Datasheet

LTC4417
13
4417f
applicaTions inForMaTion
Reduction of the valid operating range can be used to
prevent disconnected high impedance input supplies
from reconnecting. For example, if 3 series connected AA
Alkaline batteries with a total series resistance of 675mΩ
is used to source 500mA, the voltage drop due to the se-
ries resistance would be 337.5mV. Once the batteries are
discharged and are disconnected due to a UV fault, the AA
battery stack would recover the 337.5mV drop across the
internal series resistance. Using the 30mV fixed internal
hysteresis allows only 81mV of hysteresis at the input
pin, possibly allowing the input supply to revalidate and
reconnect. Using external hysteresis, the hysteresis volt-
age can be increased to 400mV, reducing or eliminating
the reconnection issue, as shown in Figure 4.
FILTERING NOISE ON OV AND UV PINS
The LTC4417 provides an 8µs OV/UV fault filter time. If
the 8µs filter time is not sufficient, add a filter capacitor
between the OV or UV pin and GND to extend the fault
filter time and ride through transient events. A UV pin fault
filter time extension capacitor, C
UVF
, is shown in Figure 5.
Use Equation (6) to select C
UVF
for the UV pin and Equa-
tion (7) to select C
OVF
for the OV pin.
C
UVF
= t
DELAY
•
R1+R2+R3
R3 • (R1+R2)
• ln
V
i
– V
f
1V – V
f
⎡
⎣
⎢
⎤
⎦
⎥
(6)
C
OVF
= t
DELAY
•
R1+R2+R3
R1•(R2+R3)
• ln
V
i
– V
f
1V – V
f
⎡
⎣
⎢
⎤
⎦
⎥
(7)
where the final input voltage V
f
and the initial voltage V
i
are the resistively divided down values of the input supply
step, as shown in Figure 6.
Connecting HYS to GND, as shown in Figure 5, selects
an internal 30mV fixed hysteresis, resulting in 3% of the
input supply voltage.
Figure 6. Fault Filter Time Extension
4417 F05
WITHOUT FAULT FILTER
TIME EXTENSION
1V V
OVUV(THR)
V
IN(INIT)
V
IN(FINAL)
t
DELAY
WITH FAULT FILTER
TIME EXTENSION
INPUT SUPPLY STEP
V
i
=
V
IN(INIT)
•(R1+ R2)
R1+R2+ R3
V
f
=
V
IN(FINAL)
•(R1+ R2)
R1+ R2+ R3
Figure 4. Setting a Higher UV Hysteresis to Prevent
Unwanted Reconnections
V1
400mV HYSTERESIS
FULLY CHARGED 3 × AA BATTERY
VALID UV
RANGE
FOR 81mV
HYSTERESIS
VALID UV
RANGE
FOR 400mV
HYSTERESIS
V1 UV FAULT AND
DISCONNECTS
337.5mV RECOVERY
WHEN LOAD IS
DISCONNECTED
81mV HYSTERESIS
2.7V UV THRESHOLD
4417 F06
C
UVF
UV1
V1 INPUT
SUPPLY
LTC4417
V1
1V
1.03V
VALID1
HYS
4417 F04
UV1
VALID
OV1
OV
UV
R1
R2M2
R3
GND
OPTIONAL
FILTER
CAPACITOR
OPTIONAL
DISCONNECT
+
–
OV1
VALID
M1
R
P
V
OUT
+
–
256ms
TIMER
1V
0.97V
Figure 5. LTC4417 Internal Hysteresis with Optional Filter
Capacitor and Manual Disconnect MOSFET
Extending the filter time delay will result in a slower
response to fast UV and OV faults. Extending the UV pin
fault filter time delay will also add delay to the OV pin. If
this is not desirable, separate the single resistive string
into two resistive strings, as shown in Figure 3.
PRIORITY REASSIGNMENT
A connected input supply can be manually disconnected
by artificially creating a UV fault. An example is shown in
Figure 5. When N-channel MOSFET, M2, is turned on, the