TPS544B20_16 Datasheet, PDF (56/74 Pages) Texas Instruments – Synchronous Buck Converters

TPS544B20, TPS544C20
SLUSB69A – MAY 2014 – REVISED FEBRUARY 2016
www.ti.com
Including a high-frequency bypass capacitor is a lossless ringing reduction technique which helps minimizes the
outboard parasitic inductances in the power stage. These capacitors store energy during the low-side MOSFET
on-time, and discharge once the high-side MOSFET is turned on. For this example a 4.7-nF, 25-V, 0402 sized
high-frequency capacitor is selected. The placement of this capacitor (shown in Figure 46) is critical to its
effectiveness.
Additionally, an R-C snubber circuit is added to this example. To balance efficiency and spike levels, a 1-nF
capacitor and a 3-Ω resistor are chosen. In this example an 0805 resistor is chosen, which is rated for 0.125 W,
nearly twice the estimated power dissipation. Figure 33 and Figure 34 show the effect of the R-C snubber on the
rising edge of the SW pin. See SLUP100 for more information about snubber circuits.
VIN
VIN
SW
SW
V_IN = 12 V
I_OUT = 20 A
Snubber = 1nF + 3Ω
Figure 33. SW Rising Edge
V_IN = 12 V
I_OUT = 20 A
Snubber = Open
Figure 34. SW Rising Edge
9.2.2.9 Temperature Sensor
This application design uses a surface-mount MMBT3904SL for the temperature sensor, Q_T. In this example, the
sensor monitors the PCB temperature where it is generally the highest, next to the power inductor. Placement of
the temperature sensor and routing back to the TSNS pin are critical design features to reduce noise its
temperature measurements. In this example, the temperature sensor is placed on the V_OUT side of the power
inductor to avoid switching noise from the SW plane, and routed back to the TSNS and AGND pin. Additionally, a
330-pF capacitor, C_T, is placed from TSNS to AGND near the TSNS pin.
Disable external temperature sensing by terminating TSNS to AGND with a 0-Ω resistor. This termination forces
the temperature readings to –40 °C, and prevents external over-temperature fault trips.
9.2.2.10 Key PMBus Parameter Selection
Several of the key design parameters for the TPS544B20 and TPS544C20 device can be configured according
to the PMBus protocol, and stored to its non-volatile memory (NVM) for future use.
9.2.2.10.1 Enable, UVLO and Sequencing
Use the ON_OFF_CONFIG (02h) command to select the turn-on behavior of the converter. For this example, the
CNTL pin was used to enable or disable the converter, regardless of the state of OPERATION (01h), as long as
input voltage is present, and above the UVLO threshold.
The minimum input voltage, V_IN(min) , for this example is 4.5 V. The VIN_ON command was set to 4.25 V, and the
VIN_OFF command was set to 4.0 V, giving 250 mV of hysteresis. If VIN falls below VIN_OFF, power conversion
stops, until it is raised above VIN_ON.
This example lacks specific turn-on or turn-off delay requirements, so SEQUENCE_TON_TOFF_DELAY was
used to set both the turn-on and turn-off delays to 0 × the soft-start time, the delay between enabling power
conversion, and the rise of the output voltage is approximately 400 µs. See the Soft-Start section for more
information.
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