Datasheet

TPS54060
www.ti.com SLVS919A –JANUARY 2009–REVISED JULY 2010
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, we will start with the following known parameters:
Output Voltage 3.3 V
Transient Response 0 to 1.5A load step ΔVout = 4%
Maximum Output Current 0.5 A
Input Voltage 34 V nom. 12V to 48 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 TPS54060. For this example, the output voltage is 3.3 V
and the maximum input voltage is 48 V, which allows for a maximum switch frequency up to 616 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 48 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 923 kHz.
Choosing the lower of the two values and adding some margin a switching frequency of 500kHz 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 R
3
shown in Figure 50.
Figure 50. High Frequency, 3.3V Output Power Supply Design with Adjusted UVLO.
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