max9120exkt Maxim Integrated Products, Inc., max9120exkt Datasheet - Page 10

• Manufacturer Part Number: max9120exkt
• Description: Sc70, 1.6v, Nanopower, Beyond-the-rails Comparators With/without Reference
• Manufacturer: Maxim Integrated Products, Inc.

SC70, 1.6V, Nanopower, Beyond-the-Rails
Comparators With/Without Reference
The internal reference in the MAX9117/MAX9118 has an
output voltage of +1.252V with respect to V
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.
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.
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
falling input voltage (V
between the trip points is the hysteresis (V
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
Table 1. Battery Applications Using MAX9117–MAX9120
10
Nickel-Cadmium
Nickel-Metal-
Lithium-Ion
BATTERY
(2 Cells)
(2 Cells)
(2 Cells)
Alkaline
Hydride
(1 Cell)
______________________________________________________________________________________
TYPE
Low-Voltage, Low-Power Operation
Reference (MAX9117/MAX9118)
Applications Information
RECHARGEABLE
THF
Yes
Yes
Yes
No
) (Figure 2). The difference
Internal Hysteresis
THR
V
FRESH
) and one for the
3.0
2.4
3.5
2.4
(V)
EE
HB
. Its typical
V
). When
END-OF-LIFE
(V)
1.8
1.8
2.7
1.8
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.
The MAX9117/MAX9119 have a 4mV internal hysteresis
band (V
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
Additional Hysteresis (MAX9117/MAX9119)
CAPACITY,
Figure 1. MAX9117/MAX9118 Voltage Reference Output
Equivalent Circuit
AA SIZE
(mA-h)
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
Considering the two possible output states in solving
for R3 yields two formulas: R3 = V
(V
ing resistor values. For example, when using the
2000
1000
1000
750
CC
HB
- V
). Additional hysteresis can be generated
REF
MAX9117/MAX9118
OPERATING TIME
) / I
V
1.25 x 10
1.25 x 10
R3
BIAS
2.5 x 10
937,500
120nA
. Use the smaller of the two result-
(hr)
6
6
6
V
CC
V
EE
MAX9119/MAX9120
OPERATING TIME
REF
REF
1.875 x 10
REF
2.5 x 10
2.5 x 10
5 x 10
- V
/ I
(hr)
R3
OUT
6
or R3 =
6
6
6
) / R3.