TPS54360-Q1 Datasheet, PDF (21/42 Pages) Texas Instruments – 4.5-V to 60-V Input, 3.5-A, Step-Down DC-DC Converter with Eco-mode

TPS54360-Q1
www.ti.com
APPLICATION INFORMATION
SLVSBZ2 – SEPTEMBER 2013
Design Guide — Step-By-Step Design Procedure
This guide illustrates the design of a high frequency switching regulator using ceramic output capacitors. A few
parameters must be known in order to start the design process. These requirements are typically determined at
the system level. Calculations can be done with the aid of WEBENCH or the excel spreadsheet (slvc452) located
on the product page. This example is designed to the following known parameters:
Output Voltage
Transient Response 0.875 A to 2.625 A load step
Maximum Output Current
Input Voltage
Output Voltage Ripple
Start Input Voltage (rising VIN)
Stop Input Voltage (falling VIN)
5V
ΔV_OUT = 4 %
3.5 A
12 V nom. 8.5 V to 60 V
0.5% of V_OUT
8V
6.25 V
Selecting the Switching Frequency
The first step is to choose a switching frequency for the regulator. Typically, the designer uses the highest
switching frequency possible because this produces the smallest solution size. High switching frequency allows
for lower value inductors and smaller output capacitors compared to a power supply that switches at a lower
frequency. The switching frequency that can be selected is limited by the minimum on-time of the internal power
switch, the input voltage, the output voltage and the frequency foldback protection.
Equation 7 and Equation 8 must be used to calculate the upper limit of the switching frequency for the regulator.
Choose the lower value result from the two equations. Switching frequencies higher than these values results in
pulse skipping or the lack of overcurrent protection during a short circuit.
The typical minimum on time, t_onmin, is 135 ns for the TPS54360-Q1. For this example, the output voltage is 5 V
and the maximum input voltage is 60 V, which allows for a maximum switch frequency up to 710 kHz to avoid
pulse skipping from Equation 7. To ensure overcurrent runaway is not a concern during short circuits use
Equation 8 to determine the maximum switching frequency for frequency foldback protection. With a maximum
input voltage of 60 V, assuming a diode voltage of 0.7 V, inductor resistance of 25 mΩ, switch resistance of 92
mΩ, a current limit value of 4.7 A and short circuit output voltage of 0.1 V, the maximum switching frequency is
902 kHz.
For this design, a lower switching frequency of 600 kHz is chosen to operate comfortably below the calculated
maximums. To determine the timing resistance for a given switching frequency, use Equation 5 or the curve in
Figure 6. The switching frequency is set by resistor R₃ shown in Figure 34. For 600 kHz operation, the closest
standard value resistor is 162 kΩ.
f_{SW(max skip)} =
1
135ns
´
æ 3.5 A x 25 mW + 5 V + 0.7 V ö
ç
è
60
V
-
3.5 A x 92 mW
+
0.7
V
÷
ø
=
710 kHz
(23)
f_SW(shift) =
8
135 ns
´
æ 4.7 A x 25 mW + 0.1 V + 0.7 V ö
ç
è
60 V
-
4.7 A x 92 mW
+
0.7 V
÷
ø
=
902 kHz
(24)
RT (kW) =
92417
600 (kHz)^0.991
= 163 kW
(25)
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