LTC693 LINER [Linear Technology], LTC693 Datasheet - Page 8

• Manufacturer Part Number: LTC693
• Description: Microprocessor Supervisory Circuits
• Manufacturer: LINER [Linear Technology]

Microprocessor Reset
The LTC692/LTC693 use a bandgap voltage reference and
a precision voltage comparator C1 to monitor the 5V
supply input on V
falls below the reset voltage threshold, the RESET output
is forced to active low state. The reset voltage threshold
accounts for a 10% variation on V
becomes active low when V
typical). On power-up, the RESET signal is held active low
for a minimum of 140ms after reset voltage threshold is
reached to allow the power supply and microprocessor to
stabilize. The reset active time is adjustable on the LTC693.
On power-down, the RESET signal remains active low
even with V
microprocessor in stable shutdown condition. Figure 1
shows the timing diagram of the RESET signal.
The precision voltage comparator, C1, typically has 40mV
of hysteresis which ensures that glitches at the V
not activate the RESET output. Response time is typically
10 s. To help prevent mistriggering due to transient loads,
V
leads trimmed as short as possible.
The LTC693 has two additional outputs: RESET and LOW
LINE. RESET is an active high output and is the inverse of
RESET. LOW LINE is the output of the precision voltage
comparator C1. When V
threshold, LOW LINE goes low. LOW LINE returns high as
soon as V
LTC692/LTC693
A
8
CC
PPLICATI
pin should be bypassed with a 0.1 F capacitor with the
CC
CC
rises above the reset voltage threshold.
as low as 1V. This capability helps hold the
LOW LINE
O
RESET
CC
V
U
CC
(see BLOCK DIAGRAM). When V
S
CC
I FOR ATIO
U
falls below the reset voltage
CC
V2
falls below 4.50V (4.40V
CC
t1
, so the RESET output
W
Figure 1. Reset Active Time
CC
U
pin do
V1
CC
V2
Battery Switchover
The battery switchover circuit compares V
V
When V
switchover comparator, C2, connects V
a charge pumped NMOS power switch, M1. When V
falls to 50mV above V
through a PMOS switch, M2. C2 has typically 20mV of
hysteresis to prevent spurious switching when V
remains nearly equal to V
is approximately 20 s.
During normal operation, the LTC692/LTC693 use a charge
pumped NMOS power switch to achieve low dropout and
low supply current. This power switch can deliver up to
50mA to V
of 5 . The V
of 0.1 F or greater to ensure stability. Use of a larger
bypass capacitor is advantageous for supplying current to
heavy transient loads.
When operating currents larger than 50mA are required
from V
ential) is desired, the LTC693 should be used. This prod-
uct provides BATT ON output to drive the base of the
external PNP transistor (Figure 2). If higher currents are
needed with the LTC692, a high current Schottky diode
can be connected from the V
supply the extra current.
BATT
t1 = RESET ACTIVE TIME
V1 = RESET VOLTAGE THRESHOLD
V2 = RESET VOLTAGE THRESHOLD +
RESET THRESHOLD HYSTERESIS
input, and connects V
OUT
t1
CC
, or a lower dropout (V
OUT
rises to 70mV above V
OUT
from V
pin should be bypassed with a capacitor
CC
BATT
and has a typical “on” resistance
BATT
, C2 connects V
OUT
. The response time of C2
CC
CC
V1
to whichever is higher.
pin to the V
LTC692/3 • F01
– V
OUT
BATT
OUT
to V
voltage differ-
, the battery
OUT
CC
OUT
CC
to V
through
to the
pin to
BATT
CC
CC