ASM690A/692A

ASM802L/802M

April 2008

ASM805L

rev 1.7

µP Power Supply Supervisor With Battery Backup Switch

5 of 15

Notice: The information in this document is subject to change without notice.

Table 2: Maximum Battery Voltages

Part Number

MAXIMUM Battery Voltage (V)

ASM690A

4.80

ASM802L

4.80

ASM805L

4.80

ASM692A

4.55

ASM802M

4.55

Although most batteries that meet the requirements of

Table2

are acceptable, lithium batteries are very effective

backup source due to their high-energy density and very

low selfdischarge rates.

Battery replacement while Powered

Batteries can be replaced even when the device is in a

used, the pin should be grounded.

Backup Power Sources - SuperCap™

Capacitor storage, with very high values of capacitance,

can be used as a back-up power source instead of

the fractional farad range. A 0.1 farad SuperCap™ would

provide a useful backup power source. Like the battery

supply, it is important that the capacitor voltage remain

below the maximum voltages shown in Table 2. Although

the circuit of Figure 4 shows the most simple way to

connect the SuperCap™, this circuit cannot insure that an

over voltage condition will not occur since the capacitor

voltage condition does not occur, the circuit of Figure 5 is

preferred. In this circuit configuration, the diode-resistor

pair clamps the capacitor voltage at one diode drop below

the ASM692A/802M or within ±10% of 5V for the

ASM690A/802L/805L to insure that the storage capacitor

does not achieve an over voltage state.

Operation without a Backup Power Source

When operating without a back-up power source, the

drop due to the ON-resistance of the PMOS switch.