Q: Why do we need to follow the recommended power sequence in the datasheet?
What will happen if the sequence is not followed?
A: The recommended power supply sequence specify the correct order to power the devices. This is done for multiple reasons such as,
- Unexpected glitches: Some pins are powered unintentionally, and this could power internal blocks, and in turn, may cause some pins to have an unknown state, hence, glitches may occur.
- Communication problems: The device do not respond to commands.
- Inaccurate outputs: Without proper initialization, the part may not be initialized correctly generating discrepancies between the expected output and the measured output.
- Undesirable voltages on pins: This is primarily caused when the ESD protection diode is turned-on and supply other pins.
- Device latch-up: There are rare and unusual cases that any pin enters into a latch-up condition.
Q: What is the recommended power sequence for DACs?
A: Some datasheets indicate the recommended power sequence but for datasheets wherein it is not specified, the rule of thumb is in sequential order:
- Ground (digital and analog)
- Digital supply
- Control digital pins (e.g. RSTSEL)
- Analog supplies (AVdd and AVss)
- Interface lines
Q: How is it internally initialized in the device?
A: the POR (power-on reset) initialize the circuit in a known state after the supplies are applied.
During the rise time of the supplies, the POR's RESET output is enabled by default, the RESET only gets de-asserted when the supplies are high enough to ensure proper operation of the internal blocks. In this manner, the POR circuit's purpose is to make sure that the supply for the digital block is high enough to properly start the initialization process of the device.
More information can be found on this discussion about power-up and power-down events of semiconductor devices.
Q: What will happen if one the device's pins is greater than the digital and/or the analog supply? Will it harm the device?
A: Most pins of a device have an ESD protection diode connected to the supplies. Ideally, these diodes are only expected to turn-on or conduct when there are electrical surges on a pin, thus, limiting the voltage on the pin to a diode drop above the positive supply or below the negative supply.
However, there are cases wherein a pin (let's say a digital pin) is biased before the analog and digital supplies are present, if this happens, the ESD protection diode conducts and the supply pins of the device are powered.
Given that, the device may be powered-up unintentionally, worse of all, all other devices electrically connected to the supply pins may also be powered-up and this should be avoided as it can cause the following:
1) Damage the ESD protection diodes when the current is high and the duration is long enough: This happens when the system’s required supply current is high or if there is a low impedance load connected to the supply. And the output impedance of the LDO must be considered when it has no input and when it’s on standby mode.
2) Unexpected behavior of the circuit or system: Unintentional power-up of the device and other devices in the circuit may cause some misbehavior, as the supply may be not high enough to guarantee proper operation.
3) Damaged device: If the supply impedance is low, the current may be high enough to cause damage to the device connected to the I/O pin where the current is sourced from.
Q: What is the difference between the supply/reference voltage ranges in the specification table of the datasheet versus the absolute maximum rating?
A: The supply and reference voltages indicated in the specification table ensures that the device is both functional and the parameters are within the limits as specified in the table. It is recommended to operate within these voltage ranges, as it is the supply that guarantee reliability on the device.
If these voltages exceed the recommended range, but less than what is specified in the absolute maximum rating, then the device is still functional but it is not guaranteed that the parameters are still within the specified limits, and some internal degradation such an electro migration may occur, reducing the life span of the device.
Lastly, if these voltages are equal or exceeds the absolute maximum rating the device can be damaged.