Multichannel Remote/Local
Temperature Sensors
MAX1668/MAX1805/MAX1989
†
Pin Description
PIN
MAX1668/
MAX1989
1, 3, 5, 7
FUNCTION
MAX1805
NAME
Combined Current Source and A/D Positive Input for Remote-Diode Channel. Do not
leave DXP floating; connect DXP to DXN if no remote diode is used. Place a 2200pF
capacitor between DXP and DXN for noise filtering.
Combined Current Sink and A/D Negative Input. DXN is normally biased to a diode volt-
age above ground.
Supply Voltage Input, 3V to 5.5V. Bypass to GND with a 0.1µF capacitor. A 200Ω series
resistor is recommended but not required for additional noise filtering.
SMBus Address Select Pin (Table 8). ADD0 and ADD1 are sampled upon power-up.
Excess capacitance (>50pF) at the address pins when floating can cause address-
recognition problems.
SMBus Slave Address Select Pin
SMBus Alert (Interrupt) Output, Open Drain
SMBus Serial-Data Input/Output, Open Drain
SMBus Serial-Clock Input
Hardware Standby Input. Temperature and comparison threshold data are retained in
standby mode. Low = standby mode, high = operate mode.
Ground
No Connection. Not internally connected. Can be used for PC board trace routing.
1, 3
DXP_
2, 4, 6, 8
9
10
11
12
13
14
15
16
2, 4
9
10
11
12
13
14
15
16
5–8
DXN_
V
CC
ADD1
ADD0
ALERT
SMBDATA
SMBCLK
STBY
GND
N.C.
—
_______________Detailed Description
The MAX1668/MAX1805/MAX1989 are temperature
sensors designed to work in conjunction with an exter-
nal microcontroller (µC) or other intelligence in thermo-
static, process-control, or monitoring applications. The
µC is typically a power-management or keyboard con-
troller, generating SMBus serial commands by “bit-
banging” general-purpose input-output (GPIO) pins or
through a dedicated SMBus interface block.
These devices are essentially 8-bit serial analog-to-digi-
tal converters (ADCs) with sophisticated front ends.
However, the MAX1668/MAX1805/MAX1989 also contain
a switched current source, a multiplexer, an ADC, an
SMBus interface, and associated control logic (Figure 1).
In the MAX1668 and MAX1989, temperature data from
the ADC is loaded into five data registers, where it is
automatically compared with data previously stored in
10 over/undertemperature alarm registers. In the
MAX1805, temperature data from the ADC is loaded into
three data registers, where it is automatically compared
with data previously stored in six over/undertemperature
alarm registers.
ADC and Multiplexer
The ADC is an averaging type that integrates over a
64ms period (each channel, typical), with excellent
noise rejection.
The multiplexer automatically steers bias currents
through the remote and local diodes, measures their
forward voltages, and computes their temperatures.
Each channel is automatically converted once the con-
version process has started. If any one of the channels
is not used, the device still performs measurements on
these channels, and the user can ignore the results of
the unused channel. If any remote-diode channel is
unused, connect DXP_ to DXN_ rather than leaving the
pins open.
The DXN_ input is biased at 0.65V above ground by an
internal diode to set up the A/D inputs for a differential
measurement. The worst-case DXP_ to DXN_ differential
input voltage range is 0.25V to 0.95V.
Excess resistance in series with the remote diode caus-
es about +0.5°C error per ohm. Likewise, 200µV of offset
voltage forced on DXP_ to DXN_ causes about 1°C error.
6
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