continuously source as much as 40mA. The compara-

tors eliminate power-supply glitches that commonly

occur when changing logic states, minimizing parasitic

feedback and making them easier to use. In addition,

they include internal hysteresis (±3mV) to ensure clean

output switching, even with slow-moving input signals.

The inputs can be taken above and below the supply

rails up to 300mV without damage. Input voltages

beyond this range can forward bias the ESD-protection

diodes and should be avoided.

The MAX951–MAX954 comparator outputs swing rail to

using a +5V ±10% supply.

The MAX951–MAX954 comparator continuously outputs

source currents as high as 40mA and sink currents of

over 5mA, while keeping quiescent currents in the

microampere range. The output can source 100mA (at

maximum power dissipation is not exceeded. The out-

put stage does not generate crowbar switching currents

during transitions; this minimizes feedback through the

supplies and helps ensure stability without bypassing.

Reference

The internal reference in the MAX951/MAX952 has an

in the -40°C to +85°C temperature range. It is comprised

of a trimmed bandgap reference fed by a proportional-

to-absolute-temperature (PTAT) current source and

buffered by a micropower unity-gain amplifier. The REF

output is typically capable of sourcing and sinking 20µA.

Do not bypass the reference output. The reference is

stable for capacitive loads less than 100pF.

__________Applications Information

The micropower MAX951–MAX954 are designed to

extend battery life in portable instruments and add

functionality in power-limited industrial controls.

Following are some practical considerations for circuit

design and layout.

Comparator Hysteresis

Hysteresis increases the comparator’s noise immunity

by increasing the upper threshold and decreasing the

lower threshold. The comparator in these devices con-

tain a ±3mV wide internal hysteresis band to ensure

clean output switching, even with slow-moving signals.

When necessary, hysteresis can be increased by using

external resistors to add positive feedback, as shown in

Figure 2. This circuit increases hysteresis at the

expense of more supply current and a slower

response. The design procedure is as follows:

1) Set R2. The leakage current in COMPIN+ is less

than 5nA (up to +85°C), so current through R2 can

be as little as 500nA and still maintain good accura-

cy. If R2 = 2.4M

Ω, the current through R2 at the

2) Choose the width of the hysteresis band. In this

3) Determine R1. If the supply voltage is 5V, then R1 =

24k

Ω.

4) Check the hysteresis trip points. The upper trip point is

or 1.22V in our example. The lower trip point is 50mV

less, or 1.17V in our example.

If a resistor divider is used for R1, the calculations

should be modified using a Thevenin equivalent

model.

Ultra-Low-Power, Single-Supply

Op Amp + Comparator + Reference

_______________________________________________________________________________________

9

REF

RB

RA

R2

COMPOUT

REF

R1

R2

COMPOUT

Figure 2. External Hysteresis

R1 R2

V

2

V

2

EHYST

IHYST

DD

IHYST

=

−

[

]

+

(

)

V

V

V

R1 + R2

R2

V

V

IN H

REF

IHYST

(

) (

)

+