Mic ro p ro c e s s o r S u p e rvis o ry Circ u it
MAX791
Leakage current through the SuperCap charging diode
a nd MAX791 inte rna l p owe r d iod e e ve ntua lly d is -
V
IN
+5V
charges the SuperCap to V . Also, if V and VBATT
CC
CC
start from 0.5V above the reset threshold and power is
R1
R2
V
los t a t V , the Sup e rCa p on VBATT d is c ha rg e s
CC
CC
through V
until VBATT reaches the reset threshold;
CC
PFI
the MAX791 then switches to battery-backup mode and
the current through V goes to zero (Figure 10).
CC
C1*
R3
Us in g S e p a ra t e P o w e r S u p p lie s
MAX791
GND
fo r VBATT a n d V
CC
If using separate power supplies for V
and VBATT,
CC
PFO
VBATT must be less than 0.3V above V when V is
CC
CC
above the reset threshold. As described in the previous
section, if VBATT exceeds this limit and power is lost at
TO µP
*OPTIONAL
V
CC
, current flows continuously from VBATT to V via
CC
the VBATT-to-V
until the circuit is broken (Figure 10).
diode and the V
-to-V
switch
+5V
PFO
0V
OUT
OUT
CC
Alt e rn a t ive Ch ip -En a b le Ga t in g
0V
V
L
V
V
V
TRIP H
IN
Using memory devices with CE and CE inputs allows
the MAX791 CE loop to be bypassed. To do this, con-
nect CE IN to ground, pull up CE OUT to V
R1 + R2
R2
V
TRIP
= 1.25
, and
OUT
c onne c t CE OUT to the CE inp ut of e a c h me mory
device (Figure 13). The CE input of each part then con-
nects directly to the chip-select logic, which does not
have to be gated by the MAX791.
R2 || R3
R1 + R2 R3
VL - 1.25 5 - 1.25 1.25
V = 1.25 /
H
+
=
R1
R3
R2
||
Figure 14. Adding Hysteresis to the Power-Fail Comparator
Ad d in g Hys t e re s is t o t h e
P o w e r-Fa il Co m p a ra t o r
Us in g S u p e rCa p s o r Ma x Ca p s
w it h t h e MAX7 9 1
Hysteresis adds a noise margin to the power-fail com-
parator and prevents repeated triggering of PFO when
VIN is near the power-fail comparator trip point. Figure
14 shows how to add hysteresis to the power-fail com-
parator. Select the ratio of R1 and R2 such that PFI
sees 1.25V when VIN falls to the desired trip point
VBATT has the same operating voltage range as V
,
CC
and the battery-switchover threshold voltages are typi-
c a lly ± 30mV c e nte re d a t VBATT, a llowing us e of a
SuperCap and a simple charging circuit as a backup
source (Figure 12).
(V
). Resistor R3 adds hysteresis. It will typically be
TRIP
an order of magnitude greater than R1 or R2. The cur-
re nt throug h R1 a nd R2 s hould b e a t le a s t 1µA to
ensure that the 25nA (max) PFI input current does not
shift the trip point. R3 should be larger than 10kΩ to
prevent it from loading down the PFO pin. Capacitor C1
adds additional noise rejection.
If V
is above the reset threshold and VBATT is 0.5V
CC
above V , current flows to V
until the voltage at VBATT is less than 0.5V above V
For example, with a SuperCap connected to VBATT
and through a diode to V , if V quickly changes
and V from VBATT
CC
OUT
CC
.
CC
CC
CC
from 5.4V to 4.9V, the capacitor discharges through
a nd V until VBATT re a c he s 5.3V typ ic a l.
V
OUT
CC
______________________________________________________________________________________ 15
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
