TPS54318 Datasheet, PDF (19/36 Pages) Texas Instruments – 2.95V To 6V Input 3A Output 2MHz Synchronous Step Down Switcher With Integrated FETs (SWIFT)

TPS54318
www.ti.com ......................................................................................................................................................................................... SLVS975 – SEPTEMBER 2009
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.
This design is available as the HPA512 evaluation module (EVM). A few parameters must be known in order to
start the design process. These parameters are typically determined on the system level. For this example, we
start with the following known parameters:
Output Voltage
Transient Response 1 to 2A load step
Maximum Output Current
Input Voltage
Output Voltage Ripple
Start Input Voltage (rising VIN)
Stop Input Voltage (falling VIN)
Switching Frequency (Fsw)
1.8 V
ΔV_OUT = 3%
3A
3.3 V nom. 3 V to 6 V
< 30 mV_PP
3.1 V
2.8 V
1000 kHz
SELECTING THE SWITCHING FREQUENCY
The first step is to decide on a switching frequency for the regulator. Typically, you want to choose the highest
switching frequency possible since this produces 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. However, the highest switching frequency causes extra switching losses, which hurt the converter’s
performance. The converter is capable of running from 200 kHz to 2 MHz. Unless a small solution size is an
ultimate goal, a moderate switching frequency of 1MHz is selected to achieve both a small solution size and a
high efficiency operation. Using Equation 5, R4 is calculated to be 180 kΩ. A standard 1% 182 kΩ value was
chosen in the design.
Figure 33. High Frequency, 1.8 V Output Power Supply Design with Adjusted UVLO
OUTPUT INDUCTOR SELECTION
The inductor selected works for the entire TPS54318 input voltage range. To calculate the value of the output
inductor, use Equation 18. K_IND is a coefficient that represents the amount of inductor ripple current relative to the
maximum output current. The inductor ripple current is filtered by the output capacitor. Therefore, choosing high
inductor ripple currents impacts the selection of the output capacitor since the output capacitor must have a
ripple current rating equal to or greater than the inductor ripple current. In general, the inductor ripple value is at
the discretion of the designer; however, K_IND is normally from 0.1 to 0.3 for the majority of applications.
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