak current limit in this scenario is only limited by the minimum on-
time of the TPS54116-Q1 and the parasitic DC voltage drops in the circuit. The peak current during a hard short
will vary with the switching frequency and only exceeds the current limit when using the TPS54116-Q1 with
higher switching frequencies like 2.1 MHz. To protect the inductor in a hard output short the inductor should be
rated for this current.
L1 = ^{Vinmax - Vout} ´
Vout
Io ´ Kind
Vinmax ´ ¦sw
(10)
vertical spacer
Iripple = ^{Vinmax - Vout} ´
Vout
L1
Vinmax ´ ¦sw
(11)
vertical spacer
ILrms =
Io² + ¹
12
´
æ Vo ´ (Vinmax - Vo) ö²
çè _{Vinmax ´ L1 ´ ¦sw} ÷ø
(12)
vertical spacer
ILpeak = Iout + ^Iripple
2
(13)
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