MAX4243 Maxim, MAX4243 Datasheet - Page 14

• Manufacturer Part Number: MAX4243
• Description: The MAX4240–MAX4244 family of micropower op amps operate from a single +1
• Manufacturer: Maxim

Single/Dual/Quad, +1.8V/10µA, SOT23,
Beyond-the-Rails Op Amps
Although optimized for use as operational amplifiers,
the MAX4240–MAX4244 can also be used as rail-to-rail
I/O comparators. Typical propagation delay depends
on the input overdrive voltage, as shown in Figure 9.
External hysteresis can be used to minimize the risk of
output oscillation. The positive feedback circuit, shown
in Figure 10, causes the input threshold to change
when the output voltage changes state. The two thresh-
olds create a hysteresis band that can be calculated by
the following equations:
The MAX4240–MAX4244 contain special circuitry to
boost internal drive currents to the amplifier output
stage. This maximizes the output voltage range over
which the amplifiers are linear. In an open-loop com-
parator application, the excursion of the output voltage
is so close to the supply rails that the output stage tran-
sistors will saturate, causing the quiescent current to
increase from the normal 10µA. Typical quiescent cur-
rents increase to 35µA for the output saturating at V
and 28µA for the output at V
Figure 9. Propagation Delay vs. Input Overdrive
14
V
V
V
HYST
LO
HI
______________________________________________________________________________________
= V
= [(R2 / R1 x V
10,000
= V
1000
(1 + R1 / R2 + R2 / R
100
IN
10
HI
x R2 / (R1 + (R1 x R2 / R
0
- V
Using the MAX4240–MAX4244
10 20 30 40 50 60 70 80 90 100
t
PD
LO
t
PD
+; V
+; V
CC
CC
= +1.8V
= +5V
IN
) + (R2 / R
V
EE
OD
t
PD
t
PD
(mV)
HYST
.
-; V
-; V
CC
CC
= +5V
= +1.8V
as Comparators
)
HYST
HYST
) + R2)
) x V
CC
CC
] /
The MAX4240–MAX4244 are ideal for applications
powered from a 2-cell battery stack. Figure 11 shows
an application circuit in which the MAX4240 is used for
monitoring the current of a 2-cell battery stack. In this
circuit, a current load is applied, and the voltage drop
at the battery terminal is sensed.
The voltage on the load side of the battery stack is
equal to the voltage at the emitter of Q1, due to the
feedback loop containing the op amp. As the load cur-
rent increases, the voltage drop across R1 and R2
increases. Thus, R2 provides a fraction of the load cur-
rent (set by the ratio of R1 and R2) that flows into the
emitter of the PNP transistor. Neglecting PNP base cur-
rent, this current flows into R3, producing a ground-ref-
erenced voltage proportional to the load current. Scale
R1 to give a voltage drop large enough in comparison
to V
The output voltage of the application can be calculated
using the following equation:
For a 1V output and a current load of 50mA, the choice
of resistors can be R1 = 2Ω, R2 = 100kΩ, R3 = 1MΩ.
The circuit consumes less power (but is more suscepti-
ble to noise) with higher values of R1, R2, and R3.
Figure 10. Hysteresis Comparator Circuit
INPUT
V
OUTPUT
OH
OS
V
of the op amp, in order to minimize errors.
as Ultra-Low-Power Current Monitors
IN
V
V
HI
LO
V
OUT
R1
R2
Using the MAX4240–MAX4244
= [I
LOAD
V
EE
x (R1 / R2)] x R3
R
V
V
HYST
CC
EE
MAX4240
MAX4241
MAX4242
MAX4243
MAX4244
V
OUT
HYSTERESIS
V
V
OH
OL