Rainbow Electronics MAX4244 - Manual (Page 11)

In the region where the differential input voltage
approaches 1.8V, the input resistance decreases expo-
nentially from 45M
Ω
to 4.4k
Ω
as the diode block begins
conducting. Conversely, the bias current increases with
the same curve.
Rail-to-Rail Output Stage
The MAX4240–MAX4244 output stage can drive up to a
10k
Ω
load and still swing to within 40mV of the rails.
Figure 3 shows the output voltage swing of a MAX4240
configured as a unity-gain buffer, powered from a single
+2V supply voltage. The output for this setup typically
swings from (V
EE + 6mV) to (V CC - 8mV) with a 100k
Ω
load.
__________Applications Information
Power-Supply Considerations
The MAX4240–MAX4244 operate from a single +1.8V
to +5.5V supply (or dual ±0.9V to ±2.75V supplies) and
consume only 10µA of supply current per amplifier. A
high power-supply rejection ratio of 90dB allows the
amplifiers to be powered directly off a decaying battery
voltage, simplifying design and extending battery life.
The MAX4240–MAX4244 are ideally suited for use with
most battery-powered systems. Table 1 lists a variety of
typical battery types showing voltage when fresh, volt-
age at end-of-life, capacity, and approximate operating
time from a MAX4240/MAX4241, assuming nominal
conditions for both normal and shutdown modes.
Although the amplifiers are fully guaranteed over tem-
perature for operation down to a +1.8V single supply,
even lower-voltage operation is possible in practice.
Figures 4 and 5 show the PSRR and supply current as
a function of supply voltage and temperature.
Power-Up Settling Time
The MAX4240–MAX4244 typically require 200µs to
power up after V
CC is stable. During this start-up time,
the output is indeterminant. The application circuit
should allow for this initial delay.
Shutdown Mode
The MAX4241 (single) and MAX4243 (dual) feature a
low-power shutdown mode. When the shutdown pin
(SHDN) is pulled low, the supply current drops to 1µA
per amplifier, the amplifier is disabled, and the outputs
enter a high-impedance state. Pulling SHDN high or
leaving it floating enables the amplifier. Take care to
ensure that parasitic leakage current at the SHDN pin
does not inadvertently place the part into shutdown
mode when SHDN is left floating. Figure 6 shows the
output voltage response to a shutdown pulse. The logic
threshold for SHDN is always referred to V
CC / 2 (not to
MAX4240–MAX4244
Single/Dual/Quad, +1.8V/10µA, SOT23,
Beyond-the-Rails Op Amps
______________________________________________________________________________________ 11
1V/div OUT
IN 1V/div
MAX4240-44 fig03
200 µ s/div
R L = 100k Ω TIED TO V EE
V IN = 2.0V
f IN = 1kHz
Figure 3. Rail-to-Rail Input/Output Voltage Range
100
60
1.0 1.2 2.0
70
90 MAX4240-44 fig04
SUPPLY VOLTAGE (V)
PSRR (dB)
1.4 1.6
80
1.8
T A = +85°C
T A = +25°C
T
A
= -40°C
Figure 4. Power-Supply Rejection Ratio vs. Supply Voltage
12
0
1.0 1.2 2.0
2
4
10
MAX4240-44 fig05
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (
µ
A)
1.4 1.6
6
8
1.8
T
A
= +85°C
T A = +25°C
T A = -40°C
Figure 5. Supply Current vs. Supply Voltage