LTC2924 A Simple and Compact Power Sequencing Solution
Many complex systems such as telecommunications equipment, storage modules, optical systems, network equipment, servers, and base stations employ FPGAs and other digital ICs that require multiple voltage rails. These voltage rails must be started and shut down in a specific order. Otherwise, the IC will be damaged.
The LTC2924 is a simple and compact power sequencing solution in a 16-pin SSOP package (see Figure 1 and Figure 2).
Figure 1: Typical Application Using External N-Channel MOSFETs
Figure 2: Four Power Sequencers with Shutdown Pins
working principle
With a single LTC2924, it is easy to perform 4 power sequencing tasks, and it is also easy to cascade multiple LTC2924s to order an unlimited number of power supplies. With a slight drop in the allowable functionality, six power supplies can be sequenced using a single LTC2924.
The LTC2924 controls the start and stop sequence of the four supplies and the ramp rate through the output pins (OUT1 to OUT4). Each OUT pin uses a 10μA current source connected to an internal charge pump and a low resistance switch connected to GND. This combination gives these outputs enough flexibility to connect them directly to multiple power shutdown pins or external N-channel MOSFETs.
The LTC2924 monitors the output voltage of each sequenced power supply through four input pins (IN1 to IN4). These inputs use precision comparators and a trimmed bandgap voltage reference to provide better than 1% accuracy. The 4-channel power-up and power-down voltage thresholds are set using resistive dividers. The power-on and power-off thresholds for each channel can be individually selected.
The LTC2924's timer pin (TMR) is used to provide an optional delay between the completion of a power supply startup and the start of the next power supply. The delay time is selected by placing a capacitor between the TMR pin and ground (delay = 200ms/μF), and floating the TMR pin will remove any delay. The start delay and stop delay can be different. Figure 3 shows a simple circuit with a shutdown delay that is half the startup delay.
Fig. 3: Setting different ON/OFF delays
The LTC2924 also includes a Power Good Timer (PGT). Whenever a power supply is enabled, the LTC2924 will start the PGT. If either of the power supplies fails to reach its specified nominal voltage within the allotted time interval, a power-on failure is detected.
in conclusion
The LTC2924 is suitable for numerous power sequencing and monitoring applications. The LTC2924-based power supply sequencing solution consumes minimal board space due to the minimal number of external components required and the 16-pin narrow SSOP package.
The power enable pin does not require a designer configuration, but it is still versatile enough to directly drive the shutdown pin or external N-channel MOSFET. The soft start of the power supply can be achieved simply by adding a capacitor. If you need to sort more than four power supplies, you can cascade multiple LTC2924s to order an almost infinite number of power supplies. Adapting the LTC2924 to a specific application requires no software, and the design can be fine-tuned during system integration simply by changing the resistance and capacitance values. Due to its simple design, low component cost, and small footprint, the LTC2924 is an excellent choice for power sequencing and monitoring applications.
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