TPS54561-Q1_15 Datasheet, PDF (31/51 Pages) Texas Instruments – TPS54561-Q1 4.5-V to 60-V Input, 5-A, Step-Down DC-DC Converter With Eco-mode™

www.ti.com
TPS54561-Q1
SLVSC60 – SEPTEMBER 2014
Sizing of the output capacitor must be such as to absorb energy stored in the inductor when transitioning from a
high to low load current. The catch diode of the regulator cannot sink current, so energy stored in the inductor
can produce an output voltage overshoot when the load current rapidly decreases. Figure 49 shows a typical
load-step response. The excess energy absorbed in the output capacitor increases the voltage on the capacitor.
Sizing of the capacitor must be such as to maintain the desired output voltage during these transient periods.
Equation 36 calculates the minimum capacitance required to keep the output voltage overshoot to a desired
value, where L_(O) is the value of the inductor, I_OH is the output current under heavy load, I_OL is the output under
light load, V_P is the peak output voltage, and V_(int) is the initial voltage. For this example, the worst-case load step
is from 3.75 A to 1.25 A. The output voltage increases during this load transition, and the stated maximum in our
specification is 4% of the output voltage. This makes V_(P) = 1.04 × 5 V = 5.2 V. V_(int) is the initial capacitor voltage
which is the nominal output voltage of 5 V. Using these numbers in Equation 36 yields a minimum capacitance of
44.1 µF.
Equation 37 calculates the minimum output capacitance needed to meet the output-voltage ripple specification,
where f_(SW) is the switching frequency, V_O(RIPPLE) is the maximum allowable output voltage ripple, and I_O(RIPPLE) is
the inductor ripple current. Equation 37 yields 19.9 µF.
Equation 38 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple
specification. Equation 38 indicates the ESR should be less than 15.7 mΩ.
The most stringent criterion for the output capacitor is 62.5 µF, required to maintain the output voltage within
regulation tolerance during a load transient.
Capacitance de-ratings for aging, temperature, and dc bias increase this minimum value. For this example, the
selection is three 47-µF, 10-V ceramic capacitors with 5 mΩ of ESR. The derated capacitance is 87.4 µF, well
above the minimum required capacitance of 62.5 µF.
Capacitors generally have a maximum ripple-current rating. Filtering a ripple current equal to or below that
maximum ripple current does not degrade capacitor reliability. Some capacitor data sheets specify the root-
mean-square (rms) value of the maximum ripple current. Use Equation 39 to calculate the rms ripple current that
the output capacitor must support. For this example, Equation 39 yields 459 mA.
C_(O) >
2 ´ DI_O
f_{(SW )} ´ DV_O
=
2 ´ 2.5 A
400 kHz ´ 0.2
V
= 62.5
μF
(35)
(⁽ )⁾ (^{( )}) ^{( ( ) ) C(O)}
>
L_(O) ´
æçè _IOH
æçè _V(P)
2_-
2_-
I_OL ^{2 ö÷ø}
V_(int) ^{2 ö÷ø}
=
7.2
mH ´
3.75 A² - 1.25 A²
5.2 V² - 5 V²
= 44.1 mF
(36)
C_(O) >
1
8 ´ f_{(SW )}
´
1
æ
ççè
V_O(RIPPLE)
I_O(RIPPLE)
ö
÷÷ø
=
8
´
1
400
kHz
´
1
æ
çè
25 mV
1.591 A
ö
÷ø
= 19.9 mF
(37)
R_(ESR) <
V_O(RIPPLE)
I_O(RIPPLE)
25 mV
=
1.591 A
= 15.7 mW
(38)
( ) _{I(CO)RMS =}
V_O ´ V_I min - V_O
=
12 ´ V_I min ´ L_(O)´ f_{(SW )}
5 V ´ (60 V - 5 V)
= 459 mA
12 ´ 60 V ´ 7.2 mH ´ 400 kHz
(39)
8.2.2.4 Catch Diode
The TPS54561-Q1 device requires an external catch diode between the SW pin and GND. The selected diode
must have a reverse voltage rating equal to or greater than maximum input voltage. The peak current rating of
the diode must be greater than the maximum inductor current. Schottky diodes are typically a good choice for the
catch diode because of the low forward voltage of these diodes. The lower the forward voltage of the diode, the
higher the efficiency of the regulator.
Typically, diodes with higher voltage and current ratings have higher forward voltages. A diode with a minimum of
60-V reverse voltage is preferable, to allow input voltage transients up to the rated voltage of the TPS54561-Q1
device.
Copyright © 2014, Texas Instruments Incorporated
Product Folder Links: TPS54561-Q1
Submit Documentation Feedback
31