TK65418M データシートの表示（PDF） - Toko America Inc

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TK65418M

STEP-DOWN CONVERTER WITH BATTERY MONITOR

Toko America Inc  

ADVANCED INFORMATION

TK654xx

THEORY OF OPERATION (CONT.)

cycle will be dependent upon the duration of the preceding

"on" cycle and the sensed input and the output voltages.

The approximate duration of the "off" cycle can be predicted

as follows:

TOFF = TON X (VIN - VOUT) / VOUT

During the majority of the "off" cycle, a power switch is

turned on. The turn-on point of the switch is synchronized

with the initiation of the "off" cycle. The turn-off point is near

the end of the "off" cycle. The addition of this switch

reduces the voltage drop across the external rectifying

device and provides significant improvements in converter

efficiency.

BATTERY MONITOR SECTION

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

Battery Monitor of the TK654xx introduces a small feedback

current (-ILBI) 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 (-ILBI) and

the values of the external resistor network. Figure 2

illustrates the typical relationship between the battery

voltage (VIN), the feedback current (-ILBI) and the voltage

on the monitoring pin (VLBI).

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 upper threshold of

the voltage monitor. 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 (VOFF) lesser than

the first.

Typically, when the battery voltage is relatively high, the

voltage on the LBI input pin will be a resistively divided

version of the battery voltage.

As the battery voltage drops, the voltage on the VLBI pin will

drop proportionately. When the voltage on the LBI input pin

passes through approximately 1.22 V, the output of the

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

state and the synchronous rectifier N-channel MOSFET

will stop being switched. There is about 50 mV of hysteresis

between the LBO trip point and its reset point. Shortly after

the LBO output has been asserted, as the input voltage

continues to drop, the magnitude of the current out of the

VLBI pin will begin to flow and proportionately increase as

the input voltage decreases. An op-amp feedback loop

internal to the Battery Monitor of the TK654xx will attempt

to maintain the voltage on the VLBI pin at a constant value

of approximately 1.22 V (thus, the plateau). As the battery

voltage continues to drop, there comes a point where the

feedback current stops increasing at about 720 nA. At this

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

drop with the input voltage and the TK654xx converter will

turn off.

For details on how to properly select the resistor divider,

refer to the “Design Considerations” section.

VLBO

VIN

VOFF

VLBI

-ILBI

0 nA

1.22 V

720 nA

LBO

OFF

FIGURE 2: BATTERY MONITOR OPERATION

GRAPHS

September 1999 TOKO, Inc.

Page 11

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