MAX808LCSA View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX808LCSA

8-Pin µP Supervisory Circuits with ±1.5% Reset Accuracy

Maxim Integrated 

50Ω DRIVER

VRST(max)

VCC

MAX808

CE IN

CE OUT

GND

8-Pin µP Supervisory Circuits

with ±1.5% Reset Accuracy

50pF CLOAD

+5V

1N4148

0.47F

VCC

BATT

OUT

MAX801

MAX808

GND

Figure 7. MAX808 CE Gate Test Circuit

Figure 8. Using the MAX801/MAX808 with a SuperCap

Chip-Enable Output

In enabled mode, CE OUT’s impedance is equivalent to

75Ω in series with the source driving CE IN. In disabled

mode, the 75Ω transmission gate is off and CE OUT is

actively pulled to the higher of VCC or VBATT. The

source turns off when the transmission gate is enabled.

__________Applications Information

The MAX801/MAX808 are not short-circuit protected.

Shorting OUT to ground, other than power-up transients

such as charging a decoupling capacitor, may destroy

the device. If long leads connect to the IC’s inputs,

ensure that these lines are free from ringing and other

conditions that would forward bias the IC’s protection

diodes. Bypass OUT, VCC, and BATT with 0.1µF

capacitors to GND.

The MAX801/MAX808 operate in two distinct modes:

1) Normal Operating Mode, with all circuitry powered

up. Typical supply current from VCC is 68µA (48µA

for the MAX808), while only leakage currents flow

from the battery.

2) Battery-Backup Mode, where VCC is below VBATT

and VRST. The supply current from the battery is typ-

ically less than 1µA.

Using SuperCaps™ or MaxCaps™

with the MAX801/MAX808

BATT has the same operating voltage range as VCC, and

the battery-switchover threshold voltage is typically

VBATT when VCC is decreasing or VBATT + 0.05V when

VCC is increasing. This hysteresis allows use of a

SuperCap (e.g., around 0.47F) and a simple charging

circuit as a backup source (Figure 8). Since VBATT can

exceed VCC while VCC is above the reset threshold, no

special precautions are needed when using these µP

supervisors with a SuperCap.

Backup-Battery Replacement

The backup battery can be disconnected while VCC is

above the reset threshold, provided BATT is bypassed

with a 0.1µF capacitor to ground. No precautions are

necessary to avoid spurious reset pulses.

Negative-Going VCC Transients

While issuing resets to the µP during power-up, power-

down, and brownout conditions, these supervisors are

relatively immune to short-duration, negative-going VCC

transients (glitches). It is usually undesirable to reset

the µP when VCC experiences only small glitches.

The Typical Operating Characteristics show a graph of

Maximum Transient Duration vs. Reset Threshold

Overdrive, for which reset pulses are not generated.

The graph was produced using negative-going VCC

pulses, starting at 5V and ending below the reset

threshold by the magnitude indicated (reset compara-

tor overdrive). The graph shows the maximum pulse

width that a negative-going VCC transient may typically

have without causing a reset pulse to be issued. As the

amplitude of the transient increases (i.e., goes farther

below the reset threshold), the maximum allowable

pulse width decreases. Typically, a VCC transient that

goes 40mV below the reset threshold and lasts for 3µs

or less will not cause a reset pulse to be issued. A

0.1µF bypass capacitor mounted close to the VCC pin

provides additional transient immunity.

______________________________________________________________________________________ 11

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