Power Sequencing and Ramp-Rate Control in a Mixed-Signal PSC
With increasing complexity of board-level power supply management and the need for adjustable power-up sequencing and ramp-rates, designers need a flexible system management master.
Generating and maintaining the appropriate power environment is critical to proper system operation. Designs with FPGAs, DSPs, and ASICs can demand four, five, or even more power supplies to power up in a prescribed sequence and ramp-rate, in order to avoid issues such as latch-up, inrush current, or I/O contention. Fusion, with its analog quad, analog I/O structure, and analog-to-digital converter, represents a smart, simple, and flexible solution for power sequencing and ramp-rate control. Power Sequencing may require several different approaches:
|
Ramp-Rate Control
Parameters: For a given power supply voltage, Ig and Cgd define how soon it can ramp up to its full scale, based on the following: dV/dt = Ig/Cgd Ig = Drive strength of the Fusion gate driver, or source/sink current on the gate, selectable in Fusion from 1 μA, 3 μA, 10 μA, and 30 μA. Cgd = Parasitic capacitor between gate and drain of the power MOSFET transistor. |
Example
In this example, the supply voltage
is on before time 0. When the
Fusion gate is turned on, the
Voltage on the gate slews until Vt
is reached; then the load side will
show the smooth ramp-rate of
5 V/ms from 0 to to target, based
on the calculation below.
dV/dt = Ig/Cgd = 10 ľA/2 nF
= 5 V/ms
Once the target voltage is reached, the gate slews to the final value determined by the following equation: Vgs = Ig x Rpullup = 3 V
The selection of the MOSFET must be done with care and knowledge of system requirements. Care must be taken also when selecting Rpullup. If the Rpullup is too large, then the Vgs rating of the MOSFET could be exceeded, leading to catastrophic failure.
Fusion for power sequencing and ramp-rate control. Fusion not only provides the ability to control power sequencing and ramp-rate, but the parameters are programmable and reprogrammable in-system, to allow for system adjustment or calibration during test. With temperature and current monitoring and nonvolatile memory (NVM), the Fusion device is capable of full system management, data logging, and remote communication. Refer to the webcast for more information on Fusion capabilities.
