AD5420BCPZ AD [Analog Devices], AD5420BCPZ Datasheet - Page 23

• Manufacturer Part Number: AD5420BCPZ
• Description: Single Channel, 16-Bit, Serial Input, Current Source DAC
• Manufacturer: AD [Analog Devices]

Preliminary Technical Data
FEATURES
FAULT ALERT
The AD5420 is equipped with a FAULT pin, this is an open-
drain output allowing several AD5420 devices to be connected
together to one pull-up resistor for global fault detection. The
FAULT pin is forced active by any one of the following fault
scenarios;
The OPEN CCT and OVER TEMP bits of the STATUS register
are used in conjunction with the FAULT pin to inform the user
which one of the fault conditions caused the FAULT pin to be
asserted. See Table 17 and Table 18.
ASYNCHRONOUS CLEAR (CLEAR)
CLEAR is an active high clear that clears the Current output to
the bottom of its programmed range. It is necessary to maintain
CLEAR high for a minimum amount of time (see Figure 2) to
complete the operation. When the CLEAR signal is returned
low, the output remains at the cleared value until a new value is
programmed.
INTERNAL REFERENCE
The AD5420 contains an integrated +5V voltage reference with
initial accuracy of ±2mV max and a temperature drift
coefficient of ±10 ppm max. The reference voltage is buffered
and externally available for use elsewhere within the system. See
Figure 32 for a load regulation graph of the Integrated reference.
EXTERNAL CURRENT SETTING RESISTOR
Referring to Figure 34, R1 is an internal sense resistor as part of
the voltage to current conversion circuitry. The stability of the
output current over temperature is dependent on the stability of
the value of R1. As a method of improving the stability of the
1)
2)
The Voltage at I
compliance range, due to an open-loop circuit or
insufficient power supply voltage. The I
controlled by a PMOS transistor and internal
amplifier as shown in Figure 34. The internal circuitry
that develops the fault output avoids using a
comparator with “window limits” since this would
require an actual output error before the FAULT
output becomes active. Instead, the signal is generated
when the internal amplifier in the output stage has less
than approxiamately one volt of remaining drive
capability (when the gate of the output PMOS
transistor nearly reaches ground). Thus the FAULT
output activates slightly before the compliance limit is
reached. Since the comparison is made within the
feedback loop of the output amplifier, the output
accuracy is maintained by its open-loop gain and an
output error does not occur before the FAULT output
becomes active.
If the core temperature of the AD5420 exceeds approx.
150°C.
OUT
attempts to rise above the
OUT
current is
Rev. PrD | Page 23 of 29
output current over temperature an external precision 15kΩ low
drift resistor can be connected to the R
be used instead of the internal resistor R1. The external resistor
is selected via the CONTROL 1 register. See Table 12.
DIGITAL POWER SUPPLY
By default the DV
alternatively, via the DV
supply may be output on the DV
supply for other devices in the system or as a termination for
pull-up resistors. This facility offers the advantage of not having
to bring a digital supply across an isolation barrier. The internal
power supply is enabled by leaving the DV
unconnected. To disable the internal supply DV
should be tied to 0V.
EXTERNAL BOOST FUNCTION
The addition of an external boost transistor as shown in Figure
36 will reduce the power dissipated in the AD5420 by reducing
the current flowing in the on-chip output transistor (dividing it
by the current gain of the external circuit). A discrete NPN
transistor with a breakdown voltage, BV
can be used.
The external boost capability has been developed for those
users who may wish to use the AD5420 at the extremes of the
supply voltage, load current and temperature range. The boost
transistor can also be used to reduce the amount of temperature
induced drift in the part. This will minimise the temperature
induced drift of the on-chip voltage reference, which improves
drift and linearity.
AD5420
Figure 36. External Boost Configuration
CC
pin accepts a power supply of 2.7V to 5.5V,
CC
BOOST
SELECT pin an internal 4.5V power
I
OUT
0.022 F
CC
pin for use as a digital power
SET
1k
CEO
pin of the AD5420 to
CC
, greater than 60V
SELECT pin
MJD31C
2N3053
CC
OR
R
LOAD
SELECT
AD5420