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MAX807NCWE Datasheet(PDF) 14 Page - Maxim Integrated Products

Part No. MAX807NCWE
Description  Full-Featured uP Supervisory Circuit with /-1.5% Reset Accuracy
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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BATT OK Output
The BATT OK comparator monitors the backup battery
voltage, comparing it with a 2.265V reference (VCC
4V). BATT OK remains high as long as the backup bat-
tery voltage remains above 2.265V, signaling that the
backup battery has sufficient voltage to maintain the
memory of static RAM. When the battery voltage drops
below 2.265V, the BATT OK output drops low, signaling
that the backup battery needs to be changed.
Applications Information
The MAX807 is 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 condi-
tions that would forward bias the IC’s protection diodes.
There are two distinct modes of operation:
1) Normal Operating Mode, with all circuitry powered
up. Typical supply current from VCC is 70µA, 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 MAX807
BATT has the same operating voltage range as VCC, and
the battery-switchover threshold voltage is typically
VBATT when VCC is decreasing or VBATT + 0.06V when
VCC is increasing. This hysteresis allows use of a
SuperCap (e.g., order of 0.47F) and a simple charging
circuit as a backup source (Figure 13). Since VBATT can
exceed VCC while VCC is above the reset threshold,
there are no special precautions when using these µP
supervisors with a SuperCap.
Alternative Chip-Enable Gating
Using memory devices with CE and CE inputs allows
the MAX807 CE loop to be bypassed. To do this, con-
nect CE IN to ground, pull up CE OUT to OUT, and
connect CE OUT to the CE input of each memory
device (Figure 14). The CE input of each part then con-
nects directly to the chip-select logic, which does not
have to be gated by the MAX807.
Adding Hysteresis to the
Power-Fail Comparator
The power-fail comparator has a typical input hystere-
sis of 20mV. This is sufficient for most applications
where a power-supply line is being monitored through
an external voltage-divider (Figure 10).
Figure 15 shows how to add hysteresis to the power-fail
comparator. Select the ratio of R1 and R2 such that PFI
sees 2.265V when VIN falls to the desired trip point
(VTRIP). Resistor R3 adds hysteresis. It will typically be
an order of magnitude greater than R1 or R2. The cur-
rent through R1 and R2 should be at least 1µA to
ensure that the 25nA (max) PFI input current does not
shift the trip point. R3 should be larger than 10k
Ω to
prevent it from loading down the PFO pin. Capacitor C1
adds additional noise rejection.
Full-Featured µP Supervisory Circuit with
±1.5% Reset Accuracy
14
______________________________________________________________________________________
Figure 13. SuperCap or MaxCap on BATT
MAX807
0.47F
1N4148
+5V
VCC
GND
BATT
OUT
Figure 14. Alternate CE Gating
MAX807
OUT
GND
CE IN
CE
CE
CE OUT
CE
CE
CE
CE
CE
CE
*MAXIMUM Rp VALUE DEPENDS ON
THE NUMBER OF RAMs.
MINIMUM Rp VALUE IS 1k
Ω.
ACTIVE-HIGH CE
LINES FROM LOGIC
RAM 1
RAM 2
RAM 3
RAM 4
Rp*




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