10.1 Layout Guidelines

Consider these layout guidelines before starting a layout work using TPS548D21.

• It is absolutely critical that all GND pins, including AGND (pin 30), DRGND (pin 29), and PGND (pins 13, 14, 15, 16, 17, 18, 19, and 20) are connected directly to the thermal pad underneath the device via traces or plane.
• Include as many thermal vias as possible to support a 40-A thermal operation. For example, a total of 35 thermal vias are used (outer diameter of 20 mil) in the TPS548D21EVM-784 available for purchase at ti.com. (SLUUBG3)
• Placed the power components (including input/output capacitors, output inductor and TPS548D21device) on one side of the PCB (solder side). Insert at least two inner layers (or planes) connected to the power ground, in order to shield and isolate the small signal traces from noisy power lines.
• Place the VIN pin decoupling capacitors as close as possible to the PVIN and PGND pins to minimize the input AC current loop. Place a high-frequency decoupling capacitor (with a value between 1 nF and 0.1 µF) as close to the PVIN pin and PGND pin as the spacing rule allows. This placement helps suppress the switch node ringing.
• Place VDD and BP decoupling capacitors as close to the device pins as possible. Do not use PVIN plane connection for the VDD pin. Separate the VDD signal from the PVIN signal by using separate trace connections. Provide GND vias for each decoupling capacitor and make the loop as small as possible.
• Ensure that the PCB trace defined as switch node (which connects the SW pins and up-stream of the output inductor) are as short and wide as possible. In the TPS548D21EVM-784 EVM design, the SW trace width is 200 mil. Use a separate via or trace to connect SW node to snubber and bootstrap capacitor. Do not combine these connections.
• Place all sensitive analog traces and components (including VOSNS, RSP, RSN, ILIM, MODE, VSEL and FSEL) far away from any high voltage switch node (itself and others), such as SW and BOOT to avoid noise coupling. In addition, place MODE, VSEL and FSEL programming resistors near the device pins.
• The RSP and RSN pins operate as inputs to a differential remote sense amplifier that operates with very high impedance. It is essential to route the RSP and RSN pins as a pair of diff-traces in Kelvin-sense fashion. Route them directly to either the load sense points (+ and –) or the output bulk capacitors. The internal circuit uses the VOSNS pin for on-time adjustment. It is critical to tie the VOSNS pin directly tied to VOUT (load sense point) for accurate output voltage result.

10.2 Layout Example

Figure 25. EVM Top View

Figure 27. EVM Inner Layer 1

Figure 29. EVM Inner Layer 3

Figure 31. EVM Bottom Layer

Figure 26. EVM Top Layer

Figure 28. EVM Inner Layer 2

Figure 30. EVM Inner Layer 4

10.2.1 Mounting and Thermal Profile Recommendation

Proper mounting technique adequately covers the exposed thermal tab with solder. Excessive heat during the reflow process can affect electrical performance. Figure 32 shows the recommended reflow oven thermal profile. Proper post-assembly cleaning is also critical to device performance. See the Application Report, QFN/SON PCB Attachment, (SLUA271) for more information.

Figure 32. Recommended Reflow Oven Thermal Profile

Table 9. Recommended Thermal Profile Parameters

PARAMETER		MIN	TYP	MAX	UNIT
RAMP UP AND RAMP DOWN
rRAMP(up)	Average ramp-up rate, TS(max) to TP			3	°C/s
rRAMP(down)	Average ramp-down rate, TP to TS(max)			6	°C/s
PRE-HEAT
TS	Pre-heat temperature	150		200	°C
tS	Pre-heat time, TS(min) to TS(max)	60		180	s
REFLOW
TL	Liquidus temperature		217		°C
TP	Peak temperature			260	°C
tL	Time maintained above liquidus temperature, TL	60		150	s
tP	Time maintained within 5°C of peak temperature, TP	20		40	s
t25P	Total time from 25°C to peak temperature, TP			480	s