Datasheet

INA220
SBOS459D –JUNE 2009–REVISED SEPTEMBER 2010 www.ti.com
Power-Up Conditions
The Conversion Ready bit clears under these
conditions:
Power-up conditions apply to a software reset via the
1. Writing to the Configuration Register, except
RST bit (bit 15) in the Configuration Register, or the
when configuring the MODE bits for Power Down
Two-Wire bus General Call Reset.
or ADC off (Disable) modes;
BASIC ADC FUNCTIONS 2. Reading the Status Register; or
3. Triggering a single-shot conversion with the
The two analog inputs to the INA220, V
IN+
and V
IN–
,
Convert pin.
connect to a shunt resistor in the bus of interest. Bus
voltage is measured at V
BUS
pin. The INA220 is
Power Measurement
typically powered by a separate supply from +3V to
+5.5V. The bus being sensed can vary from 0V to
Current and bus voltage are converted at different
26V. There are no special considerations for
points in time, depending on the resolution and
power-supply sequencing (for example, a bus voltage
averaging mode settings. For instance, when
can be present with the supply voltage off, and
configured for 12-bit and 128 sample averaging, up to
vice-versa). The INA220 senses the small drop
68ms in time between sampling these two values is
across the shunt for shunt voltage, and senses the
possible. Again, these calculations are performed in
voltage with respect to ground from V
BUS
for the bus
the background and do not add to the overall
voltage.
conversion time.
When the INA220 is in the normal operating mode
PGA Function
(that is, MODE bits of the Configuration Register are
set to '111'), it continuously converts the shunt
If larger full-scale shunt voltages are desired, the
voltage up to the number set in the shunt voltage
INA220 provides a PGA function that increases the
averaging function (Configuration Register, SADC
full-scale range up to 2, 4, or 8 times (320mV).
bits). The device then converts the bus voltage up to
Additionally, the bus voltage measurement has two
the number set in the bus voltage averaging
full-scale ranges: 16V or 32V.
(Configuration Register, BADC bits). The Mode
control in the Configuration Register also permits
Compatibility with TI Hot Swap Controllers
selecting modes to convert only voltage or current,
The INA220 is designed for compatibility with hot
either continuously or in response to an event
swap controllers such the TI TPS2490. The TPS2490
(triggered).
uses a high-side shunt with a limit at 50mV; the
All current and power calculations are performed in
INA220 full-scale range of 40mV enables the use of
the background and do not contribute to conversion
the same shunt for current sensing below this limit.
time; conversion times shown in the Electrical
When sensing is required at (or through) the 50mV
Characteristics table can be used to determine the
sense point of the TPS2490, the PGA of the INA220
actual conversion time.
can be set to ÷2 to provide an 80mV full-scale range.
Power-Down mode reduces the quiescent current
Filtering and Input Considerations
and turns off current into the INA220 inputs, avoiding
any supply drain. Full recovery from Power-Down
Measuring current is often noisy, and such noise can
requires 40ms. ADC Off mode (set by the
be difficult to define. The INA220 offers several
Configuration Register, MODE bits) stops all
options for filtering by choosing resolution and
conversions.
averaging in the Configuration Register. These
filtering options can be set independently for either
In triggered mode, writing any of the triggered convert
voltage or current measurement.
modes into the Configuration Register (even if the
desired mode is already programmed into the
The internal ADC is based on a delta-sigma (ΔΣ)
register) triggers a single-shot conversion.
front-end with a 500kHz (±30%) typical sampling rate.
This architecture has good inherent noise rejection;
Although the INA220 can be read at any time, and
however, transients that occur at or very close to the
the data from the last conversion remain available,
sampling rate harmonics can cause problems.
the Conversion Ready bit (Status Register, CNVR bit)
Because these signals are at 1MHz and higher, they
is provided to help co-ordinate one-shot or triggered
can be dealt with by incorporating filtering at the input
conversions. The Conversion Ready bit is set after all
of the INA220. The high frequency enables the use of
conversions, averaging, and multiplication operations
low-value series resistors on the filter for negligible
are complete.
effects on measurement accuracy. In general, filtering
space
the INA220 input is only necessary if there are
transients at exact harmonics of the 500kHz (±30%)
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