DS1870 datasheet(13/28 Pages) MAXIM

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LDMOS RF Power-Amplifier Bias

Controller

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the full-scale (FS) value. The expected FS value is also

found by multiplying an all-ones digital answer by the

LSB weight.

Example: Since the FS digital reading is 65,528 (FFF8

hex) LSBs, if the LSB’s weight is 50µV, then the FS

value is 65,528 x 50µV = 3.2764V.

A binary search is used to calibrate the gain of the con-

verter. This requires forcing two known voltages to the

input pin. It is preferred that one of the forced voltages

is the null input and the other is 90% of FS. Since the

LSB of the least significant bit in the digital reading reg-

ister is known, the expected digital results can be cal-

culated for both the null input and the 90% of full scale

value.

An explanation of the binary search used to scale the

gain is best served with the following example pseudo-

code:

/* Assume that the null input is 0.5V */

/* Assume that the requirement for the LSB is 50µV */

FS = 65528 * 50e-6;

/*3.2764V */

CNT1 = 0.5 / 50e-6;

/* 1000 */

CNT2 = 0.9 X FS / 50e-6;

/* 58981.5 */

/* So the null input is 0.5V and 90% of FS is 2.949V */

Set the input’s offset register to zero

gain_result = 0h;

/* Working register for gain

calculation */

CLAMP = FFF8h;

/* This is the max ADC value*/

For n = 15 down to 0

begin

gain_result = gain_result + 2n;

Write gain_result to the input’s

gain register;

Force the 90% FS input (2.949V);

Meas2= ADC result from DS1870;

If Meas2 ≥ CLAMP

Then

gain_result = gain_result - 2n;

Else

Force the null input (0.5V)

Meas1 = ADC result from DS1870

If [(Meas2-Meas1)>(CNT2-CNT1)]

Then

gain_result = gain_result - 2n;

end;

Write gain_result to the input’s gain

The gain register is now set and the resolution of the

conversion matches the expected LSB. Customers

requiring non-zero null values (e.g., 0.5V) must next

calibrate the input’s offset. If the desired null value is

0V, leave the offset register programmed to 0000h and

skip this step.

To calibrate the offset register, program the gain regis-

ter with the gain_result value determined above. Next,

force the null input voltage (0.5V for the example) and

read the digital result from the part (Meas1). The offset

value can be calculated using the following formula:

Temperature-Monitor Offset Calibration

The DS1870’s temperature sensor comes precalibrated

and requires no further adjustment by the customer for

proper operation. However, it is possible for customers

to characterize their system and add a fixed offset to

the DS1870’s temperature reading so it is reflective of

another location’s temperature. This is not required for

biasing because the temperature offset can be

accounted for by adjusting the data’s location in the

LUTs, but this feature is available for customers who

see application benefits.

To change the temperature sensor’s offset: write the

temperature offset register to 0000h, measure the

source reference temperature (Tref), and read the tem-

perature from the DS1870 (TDS1870). Then, the follow-

ing formula can be used to calculate the value for the

temperature offset register.

Once the value is calculated, write it to the temperature

offset register.

Power-Up and Low-Voltage Operation

During power-up, the device is inactive until VCC

exceeds the digital power-on-reset voltage (VPOD). At

this voltage, the digital circuitry, which includes the I2C-

compatible interface, becomes functional. However,

EEPROM-backed registers/settings cannot be internally

read (recalled) until VCC exceeds the analog power-on

reset (VPOA), at which time the remainder or the device

becomes fully functional. Once VCC exceeds VPOA, the

Rdyb bit in byte 74h is timed to go from a 1 to a 0 and

indicates when ADC conversions begin. If VCC ever

dips below VPOA, the Rdyb bit reads as a 1 again.

Once a device exceeds VPOA and the EEPROM is

recalled, the values remain active (recalled) until VCC

falls below VPOD.

TempOffset

XOR

ref

bitwise

=× −

−

()

275

1870

Offset

Meas

= − ×



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