IMP802MCSA 查看數據表（PDF） - A1 PROs co., Ltd.

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IMP802MCSA

µP Power Supply Supervisor with Battery Backup Switch

A1 PROs co., Ltd. 

IMP690A, 692A, 802L, 802M, 805L

Application Information

Watchdog Input

As discussed in the Reset section, the Watchdog Input is used to

monitor microprocessor activity. It can be used to insure that the

microprocessor is in a continually responsive state by requiring

that the WDI pin be toggled every second. If the WDI pin is not

toggled within the 1.6 second window (minimum tWD + tRS),

a reset pulse will be asserted to return the microprocessor to the

initial start-up state. Pulses as short as 50ns can be applied to the

WDI pin. If this feature is not used, the WDI pin should be open-

circuited or the logic placed into a high-impedance state to allow

the pin to float.

Backup-Battery Switchover

A power loss can be made less severe if the system RAM contents

are preserved. This is achieved in the IMP690/692/802/805 by

switching from the failed VCC to an alternate power source con-

nected at VBATT when VCC is less than the reset threshold voltage

(VCC < VRT), and VCC is less than VBATT. The VOUT pin is normally

connected to VCC through a 2Ω PMOS switch but a brown-out or

loss of VCC will cause a switchover to VBATT by means of a 20Ω

PMOS switch. Although both conditions (VCC < VRT and VCC <

VBATT) must occur for the switchover to VBATT to occur, VOUT will

be switched back to VCC when VCC exceeds VRT irrespective of the

voltage at VBATT. It should be noted that an internal device diode

(D1 in Figure 4) will be forward biased if VBATT exceeds VCC by

more than a diode drop when VCC is switched to VOUT. Because of

this it is recommended that VBATT be no greater than VRT +0.6V.

VBATT

VCC

Table 1. Pin Connections in Battery Backup Mode

Pin

VOUT

VBATT

PFI

PFO

RESET

WDI

Connection

Connected to VBATT through internal PMOS switch

Connected to VOUT

Disabled

Logic-LOW

Logic LOW (except on IMP805 where it is HIGH)

Watchdog timer disabled

During the backup power mode, the internal circuitry of the

supervisory circuit draws power from the battery supply. While

VCC is still alive, the comparator circuits remain alive and the cur-

rent drawn by the device is typically 35µA. When VCC drops more

than 1.1V below VBATT, the internal switchover comparator, the

PFI comparator and WDI comparator will shut off, reducing the

quiescent current drawn by the IC to less than 1µA.

Backup Power Sources - Batteries

Battery voltage selection is important to insure that the battery

does not discharge through the parasitic device diode D1 (see

Figure 4) when VCC is less than VBATT and VCC > VRT.

Table 2. Maximum Battery Voltages

Part No.

IMP690A

IMP802L

IMP805L

MAXIMUM Battery Voltage

4.80

4.80

4.80

IMP692A

4.55

IMP802M

4.55

SW1 SW2

D1 D2

SW3 SW4

IMP690A IMP802L

IMP692A IMP802M

D3 IMP805L

VOUT

CONDITION

VCC > Reset Threshold

VCC < Reset Threshold and

VCC > VBATT

VCC < Reset Threshold and

VCC < VBATT

SW1/SW2 SW3/SW4

Open

Closed

Open

Closed

Closed

Open

IMP690A /IMP802L / IMP805L Reset Threshold = 4.65V

IMP692A /IMP802M Reset Threshold = 4.4V

690A_06.eps

Figure 4. Internal device configuration of battery

switch-over function

Although most batteries that meet the requirements of Table 2 are

acceptable, lithium batteries are very effective backup source due

to their high-energy density and very low self-discharge rates.

Battery Replacement while Powered

Batteries can be replaced even when the device is in a powered

state as long as VCC remains above the reset threshold voltage VRT.

In the IMP devices, a floating VBATT pin will not cause a power-

supply switchover as can occur in some other supervisory

circuits. If VBATT is not used, the pin should be grounded.

6

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