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LTC690/LTC691

LTC694/LTC695

690fe

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

APPLICATIONS INFORMATION

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 VCCremainsnearlyequaltoVBATT.

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

fromVOUT,oralowerdropout(VCC-VOUTvoltagedifferential)

is desired, the LTC691 and LTC695 should be used. These

products provide BATT ON output to drive the base of

VCC

t1 = RESET ACTIVE TIME

V1 = RESET VOLTAGE THRESHOLD

V2 = RESET VOLTAGE THRESHOLD +

RESET THRESHOLD HYSTERESIS

690 F01

RESET

LOW LINE

Figure 1. Reset Active Time

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