TK65010 데이터 시트보기 (PDF) - Toko America Inc

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TK65010

BATTERY VOLTAGE MONITOR

Toko America Inc  

TK65010

THEORY OF OPERATION

The circuit in Figure 1 illustrates a typical application

utilizing the TK65010. The TK65010 differs from most

voltage monitors due to the fact that two separate

comparator thresholds can be set utilizing a single resistive

divider and a single input pin. One comparator output is

used as an early “low battery” warning. The second

comparator can be used as a later “dead battery” warning.

BATTERY IN

R1

301 K

R2

1M

VIN

GND

VMON

IFB

LBO

DBO

LOW BATTERY

DEAD BATTERY

FIGURE 1

Under these conditions, current will be flowing into the

VMON pin and will be limited to approximately 580 nA. As

the battery voltage drops, the voltage on the VMON pin will

drop proportionately. When the voltage on the VMON pin

passes through approximately 710 mV, the output of the

first comparator (LBO) will transition from a high to a low

state. Shortly after the LBO output has been asserted, as

the input voltage continues to drop, the magnitude of the

current into the VMON pin will begin to proportionately

decrease as the input voltage decreases. An op-amp

feedback loop internal to the TK65010 will attempt to

maintain the voltage on the VMON pin at a constant value of

approximately 700 mV (thus, the plateau). As the battery

voltage continues to drop, there comes a point where the

feedback current decreases to approximately zero. A this

point, the voltage on the VMON pin will resume a proportional

drop with the input voltage. As the voltage on the VMON pin

passes through approximately 690 mV, the second

comparator output (DBO) will be asserted.

In a typical voltage monitor, which uses an external resistive

divider for setting the voltage monitor threshold, the input

bias current to the monitor pin is essentially zero. In this

type of scenario, the voltage on the monitor input would be

a resistively divided version of the battery voltage. The

TK65010 introduces a small feedback current (IFB) which

introduces a “plateau” into the transfer characteristics

between the battery voltage and the voltage monitoring

pin. The width of this plateau is dependent upon the current

range of the feedback current (IFB) and the values of the

external resistor network. Figure 2 illustrates the typical

relationship between the battery voltage (VIN), the feedback

current (IFB) and the voltage on the monitoring pin (VMON).

In selecting a resistor divider network, there are typically

two degrees of freedom when selecting values. The first

criteria in selecting the divider is the ratio of the two

resistors. Selecting the ratio defines the lower threshold of

the voltage monitor (DBO). The second degree of freedom

when selecting the resistor divider is the absolute resistance

values. This second degree of freedom can be utilized to

set a secondary monitoring threshold (LBO) greater than

the first.

For details on how to properly select the resistor divider,

refer to the “Design Considerations” section.

VLBO

VIN

VDBO

VMON

580 nA

IFB

700 mV

0 nA

LBO

DBO

Typically, when the battery voltage is relatively high, the

voltage on the VMON pin of the TK65010 will be a resistively

divided version of the battery voltage minus the offset. The

magnitude of the offset voltage will be dependent upon the

resistor values comprising the external divider and the

magnitude of the feedback current flowing into the IFB pin.

FIGURE 2

January 1999 TOKO, Inc.

Page 3

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