TPS54040A_15 Datasheet, PDF (39/52 Pages) Texas Instruments – 0.5 A, 42 V Step Down DC/DC Converter with Eco-mode™

TPS54040A
www.ti.com
SLVSB58A – MARCH 2012 – REVISED JANUARY 2014
Layout
Layout is a critical portion of good power supply design. There are several signals paths that conduct fast
changing currents or voltages that can interact with stray inductance or parasitic capacitance to generate noise
or degrade the power supplies performance. To help eliminate these problems, the VIN pin should be bypassed
to ground with a low ESR ceramic bypass capacitor with X5R or X7R dielectric. Care should be taken to
minimize the loop area formed by the bypass capacitor connections, the VIN pin, and the anode of the catch
diode. See Figure 64 for a PCB layout example. The GND pin should be tied directly to the power pad under the
IC and the power pad.
The power pad should be connected to any internal PCB ground planes using multiple vias directly under the IC.
The PH pin should be routed to the cathode of the catch diode and to the output inductor. Since the PH
connection is the switching node, the catch diode and output inductor should be located close to the PH pins,
and the area of the PCB conductor minimized to prevent excessive capacitive coupling. For operation at full rated
load, the top side ground area must provide adequate heat dissipating area. The RT/CLK pin is sensitive to noise
so the RT resistor should be located as close as possible to the IC and routed with minimal lengths of trace. The
additional external components can be placed approximately as shown. It may be possible to obtain acceptable
performance with alternate PCB layouts, however this layout has been shown to produce good results and is
meant as a guideline.
Vout
Vin
UVLO
Adjust
Resistors
Topside
Ground
Area
Input
Bypass
Capacitor
Output
Capacitor
Route Boot Capacitor
Trace on another layer to
provide wide path for
topside ground
BOOT
VIN
EN
SS/TR
RT/CLK
Catch
Diode
PH
GND
COMP
VSENSE
PWRGD
Output
Inductor
Compensation
Network
Resistor
Divider
Slow Start
Capacitor
Frequency
Set Resistor
Thermal VIA
Signal VIA
Figure 64. PCB Layout Example
Estimated Circuit Area
The estimated printed circuit board area for the components used in the design of Figure 51 is 0.55 in². This area
does not include test points or connectors.
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