CS8151

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7

DEFINITION OF TERMS

Dropout Voltage: The input−output voltage differential

at which the circuit ceases to regulate against further

reduction in input voltage. Measured when the output

voltage has dropped 100mV from the nominal value

obtained at 14V input, dropout voltage is dependent upon

load current and junction temperature.

Input Voltage: The DC voltage applied to the input

terminals with respect to ground.

Line Regulation: The change in output voltage for a

change in the input voltage. The measurement is made

under conditions of low dissipation or by using pulse

techniques such that the average chip temperature is not

significantly affected.

Load Regulation: The change in output voltage for a

change in load current at constant chip temperature.

Quiescent Current: The part of the positive input current

that does not contribute to the positive load current. The

regulator ground lead current.

Ripple Rejection: The ratio of the peak−to−peak input

ripple voltage to the peak−to−peak output ripple voltage.

Current Limit: Peak current that can be delivered to the

output.

CIRCUIT DESCRIPTION

Functional Description

To reduce the drain on the battery a system can go into a

low current consumption mode when ever its not performing

a main routine. The Wake Up signal is generated

continuously and is used to interrupt a microcontroller that

is in sleep mode. The nominal output is a 5.0 V square wave

with a duty cycle of 50% at a frequency that is determined

When the microprocessor receives a rising edge from the

Wake Up output, it must issue a watchdog pulse and check

its inputs to decide if it should resume normal operations or

remain in the sleep mode.

Figure 5. Wake Up Response to WDI

Wake Up

Response

to WDI

Wake Up

WDI

Figure 6. Wake Up Response to RESET (Low Voltage)

Wake Up

Response

to RESET

RESET

Wake Up

The first falling edge of the watchdog signal causes the

Wake Up to go low within 2.0

ms (Typ) and remain low until

the next Wake Up cycle (see Figure 5). Other watchdog

pulses received within the same cycle are ignored (Figures

2, 3, and 4).

During power up, RESET is held low until the output

voltage is in regulation. During operation, if the output

voltage shifts below the regulation limits, the RESET

toggles low and remains low until proper output voltage

regulation is restored. After the RESET delay, RESET

returns high.

The Watchdog circuitry continuously monitors the input

watchdog signal (WDI) from the microprocessor. The

absence of a falling edge on the Watchdog input during one

Wake Up cycle will cause a RESET pulse to occur at the end

of the Wake Up cycle (see Figure 3).

The Wake Up output is pulled low during a RESET

regardless of the cause of the RESET. After the RESET

returns high, the Wake Up cycle begins again (see Figure 3).

The RESET pulse width, Wake Up signal frequency and

RESET high to Wake Up delay time are all set by one

Wake Up Period = (4

RESET Delay Time = (5

RESET High to Wake Up Delay Time = (2

Capacitor temperature coefficient and tolerance as well as

the tolerance of the CS8151 must be taken into account in

order to get the correct system tolerance for each parameter.