ISL28288_11 Datasheet, PDF (12/20 Pages) Intersil Corporation – Digital DC/DC PMBus 12A Module

ISL28288, ISL28488
exceed 0.5V, external series resistors must be used to ensure the
input currents never exceed 5mA (as shown in Figure 37).
V+
-
V_IN
R_{IN +}
V_OUT
R_L
V-
FIGURE 37. INPUT ESD DIODE CURRENT LIMITING - UNITY GAIN
Rail-to-Rail Output
A pair of complementary MOSFET devices are used to achieve the
rail-to-rail output swing. The NMOS sinks current to swing the output
in the negative direction. The PMOS sources current to swing the
output in the positive direction. With a 100kΩ load they will swing to
within 4mV of the positive supply rail and within 3mV of the negative
supply rail.
Enable/Disable Feature
The ISL28288 (only MSOP package option), 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 4µA. By disabling the part, multiple
ISL28288 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. 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.
Using Only One Channel
If the application only requires one channel, the user must
configure the unused channel to prevent it from oscillating. The
unused channel will oscillate if the input and output pins are
floating. This will result in higher than expected supply currents
and possible noise injection into the channel being used. The
proper way to prevent this oscillation is to short the output to the
negative input and ground the positive input (as shown in
Figure 38).
-
ISL28288
+
FIGURE 38. PREVENTING OSCILLATIONS IN UNUSED CHANNELS
moisture and provide a humidity barrier, reducing parasitic
resistance on the board.
Current Limiting
The ISL28288 has no internal current-limiting circuitry. If the
output is shorted, it is possible to exceed the Absolute Maximum
Rating for output current or power dissipation, potentially
resulting in the destruction of the device.
Power Dissipation
It is possible to exceed the +150°C maximum junction
temperatures under certain load and power-supply conditions. It
is therefore important to calculate the maximum junction
temperature (T_JMAX) for all applications to determine if power
supply voltages, load conditions, or package type need to be
modified to remain in the safe operating area. These parameters
are related in Equation 1:
T_JMAX = T_MAX + (θ_JAxPD_MAXTOTAL)
(EQ. 1)
where:
• P_DMAXTOTAL is the sum of the maximum power dissipation of
each amplifier in the package (PD_MAX)
• PD_MAX for each amplifier is calculated in Equation 2:
PD_MAX
=
2*V_S × I_SMAX + (V_S
-
V_OUTMAX
)
×
V_{----O----U---T---M-----A---X--}
R_L
(EQ. 2)
where:
• T_MAX = Maximum ambient temperature
• θ_JA = Thermal resistance of the package
• PD_MAX = Maximum power dissipation of 1 amplifier
• V_S = Supply voltage (Magnitude of V₊ and V_-)
• I_MAX = Maximum supply current of 1 amplifier
• V_OUTMAX = Maximum output voltage swing of the application
• R_L = Load resistance
Application Circuits
THERMOCOUPLE AMPLIFIER
Thermocouples are the most popular temperature-sensing
device because of their low cost, interchangeability, and ability to
measure a wide range of temperatures. The ISL28x88 (see
Figure 39) is used to convert the differential thermocouple
voltage into single-ended signal with 10X gain. The amplifier’s
rail-to-rail input characteristic allows the thermocouple to be
biased at ground and the amplifier to run from a single 5V
supply.
Proper Layout Maximizes Performance
To achieve the maximum performance of the high input
impedance and low offset voltage, care should be taken in the
circuit board layout. The PC board surface must remain clean
and free of moisture to avoid leakage currents between adjacent
traces. Surface coating of the circuit board will reduce surface
12
FN6339.4
July 26, 2011