ISL705AEH INTERSIL [Intersil Corporation], ISL705AEH Datasheet - Page 14

• Manufacturer Part Number: ISL705AEH
• Description: Rad-Hard, 5.0V/3.3V ?-Processor Supervisory Circuits
• Manufacturer: INTERSIL [Intersil Corporation]

is at its maximum specified value. This allows the positive supply
to fluctuate within its acceptable range without signaling a reset
when configured as shown in Figure 34.
In Figure 34, the ISL705AEH is monitoring +5V through V
-5V through PFI. In this example, the trip point (V
negative supply rail is set for -4.5V. Equation 2 can be used to
select the appropriate resistor values. R1 is selected arbitrarily as
100kΩ, V
the values into Equation 2 (as shown in Equation 3) it can be
seen a resistor of 153.3kΩ is needed. The closest 1% resistor
value is 154kΩ.
Figure 4 also has a general purpose NPN transistor in which the
base is connected to the PFO pin through a 100kΩ resistor. The
emitter is tied to ground and the collector is tied to MR signal.
This configuration allows the negative voltage sense circuit to
initiate a reset if it is not within its regulation window. A pull-up
on the MR ensures no false reset triggering when the negative
voltage is within its regulation window.
Assuring a Valid RST Output
When V
current and is essentially an open circuit. As a result, this pin can
drift to undetermined voltages if left undriven. By adding a pull-down
resistor to the RST pin as shown in Figure 35, any stray charge or
leakage currents will be drained to ground and keep RST low when
V
should be large enough not to load RST and small enough to pull
RST to ground. A 100kΩ resistor would suffice, assuming there is no
load on the RST pin during that time.
DD
R2
R2
falls below 1.2V. The resistor value (R1) is not critical however, it
=
=
+5V
-5V
R1 V
---------------------------------------------
100k 1.25
----------------------------------------------------- -
DD
ISL705AEH, ISL705BEH, ISL705CEH, ISL706AEH, ISL706BEH, ISL706CEH
DD
(
V
R1
R2
falls below 1.2V, the RST output can no longer sink
DD
PFI
(
5 1.25
= 5V, V
–
–
–
V
V
PFI
–
TRIP
FIGURE 34. ±5V MONITORING
– (
PFI
4.5
)
= 1.25V, and V
)
PFI
)
ISL705AEH
=
153.3kΩ
V
14
DD
PFO
RST
MR
TRIP
100k
= (-4.5V). By plugging
TRIP
) for the
2N3904
100k
(EQ. 2)
(EQ. 3)
DD
and
Assuring a Valid RST Output
On the ISL705BEH and ISL706BEH, when V
RST output can no longer source enough current to track V
result, this pin can drift to undetermined voltages if left undriven. By
adding a pull-up resistor to the RST pin as shown in Figure 36, RST
will track V
however, it should be large enough not to exceed the sink capability
of RST pin at 1.2V. A 300kΩ resistor would suffice, assuming there
is no load on the RST pin during that time.
Selecting Pull-Up Resistor Values
The ISL705CEH and ISL706CEH have open drain active low reset
outputs (RST_OD). A pull-up resistor is needed to ensure RST_OD
is high when V
must be chosen in order not to exceed the sink capability of the
RST_OD pin. The ISL705AEH has a sink capability of 3.2mA and
the ISL706CEH has a sink capability of 1.2mA. Equation 4 may
be used to select resistor R
V
exceed V
PULL
. It is also important that the pull-up voltage does not
DD
DD
FIGURE 35. RST VALID TO GROUND CIRCUIT
FIGURE 36. RST VALID TO GROUND CIRCUIT
.
FIGURE 37. RST_OD PULL-UP CONNECTION
below 1.2V. The resistor value (R1) is not critical
DD
is in a valid state (Figure 37). The resistor value
ISL705BEH, ISL706BEH
ISL706CEH, ISL705CEH
ISL705AEH, ISL706AEH
V
PULL
DD
V
V
RST_OD
DD
DD
RST
based on the pull-up voltage
RST
R1
DD
V
300kΩ
PULL
falls below 1.2V, the
100kΩ
R
PULL
March 30, 2012
DD
FN8262.0
. As a