UCSP, 1.8V, Nanopower, Beyond-the-Rails

Comparators With/Without Reference

10

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Reference (MAX9025/MAX9026)

The MAX9025–MAX9028s’ internal +1.236V reference

has a typical temperature coefficient of 40ppm/°C over

the full -40°C to +85°C temperature range. The reference

is a very-low-power bandgap cell, with a typical 35kΩ

output impedance. REF can source and sink up to

100nA to external circuitry. For applications needing

increased drive, buffer REF with a low input-bias current

op amp such as the MAX4162. Most applications require

no REF bypass capacitor. For noisy environments or fast

from REF to GND.

Applications Information

Low-Voltage, Low-Power Operation

The MAX9025–MAX9028 are ideally suited for use with

most battery-powered systems. Table 1 lists a variety of

battery types, capacities, and approximate operating

times for the MAX9025–MAX9028, assuming nominal

conditions.

Internal Hysteresis

Many comparators oscillate in the linear region of opera-

tion because of noise or undesired parasitic feedback.

This tends to occur when the voltage on one input is

equal or very close to the voltage on the other input. The

MAX9025–MAX9028 have internal 4mV hysteresis to

counter parasitic effects and noise.

The hysteresis in a comparator creates two trip points:

the comparator’s input voltages are equal, the hystere-

sis effectively causes one comparator input to move

quickly past the other, thus taking the input out of the

region where oscillation occurs. Figure 2 illustrates the

case in which IN- has a fixed voltage applied, and IN+

is varied. If the inputs were reversed, the figure would

be the same, except with an inverted output.

Adding External Hysteresis

In applications requiring more than the internal 4mV

hysteresis of the MAX9025–MAX9028, additional hys-

teresis can be added with external components.

Because the MAX9025–MAX9028 are intended for very

low-power systems, care should be taken to minimize

power dissipation in the additional circuitry.

Regardless of which approach is taken, the external

range of battery-powered systems, the hysteresis can

change as much as 40%. This must be considered

during design.

BANDGAP

REF

Figure 1. MAX9025/MAX9026 Voltage Reference Output

Equivalent Circuit

Table 1. Battery Applications Using MAX9025–MAX9028

BATTERY

TYPE

(V)

CAPACITY,

AA SIZE

(mA-H)

MAX9025/MAX9026

OPERATING TIME

(hr)

MAX9027/MAX9028

OPERATING TIME

(hr)

Alkaline

(2 Cells)

No

3.0

1.8

2000

1.8 x 10

6

2.8 x 10

6

Nickel-

Cadmium

(2 Cells)

Yes

2.4

1.8

750

680,000

1.07 x 10

6

Lithium-Ion

(1 Cell)

Yes

3.5

2.7

1000

0.9 x 10

6

1.4 x 10

6

Nickel-Metal-

Hydride

(2 Cells)

Yes

2.4

1.8

1000

0.9 x 10

6

1.4 x 10

6