Rainbow Electronics MAX1386 - Manual (Page 23)

MAX1385/MAX1386
Dual RF LDMOS Bias Controllers
with I 2 C/SPI Interface
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ADC/DAC References
The MAX1385/MAX1386 provide an internal low-noise
2.5V reference for the ADCs, DACs, and temperature
sensor. See the
Device Configuration Register section
for information on configuring the device for external or
internal reference. Connect a voltage source to
REFADC in the 1V to AV
DD range when using an exter-
nal ADC reference. Connect a voltage source to REF-
DAC in the 0.5V to 2.5V range when using an external
DAC reference. When using an external voltage refer-
ence, bypass the reference pin with a 0.1µF capacitor
to AGND.
The internal reference has a lowpass filter to reduce
noise. The device allows 60µs (typ) and 81µs (typ) worst
case for the reference to settle before permitting an ana-
log-to-digital conversion. If reference mode 11 is select-
ed, the required settling time is longer. In this case, the
user should set at least one of DAC1PD, DAC2PD, or
FBGON in the Software Shutdown register, any of which
forces the reference to be permanently powered up.
Temperature Measurements
The MAX1385/MAX1386 measure the internal die tem-
perature and two external remote-diode temperature
sensors. Set up a temperature conversion by writing
to the Analog-to-Digital Conversion register (see the
ADCCON (Write) section). Optionally program SAFE1
and SAFE2 outputs to depend on programmed temper-
ature thresholds.
The MAX1385/MAX1386 can perform temperature mea-
surements with an internal diode-connected transistor.
The diode bias current changes from 66µA to 4µA to
produce a temperature-dependent bias voltage differ-
ence. The second conversion result at 4µA is subtract-
ed from the first at 66µA to calculate a digital value that
is proportional to the absolute temperature. The stored
data result is the aforementioned digital code minus an
offset to adjust from Kelvin to Celsius.
The reference voltage for the temperature measure-
ments is always derived from the internal reference
source. Temperature results are in degrees Celsius
(two’s-complement form).
The temperature-sensing circuits power up for the first
temperature measurement in an analog-to-digital con-
version scan. The temperature-sensing circuits remain
powered until the end of the scan to avoid a possible
67µs delay of internal reference power-up time for each
individual temperature channel. If the continuous con-
vert bit CONCONV is set high and the current ADC
channel selection includes a temperature channel, the
temperature-sensor circuits remain powered up until
the CONCONV bit is set low.
The external temperature-sensor drive current ratio has
been optimized for a 2N3904 npn transistor with an ide-
ality factor of 1.0065. The nonideality offset is removed
internally by a preset digital coefficient. Use of a tran-
sistor with a different ideality factor produces a propor-
tionate difference in the absolute measured
temperature. More details on this topic and others relat-
ed to using an external temperature sensor can be
found in Maxim Application Note 1057:
Compensating
for Ideality and Series Resistance Differences Between
Thermal Sense Diodes
and Application Note 1944:
Temperature Monitoring Using the MAX1253/MAX1254
and MAX1153/MAX1154
.
High-Side Current-Sense PGAs
The MAX1385/MAX1386 provide two high-side current-
sense amplifiers with programmable gain. The current-
sense amplifiers are unidirectional and provide a 5V to
30V input common-mode range. Both CS1+ and CS2+
must be within the specified common-mode range for
proper operation of each amplifier.
The sense amplifiers measure the load current, I
LOAD
,
through an external sense resistor, R
SENSE_
, between
the CS_+ and CS_- inputs. The full-scale sense voltage
range (V
SENSE_ = V CS_ + - V CS_ -) depends on the pro-
grammed gain, Av
PGA_
(see the DCFIG (Read/Write)
section). The sense amplifiers provide a voltage output
at PGAOUT_ according to the following equation:
These outputs are also routed to the internal 12-bit ADC
so that a digital representation of the amplified voltages
can be read through the FIFO.
The PGA scales the sensed voltages to fit the input
range of the ADC. Program the PGA with gains of 2, 10,
and 25 by setting the PGSET_ bits (see the
DCFIG
(Read/Write) section). The input stages have nominal
input offset voltages of 0mV that can be adjusted by a
trim DAC (not shown in the
Functional Diagram ) over the
-3mV to +3mV range in 25µV steps. Autocalibration can
be used to control the trim DAC to minimize the effective
channel input offset voltage (see the
PGACAL (Write)
section). The PGA feedback network is referenced to
AGND.
ALARM Output
The state of ALARM is logically equivalent to the inclu-
sive OR of SAFE1 and SAFE2. The exception to this
statement is when ALARM is configured for output inter-
rupt mode (see the
Alarm Modes section). When in out-
put-interrupt mode, ALARM stays in its asserted state
until its associated flag is cleared by reading from the
V Av V V
PGAOUT PGA CS CS _ _ _ _
( ) = Ч + − −