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Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 553
Keywords: UPS, initerrupted power supply, backup power supply, power supplies
APPLICATION NOTE 553
5V, Non-Interruptible Power Supply Delivers 1A
Apr 30, 1999
Abstract: The 5V output of Figure 1 remains uninterrupted during loss of the main 5V supply. What's
more, it maintains a ±5% output tolerance while delivering 1A for 80 minutes thereafter.
The 5V output of Figure 1 remains uninterrupted during loss of the main 5V supply. What's more, it
maintains a ±5% output tolerance while delivering 1A for 80 minutes thereafter.
Figure 1. Despite brownouts or loss of the main 5V supply, this non-interruptible power supply maintains
the 5V output within ±5%. It supplies 1A at 5V for 80 minutes with the battery shown. During normal
operation, the main supply is monitored by the V
CC
terminal of the supervisory circuit (IC1). This chip
holds Q2 on by asserting RESET high. While on, Q2 holds the dc-dc controller (IC2) in shutdown mode,
turns on Q1, and turns on Q3, which routes a trickle charge to the battery stack.
When the main supply voltage drops below IC1's reset threshold (typically 4.65V), RESET quickly turns
off Q2 and Q3, bringing IC2 out of shutdown. IC2 then boosts the non-interruptible output back to 5V.
After the main supply dips below threshold, RESET remains low for 200ms whether or not the voltage
returns above threshold. This action assures an orderly completion of the switchover.
Q1 is a low-r
DS(ON)
, p-channel MOSFET that drops only 60mV at 1A. Its connections-drain to the main
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