|

||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||

PT0212(11/07)

Ver: 4

4

monitoring.

The PT7M7436/37/38 combine a 585/615mV reference with

two comparators, logic, and timing circuitry to provide the

user with information about the charge state of the power-

supply batteries. The PT7M7436/37/38 monitor separate high-

voltage and low-voltage thresholds to determine battery status.

The outputs can be used to signal when the battery is charged,

when the battery is low, and when the battery is empty.

PT7M7436/37/38

are

also

available

with

thresholds

determined using an external resistor-divider (see Application

information Resistor-Value Selection). They can monitor

down to 0.62V, making them suitable for monitoring single-

When the power-supply voltage drops below the specified low

Output). When the voltage rises above the specified high

threshold following a 90ms (min) time-out period, the low-

battery output is deasserted (see Threshold Monitoring). This

ensures the supply voltage has stabilized before power-

converter or microprocessor activity is enabled.

The low-battery outputs are available in active-low (LBOH,

LBOL push-pull and open drain) and active-high (LBOH,

LBOL open drain) configurations. Push-pull outputs are

The open-drain devices can be pulled to a voltage independent

operate from direct battery voltage while interfacing to higher

voltage microprocessors.

The PT7M7436/37/38 dual-output voltage monitors provide

two low-battery outputs, LBOH/LBOH and LBOL/LBOL.

They will assert LBOH /LBOH when HTHIN drops below

The PT7M7436/37/38 dual-output voltage monitors assert

LBOL/LBOL when LTHIN drops below the LTHIN-

threshold and deassert LBOL/LBOL when LTHIN rises above

the LTHIN+ threshold for at least 90ms. LBOH/LBOH is

asserted when HTHIN drops below the HTHIN- threshold and

is deasserted when HTHIN rises above the HTHIN+ threshold

for at least 90ms (see AC Electrical Characteristics: Fig 1).

Hysteresis increases the comparator’s noise margin by

increasing the upper threshold or decreasing the lower

threshold. The hysteresis prevents the output from oscillating

is especially important for applications where the load on the

battery creates significant fluctuations in battery voltages. The

hysteresis for the PT7M7436/37/38 is the differences between

)

)

Use the following steps to determine values for R1, R2, and

R3 of Functional Block Diagram.

Because the PT7M7436~PT7M7438 have very high input

point:

R3 =

=

trip point:

R2 =

- R3

4) Calculate R1 based on RTOTAL, R3, and R2:

R1 + R2 + R3

R2 + R3

585mV

585mV

R1 + R2 + R3

R3