MCP6041 Datasheet, PDF (11/32 Pages) Microchip Technology – 600 nA, Rail-to-Rail Input/Output Op Amps

3.0 APPLICATIONS INFORMATION
The MCP6041/2/3/4 family of operational amplifiers
are fabricated on Microchip’s state-of-the-art CMOS
process. They are unity gain stable and suitable for a
wide range of applications requiring very low power
consumption. With these op amps, the power supply
pin needs to be by-passed with a 0.1 µF capacitor.
3.1 Rail to Rail Input
The input stage of the family of devices uses two differ-
ential input stages in parallel; one operates at low V_CM
(common mode input voltage) and the other at high
V_CM. With this topology, the MCP6041/2/3/4 family
operates with V_CM up to 300 mV past either supply rail.
The Input Offset Voltage is measured at both
V_CM = V_SS - 0.3 V and V_DD + 0.3 V to ensure proper
operation.
3.2 Output Loads and Battery Life
The MCP6041/2/3/4 op amp family has outstanding
quiescent current, which supports battery-powered
applications. There is minimal quiescent current glitch-
ing when chip select (CS) is raised or lowered. This
prevents excessive current draw and reduced battery
life, when the part is turned off or on.
Heavy resistive loads at the output can cause exces-
sive battery drain. Driving a DC voltage of 2.5 V across
a 100 kΩ load resistor will cause the supply current to
increase by 25 µA, depleting the battery 43 times as
fast as I_Q (0.6 µA typ) alone.
High frequency signals (fast edge rate) across capaci-
tive loads will also significantly increase supply current.
For instance, a 0.1 µF capacitor at the output presents
an AC impedance of 15.9 kΩ (1/2πfC) to a 100 Hz
sinewave. It can be shown that the average power
drawn from the battery by a 5.0 Vp-p sinewave
(1.77 Vrms), under these conditions, is:
EQUATION
P_SUPPLY = (V_DD – V_SS)(I_Q + V_{L(p – p)}fC_L)
= (5V) (0.6µA + 5.0V_{p – p} • 100Hz • 0.1µF)
= 3.0µW + 50µW
This will drain the battery 18 times as fast as I_Q alone.
3.3 Rail to Rail Output
The output voltage range of the MCP6041/2/3/4 family
is specified two ways. The first specification, Maximum
Output Voltage Swing, defines the maximum swing
possible under a particular output load. According to
the spec table, the output can reach ≤ 10 mV of either
supply rail when R_L = 50 kΩ. See Figure 2-32 for infor-
mation on Maximum Output Voltage Swing vs. load
resistance.
MCP6041/2/3/4
The second specification, Linear Region Output Volt-
age Swing, details the output voltage range that sup-
ports the specified Open Loop Gain (A_OL ≥ 95 dB with
R_L = 50 kΩ).
3.4 Input Voltage and Phase Reversal
The MCP6041/2/3/4 op amp family uses CMOS tran-
sistors at the input. It is designed to not exhibit phase
inversion when the input pins exceed the supply volt-
ages. Figure 2-39 shows an input voltage exceeding
both supplies with no resulting phase inversion.
The maximum operating V_CM (common mode input
voltage) that can be applied to the inputs is V_SS -0.3 V
and V_DD +0.3 V. Voltage on the input that exceed this
absolute maximum rating can cause excessive current
to flow in or out of the input pins. Current beyond ±2 mA
can cause possible reliability problems. Applications
that exceed this rating must be externally limited with
an input resistor as shown in Figure 3-1.
R_IN
MCP604X
V_IN
V_OUT
R_IN
≥
(_---M_-----a_---x_---i_-m_----u_---m_------e_---x_---p_---e_--c_---t_--e_--d_-----V_----I--N----)_{---–-----}V_---D----D--
2 mA
R_IN
≥
-^V---^S---^S----–----⁽---^M-----ⁱ--ⁿ---ⁱ-^m-----^u---^m-----^e---^x---^p---^e--^c---^t--^e--^d-----^V----^I--^N----⁾
2 mA
FIGURE 3-1:
An input resistor, R_IN,
should be used to limit excessive input current if
the inputs exceed the Absolute Maximum
specification.
3.5 Capacitive Load and Stability
Driving capacitive loads can cause stability problems
with voltage feedback op amps. A buffer configuration
(G = +1) is the most sensitive to capacitive loads.
Figure 2-27 shows how increasing the load capaci-
tance will decrease the phase margin. While a phase
margin above 60° is ideal, 45° is sufficient. As can be
seen, up to C_L = 150 pF can be placed on the
MCP6041/2/3/4 op amp outputs without any problems,
while 250 pF is usable with a 45° phase margin.
When the op amp is required to drive large capacitive
loads (C_L >150 pF), a small series resistor (R_ISO in
Figure 3-2) at the output of the amplifier improves the
phase margin. This resistor makes the output load
resistive at higher frequencies, which improves the
phase margin. The bandwidth reduction caused by the
capacitive load, however, is not changed. To select
R_ISO, start with 1 kΩ, then use the MCP6041 SPICE
 2002 Microchip Technology Inc.
DS21669B-page 11