User's Manual
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| Coyote (BL2500) User's Manual |
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Appendix B. Power Supply
Appendix B describes the power circuitry provided on the Coyote.
B.1 Power Supplies
Power is supplied to the Coyote via the friction-lock connector terminal at J2. The Coyote has an onboard +5 V switching power regulator from which a +3.3 V linear regulator draws its supply. Thus both +5 V and +3.3 V are available. The Coyote is protected against reverse polarity by a diode at D1 as shown in Figure B-1.
The input voltage range is from 8 V to 40 V DC.
There is provision on the printed-circuit board for a transorb to be installed at TVS1 in parallel with C1 to provide suppression for positive noise pulses above 51 V. This part is only needed when the Coyote will be used in industrial environments where a clean source of power cannot be guaranteed, and is not part of the normal factory build.
B.2 Batteries and External Battery Connections
The SRAM and the real-time clock have battery backup. Power to the SRAM and the real-time clock (VRAM) on the Coyote's RabbitCore module is provided by two different sources, depending on whether the main part of the Coyote is powered or not. When the Coyote is powered normally, and Vcc is within operating limits, the SRAM and the real-time clock are powered from Vcc. If power to the board is lost or falls below 2.93 V, the VRAM and real-time clock power will come from the battery. The reset generator circuit controls the source of power by way of its /RESET output signal.
A soldered-in 1000 mA·h lithium battery provides power to the real-time clock and SRAM when external power is removed from the circuit board. The drain on the battery is less than 10 µA when there is no external power applied to the Coyote, and so the expected shelf life of the battery is more than
The drain on the battery is typically less than 4 µA when external power is applied, and so the expected battery in-service life is
Since the nominal shelf life of the lithium battery is 10-20 years, the in-service life should not be of concern.
B.2.1 Power to VRAM Switch
The VRAM switch on the Coyote's RabbitCore module, shown in Figure B-2, allows the battery backup to provide power when the external power goes off. The switch provides an isolation between Vcc and the battery when Vcc goes low. This prevents the Vcc line from draining the battery.
The field-effect transistor provides a very small voltage drop between Vcc and VRAM (<100 mV, typically 10 mV) so that the board components powered by Vcc will not have a significantly different voltage than VRAM.
When the Coyote is not in reset, the RESOUT line will be high. This allows VRAM to nearly equal Vcc.
When the Coyote is in reset, the RESOUT line will go low. This provides an isolation between Vcc and VRAM.
B.2.2 Reset Generator
The Coyote's RabbitCore module uses a reset generator to reset the Rabbit 3000 microprocessor when the voltage drops below the voltage necessary for reliable operation. The reset typically occurs at 2.93 V (2.63 V for the BL2510).
B.3 Chip Select Circuit
The current drain on the battery in a battery-backed circuit must be kept at a minimum. When the Coyote is not powered, the battery keeps the SRAM memory contents and the real-time clock (RTC) going. The SRAM has a powerdown mode that greatly reduces power consumption. This powerdown mode is activated by raising the chip select (CS) signal line. Normally the SRAM requires Vcc to operate. However, only 2 V is required for data retention in powerdown mode. Thus, when power is removed from the circuit, the battery voltage needs to be provided to both the SRAM power pin and to the CS signal line. The CS control circuit accomplishes this task for the SRAM's chip select signal line.
B.4 Power to Peripheral Cards
DCIN and Vcc are available on friction-lock connector terminals J7 and J8 to power peripheral cards that may be used with the Coyote.
Keep in mind that the Coyote draws 377 mA from the Vcc supply, and that the diode at D1 (shown in Figure B-1) can handle at most 1 A at VRAW, so that leaves the remaining current capacity to be shared among the DCIN and Vcc pins on friction-lock connector terminals J7 and J8. Table B-1 lists the available current at DCIN based on the current drawn at Vcc.
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