ISL21400 Intersil Corporation, ISL21400 Datasheet - Page 14

• Manufacturer Part Number: ISL21400
• Description: Programmable Temperature Slope Voltage Reference
• Manufacturer: Intersil Corporation

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
help with power-up as well as V
Noise Performance
The output noise voltage in a 0.1Hz to 10Hz bandwidth is
typically 90µV
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:
The first term controls the output DC value, and the second
term controls the Temp slope, where
DC OUTPUT CONTROL DISCUSSION
The equation above yields this expression, Equation 2, for
Reference output:
Note that the DC term is dependent on the 1.20V reference
voltage, which is constant, the overall gain, A
Reference gain, A
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
greater than 4.8V by the output dropout plus any overhead
for output loading (the specification for V
A
A
V
V
CC
REF
TS
OUT
OUT
=
. Adding a small 0.1μF capacitor at the device V
(DC)
=
⎛
⎝
=
=
(
----------------------------------
--------- -
255
2 m
⎧
⎨
⎩
n
{
•
A
A
=
V
255
V
(ranges from 0 to 1)
A
) 255
•
•
P-P
V
–
V
V
Reference Term
•
REF
REF
. The noise measurement is made with a
V
REF
REF
⎞ (ranges from -1 to +1)
⎠
•
•
--------- -
255
. Since the product A
A
•
n
REF
A
⎫
⎬
⎭
REF
+
}
⎧
⎨
⎩
14
+
A
{
V
OUT
A
•
+
V
V
•
Temp Slope Term
TS
V
load changes.
TS
•
⎛
⎝
OUT
•
(
----------------------------------
2 m
A
V
•
TS
* A
V
255
= 5.0V is listed
} ,Eq. 2
, and the
CC
) 255
REF
–
must be
CC
ranges
⎞
⎠
⎫
⎬
⎭
(EQ. 2)
will
ISL21400
V
with V
changes with A
(A
A
TEMP SENSE CONTROL DISCUSSION
The equation above yields this expression, Equation 3, for
Temp Slope:
Since V
base temp slope of the device, K (-2.1mV/°C), and the gain
terms A
V
The product A
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
down to 16.2μV/°C/bit (A
At T = T
cause a change in V
V
from T = +25°C, V
programmed Temp Slope.
In many cases a form of Equation 4 is needed which yields a
V
we get:
EXAMPLE 1: PROGRAMMED TEMPERATURE
COMPENSATION EXAMPLE
The ISL21400 can easily compensate for known
temperature drift by programming the device for the initial
V
some simple steps. The accuracy of the final programmed
output will be limited to the data sheet specs (typically 1%
accuracy for V
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
V
V
V
V
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
V
= 1, the resolution is 4.7mV/bit.
= 4), the resolution is 4.80/255 or 18.8mV/bit. With
(
(
(DC) control. As temperature increases or decreases,
T ( )
(
CC
TS
at a specific temperature:
change with respect to temperature. By rearranging,
setting and Tempco using standard equations and
= 3.0VDC,
TS
DC
V
TS
0
=
)
)
= 5.5V). The Resolution of V
and A
= +25°C, V
)
=
=
=
= K(T-T
V
---------------------------- -
OUT
(
A
A
A
(
T T
V
A
V
V
V
V
OUT
REF
–
V
*A
V
TS
•
•
•
, so that with a 4.80V full scale output
(
•
A
K
V
TS
TS
0
. This gives a formula for the portion of
OUT
V
OS
0
TS
•
)
and Slope).
=
), the slope term is dependent on the
REF
A
)
ranges from -4 to 4, so the Temperature
OUT
OUT
=
•
TS
1.20V
=
A
will then change according to the
A
•
2.50
TS
•
, and V
V
V
(TS) = zero, no changes in A
A
(
•
= 1).
T T
REF
K
–
•
)
A
0
OUT
)
TS
=
3.00V
,(in mV/°C )
will only vary with the
OUT
(DC) control
December 14, 2006
V
= 4)
TS
FN8091.0
(EQ. 3)
(EQ. 4)
(EQ. 5)
will