ISL21400 Datasheet, PDF (14/17 Pages) Intersil Corporation – Programmable Temperature Slope Voltage Reference

ISL21400
Applications Information
Power-Up Considerations
The ISL21400 has on-chip EEPROM memory storage for
the DAC and gain settings of the device. These settings
must be recalled correctly on power-up for proper operation.
Normally there are no issues with recall, although it is always
best to provide a smooth, glitch-free power-up waveform on
V_CC. Adding a small 0.1μF capacitor at the device V_CC will
help with power-up as well as V_OUT load changes.
Noise Performance
The output noise voltage in a 0.1Hz to 10Hz bandwidth is
typically 90µV_P-P. The noise measurement is made with a
bandpass filter made of a 1 pole high-pass filter with a corner
frequency at 0.1Hz and a 2-pole low-pass filter with a corner
frequency at 12.6Hz to create a filter with a 9.9Hz
bandwidth. Load capacitance up to 5000pF can be added
but will result in only marginal improvements in output noise
and transient response. The output stage of the ISL21400 is
not designed to drive heavily capacitive loads. For high
impedance loads, an R-C network can be added to filter high
frequency noise and preserve DC control.
Output Voltage Programming Considerations
Setting and controlling the output voltage of the ISL21400
can be done easily by breaking down the components into
temperature variant and invariant, and setting them
separately. Let’s use Equation (1) above to derive separate
Reference Output and Output Temp Slope equations:
V_OUT
=
⎧
⎩⎨ ^{A V}
•
V_REF
•
----ⁿ-----
255
⎫
⎬
⎭
+
⎧
⎩⎨ ^{A V}
•
V
T
S
•
⎛
⎝
(_---2_-----•_----m_-2---5--)_{--5-–----}2_----5_---5_{--⎠⎞}
⎫
⎬
⎭
= {A_V • V_REF • A_REF} + {A_V • V_TS • A_TS} ,Eq. 2
Reference Term + Temp Slope Term
The first term controls the output DC value, and the second
term controls the Temp slope, where
A_REF
=
----ⁿ-----
255
(ranges from 0 to 1)
A_TS=
⎛
⎝
(_---2_-----•_----m_-2---5-)_{---5-–-----}2_---5_---5_{--⎠⎞}
(ranges from -1 to +1)
DC OUTPUT CONTROL DISCUSSION
The equation above yields this expression, Equation 2, for
Reference output:
V_OUT(DC) = A_V • V_REF • A_REF
(EQ. 2)
Note that the DC term is dependent on the 1.20V reference
voltage, which is constant, the overall gain, A_V, and the
Reference gain, A_REF. Since the product A_V * A_REF ranges
from 0 to 4, the total reference DC output can range from
0.0V to 4.8V. In order to get the 4.8V output, V_CC must be
greater than 4.8V by the output dropout plus any overhead
for output loading (the specification for V_OUT = 5.0V is listed
with V_CC = 5.5V). The Resolution of V_OUT(DC) control
changes with A_V, so that with a 4.80V full scale output
(A_V = 4), the resolution is 4.80/255 or 18.8mV/bit. With
A_V = 1, the resolution is 4.7mV/bit.
TEMP SENSE CONTROL DISCUSSION
The equation above yields this expression, Equation 3, for
Temp Slope:
V_OUT(TS) = A_V • V_TS • A_TS
(EQ. 3)
Since V_TS = K(T-T₀), the slope term is dependent on the
base temp slope of the device, K (-2.1mV/°C), and the gain
terms A_V and A_TS. This gives a formula for the portion of
V_OUT at a specific temperature:
V_OUT(TS)= A_V • K • A_TS • (T – T₀)
(EQ. 4)
The product A_V*A_TS ranges from -4 to 4, so the Temperature
Slope can range from -8.4 to +8.4mV/°C, which is
independent of the output DC voltage. The resolution of
Slope control is determined by this range (±8.4mV/°C) and
the gain terms, and will vary from 65.8μV/°C/bit (A_V = 4)
down to 16.2μV/°C/bit (A_V = 1).
At T = T₀ = +25°C, V_OUT(TS) = zero, no changes in A_TS will
cause a change in V_OUT, and V_OUT will only vary with the
V_OUT(DC) control. As temperature increases or decreases,
from T = +25°C, V_OUT will then change according to the
programmed Temp Slope.
In many cases a form of Equation 4 is needed which yields a
V_OUT change with respect to temperature. By rearranging,
we get:
V_OUT(T)=
-^V----^O----^U----^T----⁽--^T-----^S----⁾
(T – T₀)
=
A_V • K • A_TS
,(in mV/°C )
(EQ. 5)
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 specs (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_OS = 2.50
14
FN8091.0
December 14, 2006