SC70, 1.6V, Nanopower, Beyond-the-Rails

Comparators With/Without Reference

10

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Reference (MAX9117/MAX9118)

The internal reference in the MAX9117/MAX9118 has an

temperature coefficient is 100ppm/°C over the full

-40°C to +85°C temperature range. The reference is a

PNP emitter-follower driven by a 120nA current source

(Figure 1). The output impedance of the voltage refer-

ence is typically 200k

Ω, preventing the reference from

driving large loads. The reference can be bypassed with

a low-leakage capacitor. The reference is stable for any

capacitive load. For applications requiring a lower output

impedance, buffer the reference with a low-input-leak-

age op amp, such as the MAX4162.

Applications Information

Low-Voltage, Low-Power Operation

The MAX9117–MAX9120 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 MAX9117–MAX9120, assuming nominal

conditions.

Internal Hysteresis

Many comparators oscillate in the linear region of oper-

ation because of noise or undesired parasitic feed-

back. This tends to occur when the voltage on one

input is equal or very close to the voltage on the other

input. The MAX9117–MAX9120 have internal 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.

Additional Hysteresis (MAX9117/MAX9119)

The MAX9117/MAX9119 have a 4mV internal hysteresis

with three resistors using positive feedback (Figure 3).

Unfortunately, this method also slows hysteresis re-

sponse time. Use the following procedure to calculate

resistor values.

1) Select R3. Leakage current at IN is under 2nA, so the

current through R3 should be at least 0.2µA to mini-

mize errors caused by leakage current. The current

Considering the two possible output states in solving

ing resistor values. For example, when using the

120nA

REF

Figure 1. MAX9117/MAX9118 Voltage Reference Output

Equivalent Circuit

Table 1. Battery Applications Using MAX9117–MAX9120

BATTERY

TYPE

RECHARGEABLE

(V)

(V)

CAPACITY,

AA SIZE

(mA-h)

MAX9117/MAX9118

OPERATING TIME

(hr)

Alkaline

(2 Cells)

No

3.0

1.8

2000

Nickel-Cadmium

(2 Cells)

Yes

2.4

1.8

750

937,500

1000

2.7

3.5

Yes

Lithium-Ion

(1 Cell)

Nickel-Metal-

Hydride

(2 Cells)

Yes

2.4

1.8

1000

MAX9119/MAX9120

OPERATING TIME

(hr)