TPS54531_15 Datasheet, PDF (18/32 Pages) Texas Instruments – TPS54531 5-A, 28-V Input, Step-Down SWIFT™ DC-DC Converter With Eco-mode™

TPS54531
SLVSBI5A – MAY 2013 – REVISED OCTOBER 2014
www.ti.com
-G_PWRSTG
R3 = ^{10 20}
´ ^VOUT
gm_ea
V_ref
(17)
To maximize phase gain, the compensator zero is placed one decade below the crossover frequency of 20 kHz.
Use Equation 18 to calculate the required value for C6.
1
C6 =
2 × p × R3 × ^FCO
10
(18)
To maximize phase gain the high frequency pole is placed one decade above the crossover frequency of 20 kHz.
The pole can also be useful to offset the ESR of aluminum electrolytic output capacitors. Use Equation 19 to
calculate the value for C7.
1
C7 =
2 × p × R3 ×10 × F_CO
(19)
For this design, the calculated values are as follows:
R3 = 37.4 kΩ
C6 = 2200 pF
C7 = 22 pF
9.2.2.7 Bootstrap Capacitor
Every TPS54531 design requires a bootstrap capacitor, C4. The bootstrap capacitor value must be 0.1 μF. The
bootstrap capacitor is located between the PH and BOOT pins. The bootstrap capacitor should be a high-quality
ceramic type with X7R or X5R grade dielectric for temperature stability.
9.2.2.8 Catch Diode
The TPS54531 device sis designed to operate using an external catch diode between the PH and GND pins.
The selected diode must meet the absolute maximum ratings for the application. The reverse voltage must be
higher than the maximum voltage at the PH pin, which is V_IN(MAX) + 0.5 V. Peak current must be greater than
I_O(MAX) plus on half the peak-to-peak inductor current. The forward-voltage drop should be small for higher
efficiencies. The catch diode conduction time is (typically) longer than the high-side FET on time, so attention
paid to diode parameters can make a marked improvement in overall efficiency. Additionally, check that the
selected device is capable of dissipating the power losses. For this design, a CDBC540-G was selected, with a
reverse voltage of 40 V, forward current of 5 A, and a forward-voltage drop of 0.55 V.
9.2.2.9 Slow-Start Capacitor
The slow-start capacitor determines the minimum amount of time required for the output voltage to reach the
nominal programmed value during power up which is useful if a load requires a controlled voltage slew rate. The
slow-start capacitor is also used if the output capacitance is very large and requires large amounts of current to
quickly charge the capacitor to the output voltage level. The large currents necessary to charge the capacitor
may make the TPS54531 device reach the current limit. Excessive current draw from the input power supply may
cause the input voltage rail to sag. Limiting the output voltage slew rate solves both of these problems. Use
Equation 3 to calculate the value of the slow-start capacitor. For the example circuit, the slow-start time is not too
critical because the output capacitor value is 2 × 47 μF which does not require much current to charge to 5 V.
The example circuit has the slow-start time set to an arbitrary value of 4 ms which requires a 10-nF capacitor.
For the TPS54531 device, I_SS is 2 µA and V_ref is 0.8 V.
18
Submit Documentation Feedback
Product Folder Links: TPS54531
Copyright © 2013–2014, Texas Instruments Incorporated