LMV7231 datasheet(14/18 Pages) NSC

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INPUT/OUTPUT VOLTAGE RANGE ABOVE V+

The LMV7231 Hex Window Comparator with 1.5% precision

can accurately monitor up to 6 power rails or batteries at one

time. The input and output voltages of the device can exceed

the supply voltage, V+, of the comparator, and can be up to

the absolute maximum ratings without causing damage or

performance degradation. The typical µC input pin with crow-

bar diode ESD protection circuitry will not allow the input to

go above V+, and thus its usefulness is limited in power sup-

ply supervision applications.

The supply independent inputs of the window comparator

blocks allow the LMV7231 to be tolerant of system faults. For

example if the power is suddenly removed from the LMV7231

due to a system malfunction yet there still exists a voltage on

the input, this will not be an issue as long as the monitored

input voltage does not exceed absolute maximum ratings.

Another example where this feature comes in handy is a bat-

tery sense application such as the one in Figure 6. The boards

may be sitting on the shelf unbiased with V+ grounded, and

yet have a fully charged battery on board. If the comparator

measuring the battery had crowbar diodes, the diode from –

IN to V+ would turn on, sourcing current from the battery

eventually draining the battery. However, when using the

LMV7231 no current, except the low input bias current of the

device, will flow into the chip, and the battery charge will be

preserved.

30114958

FIGURE 6. Battery Sense Application

The output pin voltages of the device can also exceed the

supply voltage, V+, of the comparator. This provides extra

flexibility and enables designs which pull up the outputs to

higher voltage levels to meet system requirements. For ex-

ample it’s possible to run the LMV7231 at its minimum oper-

ating voltage, V+ = +2.2V, but pull up the output up to the

absolute maximum ratings to bias a blue LED, with a forward

voltage of V

F = +4V.

In a power supply supervision application the hardwired

LMV7231 is a sound solution compared to the uC with soft-

ware alternative for several reasons. First, startup is faster.

During startup you don’t need to account for code loading

time, oscillator ramp time, and reset time. Second, operation

is quick. The LMV7231 has a maximum propagation delay in

the µs and isn’t affected by sampling and conversion delays

related to reading data, calculating data, and setting flags.

Third, less overhead. The LMV7231 doesn’t require an ex-

pensive power consuming microcontroller nor is it dependent

on controller code which could get damaged or crash.

POWER SUPPLY BYPASSING

Bypass the supply pin, V+, with a 0.1

μF ceramic capacitor

placed close to the V+ pin. If transients with rise/fall times of

100’s

μs and magnitudes of 100’s mV are expected on the

power supply line a RC low pass filter network as shown in

Figure 7 is recommended for additional bypassing. If no such

bypass network is used power supply transients can cause

the internal voltage reference of the comparator to temporarily

shift potentially resulting in a brief incorrect comparator out-

put. For example if an RC network with 100

Ω resistance and

μF capacitance (1.1ms rise time) is used the voltage ref-

erence will shift temporarily the amount, V

TH Power Supply

Sensitivity (V

THPSS), specified in the Electrical Characteris-

tics table.

30114959

FIGURE 7. Power Supply Bypassing

POWER SUPPLY SUPERVISION

Figure 8 shows a power supply supervision circuit utilizing the

LMV7231. This application uses the efficient, easy to use

LM25007 step-down switching regulator. This switching reg-

ulator can handle a 9V – 42V input voltage range and it’s

regulated output voltage is set to 5V with R2 = R3 = 3k

Ω.

Resistor R6 and capacitors C6, C7 are utilized to minimize

output ripple voltage per the LM25007 evaluation board ap-

plication note.

The comparator voltage window is set to 5V +/- 5% by

R7=1.15k

Ω , R8=10Ω, R9=95.3Ω. See 3 RESISTOR VOLT-

AGE DIVIDER SELECTION section in the Application Infor-

mation section of the datasheet for details on how to set the

comparator voltage window.

With components selected the output ripple voltage seen on

the LM25007 is approximately 30 - 35mV and is reduced to

about 4mV at the comparator input, +IN1, by the resistor di-

vider. This ripple voltage can be reduced multiple ways. First,

user can operate the device in continuous conduction mode

rather than discontinuous conduction mode. To do this in-

crease the load current of the device (see LM25007 datasheet

for more details). However, make sure not to exceed the pow-

er rating of the resistors in the resistor ladder. Second, ripple

can be reduced further with a bypass cap, C9, at the resistor

divider. If desired a user can select a 1uF capacitor to achieve

less than 3mV ripple at +IN1. However, there is a tradeoff and

adding capacitance at this node will lower the system re-

sponse time.

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