LM88 National Semiconductor, LM88 Datasheet - Page 6

• Manufacturer Part Number: LM88
• Description: Factory Programmable Dual Remote-Diode Thermostat
• Manufacturer: National Semiconductor

www.national.com
1.0 Functional Description
1.2 TYPICAL PIN CONNECTION
1.3 LM88 OPTIONS
1.3.1 Set-Point Values
T_SP0 and T_SP1 are dependent on the value of T_CRIT:
where:
a and b are any integer in the range of −32 to +31.
T_CRIT can be any value in the range of 0˚C to +125˚C with
a resolution of 1˚C.
1.3.2 Functionality
The LM88’s comparators can be factory programmed to do a
greater than or less than comparison. When programmed for
a greater than comparison, the comparison result is true
when the temperature measured is above the prepro-
grammed setpoint temperature. The comparison returns to
false when the temperature measured is below or equal to
the setpoint temperature minus one degree. For a less than
comparison the comparison result is true when the tempera-
ture measured is below the preprogrammed limit. The result
turns to false when the temperature measured is above or
equal to the setpoint limit plus one degree. SP0, SP1 and
CRIT comparisons can all be independently programmed to
Pin Label
D0+
D−
D1+
GND
O_CRIT
O_SP1
O_SP1
V
+
T_SP0 = T_CRIT + 4a + 1
T_SP1 = T_CRIT + 4b + 1
(Continued)
Pin Number
1
2
3
4
5
6
7
8
6
be greater than or less than. All CRIT comparisons are
required to be the same, either greater than or less than. The
comparator hysteresis can also be factory set to one, two or
three degrees. The hysteresis for all comparisons is required
to be the same.
2.0 Application Hints
2.1 OPEN-DRAIN OUTPUTS
The O_SP0, O_SP1 and 0_CRIT outputs are open-drain
outputs and do not have internal pull-ups. A “high” level will
not be observed on these pins until pull-up current is pro-
vided from some external source, typically a pull-up resistor.
Choice of resistor value depends on many system factors
but, in general, the pull-up resistor should be as large as
possible. This will minimize any internal temperature reading
errors due to internal heating of the LM88. The maximum
resistance of the pull-up needed to provide a 2.1V high level,
based on LM88 specification for High Level Output Current
with the supply voltage at 3.0V, is 430k .
2.2 THERMAL DIODE MOUNTING CONSIDERATIONS
To measure temperature the LM88 uses two remote diodes.
These diodes can be located on the die of a target IC,
allowing measurement of the IC’s temperature, independent
Typical Connection
3904-type transistor
shorted-collector base or
Pentium thermal diode anode;
2.2nF capacitor connected to
D-
3904-type transistor emitter or
Pentium thermal diode
cathode (individual traces are
required to each diode; do not
daisy chain); two 2.2nF
capacitors connected to D0+
and D1+
3904-type transistor shorted
collector-base or Pentium
thermal diode anode; 2.2nF
capacitor connected to D-
a quiet system ground
2k pull-up; system shutdown
or the THERM pin of the ICH
(I/O Controller Hub found in
PCs)
2k pull-up; general purpose
input (GPI), to determine
which diode caused the
THERM event
2k pull-up; general purpose
input (GPI), to determine
which diode caused the
THERM event
3.3V; 0.1µF bypass capacitor