MAX793 Maxim, MAX793 Datasheet - Page 15

• Manufacturer Part Number: MAX793
• Description: 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
• Manufacturer: Maxim

These µP supervisory circuits are not short-circuit pro-
tected. Shorting V
transients such as charging a decoupling capacitor,
destroys the device. Decouple both V
pins to ground by placing 0.1µF ceramic capacitors as
close to the device as possible.
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
Figure 13. Battery Switchover Timing
Table 1. Input and Output Status in
Battery-Backup Mode
__________Applications Information
V
V
V
Driving an External Switch with BATT ON
CC
OUT
BATT
PIN NAME
LOWLINE
BATT ON
BATT OK
CE OUT
= 3.6V
RESET
RESET
CE IN
WDO
BATT
OUT
PFO
V
WDI
PFI
MR
CC
V
RST
3.6V
______________________________________________________________________________________
OUT
Connected to BATT through an internal
140Ω switch
Disconnected from OUT
Pulled up to BATT
Logic low
Disabled
Logic low
Disabled, but still pulled up to V
Logic low
Disabled
Logic low
Pulled up to V
Connected to OUT
Logic low
High impedance
Pulled to BATT
to ground, excluding power-up
3.0V/3.3V Adjustable Microprocessor
3.3V
CC
STATUS
3.6V
V
SW
CC
3.3V
and BATT
CC
to V
(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
prolonging 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
the source to OUT (Figure 14b). This method orients
the body diode from V
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
-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.
SuperCaps™ are capacitors with extremely high
capacitance values (e.g., order of 0.47F) for their size.
Figure 15 shows two ways to use a SuperCap as a
backup power source. The SuperCap can be connect-
ed through a diode to the 3V input (Figure 15a); or, if a
5V supply is also available, the SuperCap can be
charged up to the 5V supply (Figure 15b), allowing a
longer backup period. Since V
while V
special precautions when using these µP supervisors
with a SuperCap.
These µP supervisors were designed for battery-
backed applications. If a backup battery is not used,
connect BATT, OUT, and V
ent µP supervisor. See the µP Supervisory Circuits
table at the end of this data sheet.
The backup power source can be removed while V
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
reset threshold, battery-backup mode cannot be
entered.
™ LITTLE FOOT is a trademark of Siliconix Inc.
SuperCap is a trademark of Baknor Industries.
CC
Supervisory Circuits
, the collector to OUT, and the base to BATT ON
CC
is above the reset threshold, there are no
Using a SuperCap™ as a Backup
Replacing the Backup Battery
CC
Backup Power Source
CC
to OUT and prevents the
Operation without a
together, or use a differ-
BATT
CC
Power Source
can exceed V
stays above the
GS
CC
down to
, and
CC
CC
CC
15
,