LMC6953 Datasheet, PDF (9/19 Pages) National Semiconductor (TI) – PCI Local Bus Power Supervisor

LMC6953
www.ti.com
APPLICATION NOTE
SNVS132D – APRIL 1998 – REVISED APRIL 2013
HOW THE LMC6953 FUNCTIONS
The LMC6953 is a power supply supervisor with its performance specifications compliant to PCI Specifications
Revision 2.1. The chip monitors power-up, power-down, brown-out, power failure and manual reset interrupt
situations.
During power-up, the LMC6953 holds RESET low for 100 ms after both 5V and 3.3V are within specified
windows. It asserts reset in 490 ns when a brown-out is detected. Brown-out occurs when 5V supply is above
5.75V over-voltage or below 4.25V under-voltage or when 3.3V supply is above 4.1V over-voltage or 2.5V under-
voltage. In case of power failure where the 5V supply falls under 3.3V supply by 300 mV maximum, reset is
asserted in 90 ns. RESET also can be asserted by sending a 5V CMOS logic low to the manual reset pin.
Each time RESET is asserted, it holds low for 100 ms after a fault condition is recovered. The 100 ms reset
delay is generated by the 0.01 μF C_EXT capacitor, and can be adjusted by changing the value of C_EXT.
It is highly recommended to place lands on printed circuit boards for 120 pF capacitors between pin 2 and
ground and also between pin 3 and ground. As power supplies may change abruptly, there can be very high
frequency noise present and the capacitors can minimize the noise,
MINIMUM SUPPLY VOLTAGE FOR RESET ASSERTION
The LMC6953 specifies V_DD = 1.55V as the minimum supply voltage to achieve consistent RESET assertion.
This ensures system stability in initialization state.
Figure 18. Output Voltage vs Supply Voltage
Figure 18 is measured by shorting pins 1, 2 and 3 together when supply voltage is from 0V to 3.3V. Then pin 3 is
connected with a constant 3.3 V_DC and pins 1 and 2 are connected to a separate power supply that continues to
vary from 3.3V to 6V.
5V AND V_DD PINS
By having the 5V and the V_DD pins separate, a capacitor can be used as a back-up power supply in event of a
sudden power supply failure. This circuit is shown in Figure 22. Under normal condition, the diode is forward-
biased and the capacitor is charged up to V_DD − 0.7V. If the power supply goes away, the diode becomes
reverse-biased, isolating the 5V and the V_DD pins. The capacitor provides power to the internal comparators for a
short duration for the LMC6953 to operate.
C_EXT SETS RESET DELAY IN LINEAR FASHION
The LMC6953 has internal delay circuitry to generate the reset delay. By choosing different values of capacitor
C_EXT, reset delay can be programmed to the desired length for the system to stabilize after a fault condition
occurs.
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