ISL28158, ISL28258
FN6377 Rev 5.00 Page 15 of 19
October 12, 2015
Applications Information
Introduction
The ISL28158 is a single CMOS rail-to-rail input, output (RRIO)
operational amplifier with an enable feature. The ISL28258 is a
dual version without the enable feature. Both devices are
designed to operate from single supply (2.4V to 5.5V) or dual
supplies (±1.2V to ±2.75V).
Rail-to-Rail Input/Output
These devices feature PMOS inputs with an input common
mode range that extends up to 0.3V beyond the V+ rail, and to
0.1V below the V- rail. The CMOS output features excellent
drive capability, typically swinging to within 6mV of either rail
with a 100k load.
Results of Over-Driving the Out put
Caution should be used when over-driving the output for long
periods of time. Over-driving the output can occur in two ways
1) The input voltage times the gain of the amplifier exceeds the
supply voltage by a large value or, 2) the output current
required is higher than the output stage can deliver. These
conditions can result in a shift in the Input Offset Voltage (VOS)
as much as 1µV/hr. of exposure under these conditions.
IN+ and IN- Input Protection
All input terminals have internal ESD protection diodes to both
positive and negative supply rails, limiting the input voltage to
within one diode beyond the supply rails. They also contain
back-to-back diodes across the input terminals (see “Pin
Descriptions” on page 14 - Circuit 1). For applications where the
input differential voltage is expected to exceed 0.5V, an external
series resistor must be used to ensure the input currents never
exceed 5mA (Figure 47).
Enable/Disable Feature
The ISL28158 offers an EN pin that disables the device when
pulled up to at least 2.0V. In the disabled state (output in a high
impedance state), the part consumes typically 10µA at room
temperature. By disabling the part, multiple ISL28158 parts can
be connected together as a MUX. In this configuration, the
outputs are tied together in parallel and a channel can be
selected by the EN pin. The loading effects of the feedback
resistors of the disabled amplifier must be considered when
multiple amplifier outputs are connected together. Note that
feed through from the IN+ to IN- pins occurs on any Mux Amp
disabled channel where the input differential voltage exceeds
0.5V (e.g., active channel
VOUT = 1V, while disabled channel VIN = GND), so the mux
implementation is best suited for small signal applications. If
large signals are required, use series IN+ resistors, or large
value RF
, to keep the feed through current low enough to
minimize the impact on the active channel. See “Limitations of
the Differential Input Protection” on page 15 for more details.
The EN pin also has an internal pull-down. If left open, the EN
pin will pull to the negative rail and the device will be enabled
by default. When not used, the EN pin should either be left
floating or connected directly to the -V pin.
Limitations of the Dif ferential Input Protection
If the input differential voltage is expected to exceed 0.5V, an
external current limiting resistor must be used to ensure the
input current never exceeds 5mA. For non-inverting unity gain
applications, the current limiting can be via a series IN+
resistor, or via a feedback resistor of appropriate value. For
other gain configurations, the series IN+ resistor is the best
choice, unless the feedback (RF) and gain setting (RG)
resistors are both sufficiently large to limit the input current to
5mA.
Large differential input voltages can arise from several
sources:
1) During open loop (comparator) operation. Used this way, the
IN+ and IN- voltages don’t track, so differentials arise.
2) When the amplifier is disabled but an input signal is still
present. An RL or RG to GND keeps the IN- at GND, while the
varying IN+ signal creates a differential voltage. Mux Amp
applications are similar, except that the active channel VOUT
determines the voltage on the IN- terminal.
3) When the slew rate of the input pulse is considerably faster
than the op amp’s slew rate. If the VOUT can’t keep up with the
IN+ signal, a differential voltage results, and visible distortion
occurs on the input and output signals. To avoid this issue,
keep the input slew rate below 0.1V/µs, or use appropriate
current limiting resistors.
Large (>2V) differential input voltages can also cause an
increase in disabled ICC.
FIGURE 47. INPUT CURRENT LIMITING
-
+
RIN
RL
VIN
VOUT