ZHCSEF7G December 2014 – February 2019 TPS659037
PRODUCTION DATA.
As in every switch-mode-supply design, the following general layout rules apply:
The goal of the previously listed guidelines is a layout that minimizes emissions, maximizes EMI-immunity, and maintains a safe operating area for the device.
To minimize the spiking at the phase-node for both, high-side (VIN – SWx) as well as low-side (SWx – PGND), the decoupling of VIN is paramount. Appropriate decoupling and thorough layout should ensure that the spikes never exceed 7V across the high-side and low-side FETs.
TI recommends the guidelines shown in Figure 8-1 regarding parasitic inductance and resistance.
Table 8-1 lists the maximum allowable parasitic (inductance measured at 100 MHz) and the achievable values in an optimized layout.
CONNECTION | MAXIMUM ALLOWABLE INDUCTANCE | MAXIMUM ALLOWABLE RESISTANCE | OPTIMIZED LAYOUT (EVM) INDUCTANCE | OPTIMIZED LAYOUT (EVM) RESISTANCE | ||
---|---|---|---|---|---|---|
PowerPlane – CIN | N/A | N/A for SOA, keep small for efficiency | N/A | N/A for SOA, keep small for efficiency | ||
CIN – SMPSx_IN | 1 nH | 3 mΩ | SMPS1 | 0.533 nH | SMPS1 | 1.77 mΩ |
SMPS2 | 0.465 nH | SMPS2 | 1.22 mΩ | |||
SMPS3 | 0.494 nH | SMPS3 | 1.37 mΩ | |||
SMPS4 | 0.472 nH | SMPS4 | 1.23 mΩ | |||
SMPS5 | 0.517 nH | SMPS5 | 1.27 mΩ | |||
SMPS6 | 0.518 nH | SMPS6 | 1.69 mΩ | |||
SMPS7 | 0.501 nH | SMPS7 | 1.27 mΩ | |||
SMPS8 | 0.509 nH | SMPS8 | 1.42 mΩ | |||
SMPS9 | 0.491 nH | SMPS9 | 1.4 mΩ | |||
CIN – SMPSx_GND | 1 nH | 2 mΩ | SMPS1 | 0.552 nH | SMPS1 | 1.21 mΩ |
SMPS2 | 0.583 nH | SMPS2 | 0.8 mΩ | |||
SMPS3 | 0.668 nH | SMPS3 | 0.93 mΩ | |||
SMPS4 | 0.57 nH | SMPS4 | 0.81 mΩ | |||
SMPS5 | 0.577 nH | SMPS5 | 0.76 mΩ | |||
SMPS6 | 0.608 nH | SMPS6 | 1.13 mΩ | |||
SMPS7 | 0.646 nH | SMPS7 | 0.83 mΩ | |||
SMPS8 | 0.67 nH | SMPS8 | 0.73 mΩ | |||
SMPS9 | 0.622 nH | SMPS9 | 0.82 mΩ | |||
SMPSx_SW – Inductor | N/A | N/A for SOA, keep small for efficiency | N/A | SMPS1 | 1.9 mΩ | |
SMPS2 | 0.89 mΩ | |||||
SMPS3 | 1.99 mΩ | |||||
SMPS4 | 0.93 mΩ | |||||
SMPS5 | 1.37 mΩ | |||||
SMPS6 | 1.11 mΩ | |||||
SMPS7 | 1.17 mΩ | |||||
SMPS8 | 1.35 mΩ | |||||
SMPS9 | 0.88 mΩ | |||||
Inductor – COUT | N/A | N/A for SOA, keep small for efficiency | N/A | N/A for SOA, keep small for efficiency | ||
COUT – GND | Use dedicated GND plane to keep inductance low | mΩ | SMPS1 | 0.552 nH | SMPS1 | 1.21 mΩ |
SMPS2 | 0.583 nH | SMPS2 | 0.8 mΩ | |||
SMPS3 | 0.668 nH | SMPS3 | 0.93 mΩ | |||
SMPS4 | 0.57 nH | SMPS4 | 0.81 mΩ | |||
SMPS5 | 0.577 nH | SMPS5 | 0.76 mΩ | |||
SMPS6 | 0.608 nH | SMPS6 | 1.13 mΩ | |||
SMPS7 | 0.646 nH | SMPS7 | 0.83 mΩ | |||
SMPS8 | 0.67 nH | SMPS8 | 0.73 mΩ | |||
SMPS9 | 0.622 nH | SMPS9 | 0.82 mΩ | |||
GND(CIN) – GND(COUT) | Use dedicated GND plane to keep inductance low | mΩ | Use dedicated GND plane to keep inductance low | mΩ |
TI recommends to measure the voltages across the high-side FET (voltage at SMPSx_IN vs. SMPSx_SW) and the low-side FET (SMPSx_SW vs. SMPSx_GND) with a high-bandwidth high-sampling rate scope with a low-capacitance probe (ideally a differential probe). Measure the voltages as close as possible to the device-pins and verify the amplitude of the spikes. A small-loop-GND-connection to the closest accessible SMPSx_GND (of the particular rail) is essential. Ideally, this measurement should be performed during start-up of the respective SMPS-rail (to take in account the inrush-current) and at high temperature.
When measuring the voltage difference between the SMPSx_IN and SMPSx_SW pins, there should be a maximum of 7V when measuring at the pins. Similarly, when measuring the voltage difference between the SMPSx_SW and SMPSx_GND pins, there should be a maximum of 7V when measuring at the pins.
For more information on cursor-positioning, see Figure 8-2 and Figure 8-3.