Datasheet

p ´ ´
O O
1
P2 = type 1
2 R (C2 + C )
p ´ ´
O O
1
P2 = type 2b
2 R3 | | R C
p ´ ´
O O
1
P2 = type 2a
2 R3 | | R (C2 + C )
1
Z1
2 R3 C1
=
p ´ ´
TPS54040
SLVS918A –MARCH 2009–REVISED SEPTEMBER 2013 www.ti.com
DETAILED DESCRIPTION (continued)
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APPLICATION INFORMATION
Design Guide — Step-By-Step Design Procedure
This example details the design of a high frequency switching regulator design using ceramic output capacitors.
A few parameters must be known in order to start the design process. These parameters are typically determined
at the system level. For this example, start with the following known parameters:
Output Voltage 5 V
Transient Response 0 to 1.5A load step ΔVout = 4%
Maximum Output Current 0.5 A
Input Voltage 34 V nom. 12V to 42 V
Output Voltage Ripple 1% of Vout
Start Input Voltage (rising VIN) 8.9 V
Stop Input Voltage (falling VIN) 7.9 V
Selecting the Switching Frequency
The first step is to decide on a switching frequency for the regulator. Typically, the user will want to choose the
highest switching frequency possible since this will produce the smallest solution size. The high switching
frequency allows for lower valued 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 and the output voltage and the frequency shift limitation.
Equation 12 and Equation 13 must be used to find the maximum switching frequency for the regulator, choose
the lower value of the two equations. Switching frequencies higher than these values will result in pulse skipping
or the lack of overcurrent protection during a short circuit.
The typical minimum on time, t
onmin
, is 130 ns for the TPS54040. For this example, the output voltage is 5 V and
the maximum input voltage is 42 V, which allows for a maximum switch frequency up to 1012 kHz when including
the inductor resistance, on resistance and diode voltage in Equation 12. To ensure overcurrent runaway is not a
concern during short circuits in your design use Equation 13 or the solid curve in Figure 41 to determine the
maximum switching frequency. With a maximum input voltage of 42 V, assuming a diode voltage of 0.5 V,
inductor resistance of 130mΩ, switch resistance of 400mΩ, a current limit value of 0.94 A and a short circuit
output voltage of 0.1V. The maximum switching frequency is approximately 1055 kHz.
Choosing the lower of the two values and adding some margin a switching frequency of 700kHz is used. To
determine the timing resistance for a given switching frequency, use Equation 11 or the curve in Figure 39.
The switching frequency is set by resistor R3 shown in Figure 50.
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