LTC2933
27
2933fa
For more information www.linear.com/LTC2933
Typical applicaTions
Two-Channel Voltage Monitoring with EEPROM Fault
Storage Power Backup
Figure 5 in the Typical Applications section illustrates an
EEPROM fault storage power backup circuit. The LTC2933
is supplied by the 12V rail, which is also monitored on V1.
The other monitored rail, 1.8V on V3, is too low to provide
adequate supply voltage, in case the 12V line collapses
to ground. In case such a fault occurs, the LTC2933 still
needs adequate power for EEPROM backup fault storage,
which takes less than 10ms. This is provided by the 22µF
capacitor connected between the V2 pin and ground, which
is charged from the 12V rail through R1. Since the V2
voltage may not exceed 6V, a 4.7V voltage-limiting Zener
diode connected between V2 and ground is necessary. In
this example, V4 through V6 are not used.
The minimum value of the charge-storage capacitor is
calculated as:
C
MIN
=
2SUP(MAX)
EEFS
V2− V2
MIN
=
1.5mA • 10ms
= 11.5µF
R1 has to limit the Zener diode reverse current to a value be-
low its maximum rating. This determines R1’s minimum
value.
R
MIN
=
−
I
Z(MAX)
=
−
0.1mA
= 73kΩ
The maximum value of R1 is determined by the V2 pin
input current and the Zener diode reverse leakage current:
R
MAX
=
−
I
Z(MIN)
+ V2 / R
IN(MIN)
=
12V − 4.7V
0.01mA + 4.7V / 400k
= 336kΩ
Low Cost Multipoint Temperature Control System
Figure 6 in the Typical Applications section illustrates a
low cost, 4-point temperature control system, which is
suited for such commercial applications as electric ovens
and dryers.
The temperature sensors are four 2N3904 diode-connected
BJTs, strategically placed inside the oven/dryer, which are
forward-biased at constant current through 10k resistors
connected to the regulated 3.3V pin. The diode voltages,
which exhibit a negative 2.2mV/°C temperature coef
-
ficient, are monitored on the V2 to V5 inputs, set to the
precision range.
The OV faults, corresponding to under-the-limit tempera
-
tures,
are mapped into GPIO1, which controls the electric
heater through a power MOSFET switch and a relay.
The UV faults, corresponding to over-the-limit tempera
-
tures, are mapped into GPIO2, which controls the cooling
fan through a power MOSFET switch.
A microprocessor is used to program the appropriate
temperature limits into the LTC2933, via the I
2
C interface.
All faults are also mapped into GPIO3, which alerts the
microprocessor on system status.
The diode connected in series with the fan 12V supply
protects the LTC2933 against inductive voltage spikes
which can propagate on its V1 supply pin through the
common 12V line.
Such a low cost system can control oven/dryer temperature
within ±10°C accuracy, over a 50°C to 150°C range, after
proper calibration.
