LTC690 Linear Technology, LTC690 Datasheet - Page 8

• Manufacturer Part Number: LTC690
• Description: Microprocessor Supervisory Circuits
• Manufacturer: Linear Technology

LTC690/LTC691
LTC694/LTC695
A
Microprocessor Reset
The LTC690 family uses a bandgap voltage reference and
a precision voltage comparator C1 to monitor the 5V
supply input on V
below the reset voltage threshold, the RESET output is
forced to active low state. The reset voltage threshold
accounts for a 5% variation on V
becomes active low when V
typical). On power-up, the RESET signal is held active low
for a minimum of 35ms for the LTC690/LTC691 (140ms
for the LTC694/LTC695) after reset voltage threshold is
reached to allow the power supply and microprocessor to
stabilize. The reset active time is adjustable on the LTC691/
LTC695. On power-down, the RESET signal remains ac-
tive low even with V
hold the 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 V
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 LTC691 and LTC695 have 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
8
CC
PPLICATI
pin should be bypassed with a 0.1 F capacitor with the
LOW LINE
O
CC
RESET
U
CC
V
(see Block Diagram). When V
CC
S
as low as 1V. This capability helps
I FOR ATIO
U
CC
V2
falls below 4.75V (4.65V
CC
t
, so the RESET output
1
W
CC
CC
falls below
Figure 1. Reset Active Time
pin do not
U
CC
falls
V1
the reset voltage threshold, LOW LINE goes low. LOW
LINE returns high as soon as V
voltage threshold.
Battery Switchover
The battery switchover circuit compares V
input, and connects V
V
comparator, C2, connects V
pumped NMOS power switch, M1. When V
50mV above V
PMOS switch, M2. C2 has typically 20mV of hysteresis to
prevent spurious switching when V
equal to V
20 s.
During normal operation, the LTC690 family uses a charge
pumped NMOS power switch to achieve low dropout and
low supply current. This power switch can deliver up to
50mA to V
5 . The V
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
tial) is desired, the LTC691 and LTC695 should be used.
These products provide BATT ON output to drive the base
V2
CC
rises to 70mV above V
t
V1 = RESET VOLTAGE THRESHOLD
V2 = RESET VOLTAGE THRESHOLD +
1
OUT
= RESET ACTIVE TIME
t
RESET THRESHOLD HYSTERESIS
1
BATT
OUT
, or a lower dropout (V
OUT
. The response time of C2 is approximately
pin should be bypassed with a capacitor of
BATT
from V
, C2 connects V
CC
OUT
and has a typical on resistance of
to whichever is higher. When
BATT
OUT
CC
CC
, the battery switchover
to V
V1
OUT
-V
rises above the reset
CC
OUT
CC
to V
690 F01
through a charge
voltage differen-
remains nearly
CC
BATT
to the V
CC
through a
falls to
BATT