12

LTC2901

2901is sn2901

Pull-Up Current vs V2 curve). Output rise time (10% to

90%) is estimated using:

The on-resistance as a function of the V2 voltage at room

temperature is estimated using:

R

V

610

21

5

•

–

capacitance, the rise time is 86

µs. If the output needs to

pull up faster and/or to a higher voltage, a smaller

external pull-up resistor may be used. Using a 10k pull-

up resistor, the rise time is reduced to 3.3

µs for a 150pF

load capacitance.

The LTC2901-2 has an active pull-up to V2 on the RST

output. The typical performance curve (RST Pull-Up Cur-

rent vs V2 curve) demonstrates that the pull-up current is

mated to be approximately 625

Ω. A 150pF load capaci-

tance makes the rise time about 206ns.

Watchdog Timer

The watchdog circuit typically monitors a

µP’sactivity.The

µP is required to change the logic state of the WDI pin on

a periodic basis in order to clear the watchdog timer and

prevent the WDO pin from going low. Whenever RST is low,

the watchdog timer is cleared and WDO is set high. The

watchdog timer is started when RST pulls high. Subsequent

edges received on the WDI pin will clear the watchdog timer.

The timer will continue to run until the watchdog timer times

out. Once the watchdog timer times out, internal circuitry

will bring the WDO pin low. WDO will remain low and the

watchdog timer will remain cleared until the next edge is

received on the WDI pin or until RST goes low.

To disable the watchdog timer, simply ground the CWT pin

(Pin 9). With CWT held at ground, any reset event will force

WDO high indefinitely. It is safe to leave the WDI pin (Pin 8)

unconnected because the weak internal pull-up (10

µA typ)

will pull WDI high. Tying WDI to V1 or ground is also

allowed, but grounding the WDI pin will force the pull-up

current to be drawn continuously.

Selecting the Reset Timing Capacitor

The reset time-out period is adjustable in order to accom-

modate a variety of microprocessor applications. The

value of this capacitor is determined by:

217.4 (pF/ms).

Leaving the CRT pin unconnected will generate a mini-

mum reset time-out of approximately 50

µs. Maximum

reset time-out is limited by the largest available low

leakage capacitor. The accuracy of the time-out period will

be affected by capacitor leakage (the nominal charging

current is 2

µA) and capacitor tolerance. A low leakage

ceramic capacitor is recommended.

Selecting the Watchdog Timing Capacitor

The watchdog time-out period is adjustable and can be

optimized for software execution. The watchdog time-out

between the CWT pin and ground. Given a specified

watchdog time-out period, the capacitor is determined by:

50 (pF/ms).

Leaving the CWT pin unconnected will generate a mini-

mum watchdog time-out of approximately 200

µs. Maxi-

mum time-out is limited by the largest available low

leakage capacitor. The accuracy of the time-out period will

be affected by capacitor leakage (the nominal charging

current is 2

µA) and capacitor tolerance. A low leakage

ceramic capacitor is recommended.

Monitoring Power Supply Controller Activity

Figure 7 demonstrates how the LTC2901 can be used to

monitor switcher activity. The monitor is configured to

supervise 3.3V, 2.5V, 1.8V and one adjustable input.

APPLICATIO S I FOR ATIO