X60250 Datasheet, PDF (7/16 Pages) Intersil Corporation – Micro Power Programmable Voltage Reference

X60250
FUNCTIONAL DESCRIPTION
The X60250 combines a micropower precision reference
with an 8-bit, 256 tap digitally controlled 100kΩ
potentiometer (DCP) which allows nonvolatile setting of
an output reference voltage. When normally configured
with the V_REFL pin tied to ground, the device provides an
output range of 0V to 1.25V with 4.90mV resolution.
The device can also be configured with an optional
100kΩ series resistor to ground, which effectively halves
the output voltage range while doubling the resolution.
Grounding the R₁ pin while floating the V_REFL pin places
the device in this mode. Output voltage setting accuracy
can be as high as 0.10% while permitting adjustment
from 0.625V to 1.25V (2.45mV resolution).
Reference Section
The reference is designed to provide an accurate, low
tempco voltage source while requiring less than 12µA
(typical) of supply current. This supply current is for the
reference section only. Keep in mind that the DCP will
increase supply current draw by VREF/RTOTAL
(typically 1.25/100k or 12.5µA). The total current drawn
by the adjustable reference circuit will be less than 25µA
(typically).
The reference output has a typical impedance of 1Ω and
can provide up to 400µA of load current. It is intended to
drive the resistive load of the DCP, which is a minimum
of 85kΩ, but can also be used to drive off chip circuitry
provided the loading does not exceed the 400µA
maximum. Also, highly capacitive loads can make the
reference oscillate, so no more than 2000pF should be
placed directly on the output of the V_REFOUT pin.
The reference output produces about 200µV RMS of
noise (10kHz bandwidth) due to its micropower design.
This is easily reduced in normal applications, as shown in
the applications section for optimizing circuits for
reducing output noise levels.
DCP Section
The 256 tap DCP has an 8-bit nonvolatile wiper control
register which controls which tap is selected. The
register is changed by performing a serial data write to its
address (0h, see Serial Interface section). The resulting
wiper position will produce an output voltage at V_OUT,
depending on whether the DCP V_REFL is grounded or the
R₁ pin is grounded. The wiper consists of CMOS
transistors and has a finite resistance, typically 600Ω at
V_CC = 5V (this parameter increases with decreasing
V_CC). The wiper resistance will produce errors in
reference circuits due to I-R drops if current flows
through the wiper. However, typically these circuits will
have the wiper connected to a high impedance
comparator or amplifier input which results in very small
wiper currents and thus only a small output voltage error.
If the X60250 is used with the wiper connected to V_REFL
to produce a current source, care must be taken to avoid
exceeding the maximum output current of the reference
(typically 400µA).
Power-Up considerations
The X60250 contains EEPROM nonvolatile storage cells
which are recalled during power-up. This recall process
works best with power supply (V_CC) ramping that is
monotonic and free of excessive glitches (<100mV
disturbances give best results). The ramp rate spec
should be adhered to, although the most sensitive part of
recall is between V_CC = 1.0V and 2.5V. Effort should be
made to make sure the device receives a power-up ramp
between those voltage levels that meet the ramp rate
spec and have no glitches.
Recall of the stored wiper position happens in < 1ms
from V_CC reaching 2.5V. Note that any excursions of V_CC
below 2.5V, although temporary, can cause the wiper to
be loaded with the midpoint value (80h) until V_CC
recovers to its normal voltage.
Register Organization
There are 2 nonvolatile registers and 1 volatile register
available for storage and recall via the serial bus. They
contain the current wiper position, a general purpose
data register and a status register.
The wiper register is nonvolatile and is at address 0h and
contains 8 bits, with the 00h setting corresponding to the
tap position nearest V_REFL, and the FFh setting nearest
to V_REFOUT.
The general purpose register is nonvolatile and is at
address 1h, and contains 8 bits for use as scratchpad
memory or serial number information.
The Status register is volatile and is at address 7h. It has
one active bit, D3, which is the WEL bit. This bit must be
set to 1 berfore any nonvolatile writes are performed to
the other registers. See the register information on the
next page.
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FN8146.1
September 14, 2005