__________Applications Information
These µP supervisory circuits are not short-circuit pro-
tected. Shorting V
OUT
to ground, excluding power-up
transients such as charging a decoupling capacitor,
destroys the device. Decouple both V
CC
and BATT
pins to ground by placing 0.1µF ceramic capacitors as
close to the device as possible.
Driving an External Switch with BATT ON
BATT ON can be directly connected to the base of a
PNP transistor or the gate of a PMOS transistor. The
PNP connection is straightforward: connect the emitter
to V
CC
, the collector to OUT, and the base to BATT ON
(Figure 14a). No current-limiting resistor is required, but
a resistor connecting the base of the PNP to BATT ON
can be used to limit the current drawn from V
CC
, pro-
longing battery life in portable equipment.
If you are using a PMOS transistor, however, it must be
connected backwards from the traditional method.
Connect the gate to BATT ON, the drain to V
CC
, and
the source to OUT (Figure 14b). This method orients
the body diode from V
CC
to OUT and prevents the
backup battery from discharging through the FET when
its gate is high. Two PMOS transistors in the Siliconix
LITTLE FOOT
®
series are specified with V
GS
down to
-2.7V. The Si9433DY has a maximum 100mΩ drain-
source on-resistance with 2.7V of gate drive and a 2A
drain-source current. The Si9434DY specifies a 60mΩ
drain-source on-resistance with 2.7V of gate drive and
a 5.1A drain-source current.
Using a Super Cap as a Backup
Power Source
Super caps are capacitors with extremely high capaci-
tance values (e.g., order of 0.47F) for their size. Figure
15 shows two ways to use a super cap as a backup
power source. The super cap can be connected
through a diode to the 3V input (Figure 15a); or, if a 5V
supply is also available, the super cap can be charged
up to the 5V supply (Figure 15b), allowing a longer
backup period. Since V
BATT
can exceed V
CC
while
V
CC
is above the reset threshold, there are no special
precautions when using these µP supervisors with a
super cap.
Operation without a
Backup Power Source
These µP supervisors were designed for battery-
backed applications. If a backup battery is not used,
connect BATT, OUT, and V
CC
together, or use a differ-
ent µP supervisor.
Replacing the Backup Battery
The backup power source can be removed while V
CC
remains valid, without danger of triggering a reset
pulse, provided that BATT is decoupled with a 0.1µF
capacitor to ground. As long as V
CC
stays above the
reset threshold, battery-backup mode cannot be
entered.
MAX793/MAX794/MAX795
3.0V/3.3V Adjustable Microprocessor
Supervisory Circuits
______________________________________________________________________________________ 15
CE IN
High impedance
CE OUT
Pulled to BATT
RESET
Logic low
BATT Connected to OUT
LOWLINE
Logic low
RESET Pulled up to V
CC
WDO
Logic low
WDI Disabled
BATT OK Logic low
PFO
Logic low
MR
Disabled, but still pulled up to V
CC
PFI Disabled
V
CC
Disconnected from OUT
BATT ON Pulled up to BATT
OUT
Connected to BATT through an internal
140Ω switch
PIN NAME STATUS
Table 1. Input and Output Status in
Battery-Backup Mode
