LTC1235

9

1235fa

APPLICATIONS INFORMATION

Voltage Output

During normal operation, the LTC1235 uses a charge

pumped NMOS power switch to achieve low dropout and

low supply current. This power switch can deliver up to

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

ential) is desired, the LTC1235 provides BATT ON output

to drive the base of external PNP transistor (Figure 3).

Another alternative to provide higher current is to connect

pin to supply the extra current.

the battery which can damage lithium batteries. LTC1235

uses a charge pumped NMOS power switch to eliminate

unwanted charging current while achieving low dropout

and low supply current. Since no current goes to the

junction leakage.

Conditional Battery Backup

backup function when primary power is lost. Disabling

the battery backup function is useful in conserving the

backup battery’s life when the SRAM doesn’t need battery

backup during long term storage of a computer system,

or delivery of the computer system to the end user.

The BACKUP pin (Pin 8) is used to serve this feature on

age threshold, the status of the BACKUP pin (logic low

or logic high) is stored in the Memory Logic (see Block

comparator, C2, shuts off the NMOS power switch, M1. M2

is designed for very low dropout voltage (input-to-output

differential). This feature is advantageous for low current

applications such as battery backup in CMOS RAM and

other low power CMOS circuitry. If the stored status is

in Battery Saving Mode (see Figure 4). The supply current

in both mode is 1μA maximum.

tery is replaced (with power off), the status stored in the

Battery Backup Mode or in Battery Saving Mode. When

C2 has typically 20mV of hysteresis to prevent spurious

status stored in the Memory Logic is high. The response

time of C2 is approximately 20μs.

The LTC1235 is protected for safe area operation with short

circuit limit. Output current is limited to approximately

200mA. If the device is overloaded for a long period of

time, thermal shutdown turns the power switch off until

the device cools down. The threshold temperature for

thermal shutdown is approximately 155°C with about 10°C

of hysteresis which prevents the device from oscillating

in and out of shutdown.

The PNP switch was not chosen for the internal power

switch because it injects unwanted current into the

competitive devices and adds to the charging current of

Figure 3. Using BATT ON to Drive External PNP Transistor

+5V

+3V

0.1μF

0.1μF

LTC1235

GND

ANY PNP POWER TRANSISTOR

1235 F03

BATT ON

R1