General
Review and verify the following for the custom schematic design:
- Above sections, including relevant application notes and FAQ links
- Pin attributes, signal description and electrical specifications
- Electrical characteristics and additional available information
- Implementation of on-board supply or provision provided to connect external supply
- An LDO is recommended (use of FET switch or load switch is not recommended or allowed)
- Choose on-board LDO that supports a minimum of 400mA current, has excellent load current transient response, and quick output discharge (active discharge)
- Required bulk and bypass capacitors are provided (follow EVM schematics)
- On-board LDO has provision to be enabled by processor IO
- When external supply is connected, add bulk and decoupling capacitor provision on the processor board near to the processor VPP pin and provided a TP to connect the external supply
- External supply follows the recommended power sequence and slew rate requirements as per the data sheet
- The external supply timing is controlled by the processor IO
- Leave the processor VPP supply pin floating (HiZ) or grounded during power-up sequences, power-down sequences, and normal device operation
Schematic Review
Follow the below list for the custom schematic design:
- A dedicated LDO or PMIC output is used
- Nominal voltage connected to VPP is 1.8V and supports current requirements as per data sheet requirements
- Selected LDO specifications including load current transient response is similar to the LDO used on the EVM schematics
- Processor IO is used to control the EN of the LDO and the required pull is provided
- Verify the if EN pull holds the LDO is in off-state during power cycling
- When an adjustable LDO is used, verify the output voltage configuration, output voltage slew and use of over voltage protection
- Series resistor is provided to isolate the processor VPP supply from the LDO output for testing the timing or LDO output
- Supply rail connected follows the processor ROC
Additional
- Always provide provision on the processor board to connect VPP supply (on-board or external supply).
- Select an LDO with fast transient response and connect LDO output to the processor VPP pin with a low loop inductance path to source the high transient load current, where the VPP pin never drops below the minimum operating voltage.
- Enable the VPP only during eFuse programming. Connecting the VPP supply to a continuous 1.8V supply rail is not a recommended or allowed or supported option.
- Due to the transient load current requirement during eFuse programming, using load switch or FET switch is not a recommended approach. A load switch or FET switch is likely to have too much voltage drop that is not compensated when using an LDO.
- If the use case requires use of load switch or FET switch, characterize the board by measuring the voltage on the processor VPP pin during programming and verify supply never drops below the ROC minimum limit. Several variables in the path of VPP can cause the supply to be out of the ROC when using load switch or FET and are required to be characterized before implementing. Check or test if the load switch or FET switch violates the maximum VPP supply slew rate limit (of 6000V per second) defined in the data sheet.