LTC4417_15 Datasheet, PDF (13/32 Pages) Linear Technology – Prioritized PowerPat Controller

LTC4417
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.
FULLY CHARGED 3 × AA BATTERY
V1
400mV HYSTERESIS
337.5mV RECOVERY
WHEN LOAD IS
DISCONNECTED
81mV HYSTERESIS
2.7V UV THRESHOLD
V1 UV FAULT AND
DISCONNECTS
VALID UV
RANGE
FOR 400mV
HYSTERESIS
VALID UV
RANGE
FOR 81mV
HYSTERESIS
4417 F06
Figure 4. Setting a Higher UV Hysteresis to Prevent
Unwanted Reconnections
Connecting HYS to GND, as shown in Figure 5, selects
an internal 30mV fixed hysteresis, resulting in 3% of the
input supply voltage.
V1 INPUT
SUPPLY
C_UVF
V1
R3
UV1
1V
1.03V
OPTIONAL
FILTER
CAPACITOR
LTC4417
+ ^UV
–
UV1
VALID
V_OUT
VALID1
R_P
256ms
TIMER
M1
M2
OPTIONAL
DISCONNECT
1V
R2
OV1 0.97V
R1
GND
+ ^OV
–
OV1
VALID
HYS
4417 F04
Figure 5. LTC4417 Internal Hysteresis with Optional Filter
Capacitor and Manual Disconnect MOSFET
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^{⎡⎣⎢}_1VVⁱ
–
–
V_f
V_f
⎤
⎥
⎦
(6)
C_OVF
=
t_DELAY
•
R1+ R2 + R3
R1• (R2+R3)
•
ln^{⎡⎣⎢}_1VVⁱ
–
–
V_f
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.
V_IN(INIT)
V_IN(FINAL)
INPUT SUPPLY STEP
V_i
=
V_IN(INIT) •(R1+R2)
R1+ R2 + R3
WITHOUT FAULT FILTER
TIME EXTENSION
1V V_OVUV(THR)
WITH FAULT FILTER
TIME EXTENSION
V_f
=
V_IN(FINAL) •(R1+R2)
R1+ R2 + R3
t_DELAY
4417 F05
Figure 6. Fault Filter Time Extension
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
4417f
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