LTC690 查看數據表（PDF） - Linear Technology
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LTC690 Datasheet PDF : 18 Pages
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LTC690/LTC691
LTC694/LTC695
APPLICATIONS INFORMATION
Microprocessor Reset
The LTC690 family uses a bandgap voltage reference and a
precision voltage comparator C1 to monitor the 5V supply
input on VCC (see Block Diagram). When VCC falls below
the reset voltage threshold, the RESET output is forced
to active low state. The reset voltage threshold accounts
for a 5% variation on VCC, so the RESET output becomes
active low when VCC falls below 4.75V (4.65V 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 active low
even with VCC as low as 1V. This capability helps 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 VCC pin do
not activate the RESET output. Response time is typically
10μs. To help prevent mistriggering due to transient loads,
VCC pin should be bypassed with a 0.1μF capacitor with
the 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 VCC falls
below the reset voltage threshold, LOW⎯LINE goes low.
LOW⎯LINE returns high as soon as VCC rises above the
reset voltage threshold.
Battery Switchover
The battery switchover circuit compares VCC to the VBATT
input, and connects VOUT to whichever is higher. When
VCC rises to 70mV above VBATT, the battery switchover
comparator, C2, connects VOUT to VCC through a charge
pumped NMOS power switch, M1. When VCC falls to 50mV
above VBATT, C2 connects VOUT to VBATT through a PMOS
switch, M2. C2 has typically 20mV of hysteresis to prevent
spurious switching when VCC remains nearly equal to VBATT.
The response time of C2 is approximately 20μs.
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 VOUT from VCC and has a typical on resistance of
5Ω. The VOUT pin should be bypassed with a capacitor of
0.1μF 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 VOUT, or a lower dropout (VCC-VOUT voltage differential)
is desired, the LTC691 and LTC695 should be used. These
products provide BATT ON output to drive the base of
V2
VCC
V1
V2
V1 = RESET VOLTAGE THRESHOLD
V1
V2 = RESET VOLTAGE THRESHOLD +
RESET THRESHOLD HYSTERESIS
RESET
t1
t1
t1 = RESET ACTIVE TIME
LOW LINE
690 F01
Figure 1. Reset Active Time
690fe
9
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