MAX9117–MAX9120
SC70, 1.6V, Nanopower, Beyond-the-Rails
Comparators With/Without Reference
10 ______________________________________________________________________________________
Reference (MAX9117/MAX9118)
The internal reference in the MAX9117/MAX9118 has an
output voltage of +1.252V with respect to V
EE
. Its typical
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 200kΩ, 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:
one for the rising input voltage (V
THR
) and one for the
falling input voltage (V
THF
) (Figure 2). The difference
between the trip points is the hysteresis (V
HB
). When
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
band (V
HB
). Additional hysteresis can be generated
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
through R3 at the trip point is (V
REF
- V
OUT
) / R3.
Considering the two possible output states in solving
for R3 yields two formulas: R3 = V
REF
/ I
R3
or R3 =
(V
CC
- V
REF
) / I
R3
. Use the smaller of the two result-
ing resistor values. For example, when using the
