Rainbow Electronics MAX11008 User Manual

Page 28

MAX11008

Figure 16 shows the functional diagram of the
MAX11008 DACs. Each DAC includes an input and
output register. The input registers hold the result of the
most recent write operation, and the output registers
hold the current output code for the respective DAC.
Data written to a DAC input register is transferred to its
output register by writing to the Load DAC register (see
Table 22). Alternatively, write data directly to the output
register using the DAC Input and Output Data register.

The analog output voltages of the DACs (before amplifi-
cation by the gate-drive amplifiers) are calculated with
the following equation:

where V

DACREF

is the value of the internal or external

reference voltage and CODE is the decimal value of the
12-bit code contained in the output register.

Gate-Drive Amplifiers

The gate-drive amplifiers are proportional to the analog
outputs of the 12-bit DACs and provide the necessary
gate voltage to drive the external LDMOS transistors.
Both amplifiers have a fixed gain of 2V/V and are capa-
ble of sourcing or sinking up to 2mA of current. Output
short-circuit protection prevents output currents from
exceeding ±25mA.

The gate output is equal to the DAC output voltage
amplified by 2.

GATE_

= 2 x V

DAC

See the

Software Configuration Registers

and

Temperature/APC LUT Configuration Registers

sections

for information on how the gate voltages are controlled
by temperature and APC samples.

Output Clamp

The MAX11008 features an output clamp mode that
protects the external LDMOS transistors by connecting
the gate-drive amplifier outputs (GATE_) to AGND. The
clamp mode can be controlled by the OPSAFE_ digital
inputs or by setting the appropriate ALMCLMP[1:0] bits
in the Alarm Hardware Configuration register (see
Table 14). When using the OPSAFE_ digital inputs, pull
OPSAFE_ high to enter clamp mode and pull OPSAFE_
low to exit clamp mode. The clamp can also be activat-
ed automatically from the alarm trip point setting regis-
ters; see the

Alarm Software Configuration Register

(ALMSCFIG) (Read/Write)

section.

Self-Calibration

Calibrate channel 1 and channel 2 by writing to the
PGA Calibration Control register. The MAX11008 func-
tions after power-up without a calibration. Command a
calibration after powering up the device by setting the
TRACK bit to 0 and the DOCAL bit to 1 (see Table 19).
Subsequently, set the TRACK, DOCAL, and SELFTIME
bits to 1 to enable automatic self-calibration (approxi-
mately every 13ms). This minimizes loss of perfor-
mance over temperature and supply-voltage variation.
Alternatively, run self-calibration manually to control the
timing of the operation. Set the TRACK and DOCAL bits
to 1 and the SELFTIME bit to 0 to perform manually trig-
gered self-calibration.

The self-calibration algorithm cancels offsets at the
PGA-drive amplifier inputs in approximately 50µV incre-
ments to improve accuracy. The self-calibration routine
can be commanded when the DACs are powered
down, but the results will not be accurate. For best
results, run the calibration after the DAC power-up time,
t

DPUEXT

. The ADC’s operation is suspended during a

self-calibration. The BUSY output returning low indi-
cates the end of the self-calibration routine. Wait until
the end of the self-calibration routine before requesting
an ADC conversion.

CODE

DAC

DACREF

4096

Dual RF LDMOS Bias Controller with
Nonvolatile Memory

______________________________________________________________________________________

Channel 1/Channel 2

CHANNEL 1/

CHANNEL 2 DAC

CHANNEL 1/CHANNEL 2

DAC OUTPUT REGISTERS

CHANNEL 1/CHANNEL 2

DAC INPUT REGISTERS

LDDACCH_
SET TO 1 IN

LOAD DAC

Figure 16. DAC Functional Diagram