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ISL21400
In many cases a form of Equation 6 is needed which yields a
V OUT change with respect to temperature. By rearranging,
we get Equation 7:
V OUT parameters, the system characteristics can be
recorded.
For the following example, let’s determine the voltage output,
V OUT ( T ) = ----------------------------- = A V ? K ? A TS ,(in mV/ ° C )
V OUT ( TS )
( T – T 0 )
(EQ. 7)
V OUT (DC) at +25°C, and also the change due to
temperature variation (ppm) from +25°C to +85°C.
Equations 4 and 7 will be used to calculate the answers.
= ? 1 ? 1.20 ? ---------- ?
= 1 ? – 2.1 ? ------------------------------------
A ( REF ) = ----------- = 0.625 = ----------
--------------------------------------- ? 10 = 1010ppm ? ° C
A TS = --------------------
V OUT = A V ? ? V REF ? ---------- + K ( T – T 0 ) -------------------------------- ?
?
?
= ? ? 1.20 ? ---------- + ( – 0.0021 ) ( 85 – 25 ) ---------------------------------- ?
?
?
A TS = 0.333 = ----------------------------------
EXAMPLE 1: PROGRAMMED TEMPERATURE
COMPENSATION EXAMPLE
The ISL21400 can easily compensate for known
temperature drift by programming the device for the initial
V OUT setting and Tempco using standard equations and
some simple steps. The accuracy of the final programmed
output will be limited to the data sheet specifications
(typically 1% accuracy for V OUT and Slope).
In this example, an N-Channel MOSFET gate has a
-2.8mV/°C Tempco from -10°C to +85°C. A constant bias
drain current is desired, with a target Vgs range derived from
the data sheet of 2.5V to 3.5V at +25°C.
Offset Setting : Using Equation 2 and targeting
V OUT = 3.0VDC:
V OUT ( DC ) = ( A V ? V REF ? A REF ) = 3.00V
V REF = 1.20V
A V ? A REF = 2.50
Note that A REF varies from 0 to 1, so to get 2.40, A V = 4.
2.50 n
4 255
n = 159 decimal
= 9F hex
Temperature Slope Setting : Using Equation 5, which can
solve for Slope directly:
V OUT ( T ) = A V ? K ? A TS = – 2.8mV ? ° C
– 2.8
4 ? – 2.1
( 2 ? m ) – 255
255
m = 170 decimal
= A9 hex
The ISL21400 device can be programmed with these
calculated parameters and perform temperature
compensation or direct control in the target circuit. If
parameters change for some reason, then the device can be
reprogrammed with new values and the circuit retested.
EXAMPLE 2. CALCULATING THE V OUT TEMPERATURE
SLOPE
In some applications, it may be desirable to calculate what
the output voltage and temp slope are, given the
programmed register settings. Such an application could be
a closed loop system with internal calibration procedure. By
reading the registers of the ISL21400, then calculating the
15
Given, the contents of the registers:
A V = 1
n = 178 decimal
m = 74 decimal
Using Equation 2:
V OUT ( DC ) = ( A V ? V REF ? A REF )
178
? 255 ?
= 0.8376V
Using Equation 5:
V OUT ( T ) = ( A V ? K ? A TS ) mV ? ° C
( 2 ? 74 ) – 255
255
= 0.8812mV ? ° C
Also, to solve for overall temp slope of the output:
0.8812mV ? ° C 6
872.5mV
Note that Equation 1 can be used directly to solve for output
voltage at a given temperature, in this case +85°C:
? n ( 2 ? m ) – 255 ?
255 255
1
V OUT ( +85 ° C ) =
? 178 ( 2 ? 74 ) – 255 ?
255 255
= 0.8905V
Typical Applications Circuits
LDMOS RF Power Amplifier (RFPA). The ISL21400 is used
to set the gate bias for the LDMOS transistor in a single
stage of an RFPA. Normally this is done with a DAC or digital
potentiometer with some discrete temperature
compensation circuitry. The ISL21400 simplifies this control
and allows a full range of DC bias and tempco control.
A typical circuit can be calibrated for correct bias at room
temperature (+25°C) on power-up using a microcontroller or
direct I 2 C control. The temperature of the unit can then be
increased to the highest operating range, and the
Temperature Slope setting can then be adjusted to bring the
amplifier back to correct bias. Since the Temp Slope setting
has a negligible effect on the room temperature setting, the
amplifier will be biased correctly over the operating
temperature of the unit.
FN8091.3
March 31, 2011
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