Datasheet

LTC4417
12
4417f
I
HYS/8
UV1
V1 INPUT
SUPPLY
LTC4417
V1
1V
VALID1
1V
HYS
4417 F03
I
HYS
R
HYS
124k
TO
1.24M
UV1
VALID
OV1
R1
R2
R3R7
R6
UV1
GND
I
HYS/8
V
LDO
DUAL-
RESISTIVE
CONNECTION
+
–
OV1
VALID
M1
R
P
V
OUT
+
–
256ms
TIMER
R5
R4
OV1
UV1
T-RESISTIVE
CONNECTION
OPTIONAL INDEPENDENT
HYSTERESIS
R9
R10
R8
R11
OV1
OV
UV
R12
applicaTions inForMaTion
Figure 2. OV and UV Thresholds and Hysteresis Voltage
DEFINING OPERATIONAL RANGE
To guard against noise and transient voltage events during
live insertion, the LTC4417 requires an input supply remain
in the OV/UV window for at least 256ms to be valid. The
OV/UV window for each input supply is set by a resistive
divider (for example, R1, R2 and R3 for V1 input supply)
connected from the input supply to GND, as shown in
Figure 1. When setting the resistive divider values for the
OV and UV input supply threshold, take into consideration
the tolerance of the input supply, 1.5% error in the OV
and UV comparators, tolerance of R1, R2 and R3, and the
±20nA maximum OV/UV pin leakage currents.
In addition to tolerance considerations, hysteresis reduces
the valid input supply operating range. Input supplies will
need to be within the reduced input supply operating range
to validate. Referring to Figure 2, V1 supply voltage must
be greater than UV
HYS
to exit the UV fault. If an OV fault
occurs, the V1 supply voltage must return to a voltage
lower than the OV
HYS
voltage to exit the OV fault.
REDUCED
OPERATING
WINDOW
OV/UV
WINDOW
OV
UV
V1
4417 F02
UV1 FAULT
OV1 FAULT
V1 VALID
OV
HYS
UV
HYS
Hysteresis for the OV and UV comparators are set via the
HYS pin. Two options are available. Connecting a resistor,
R
HYS
, between HYS and GND, as shown in Figure 3, sets
the hysteresis current I
OV_UV(HYS)
that is sunk into UV1,
UV2 and UV3 and sourced out of OV1, OV2 and OV3. The
value of R
HYS
is calculated with Equation (1). Choose R
HYS
to limit the hysteresis current to between 50nA and 500nA.
R
HYS
=
63mV
I
OVUV(HYS)
(1)
where 50nA ≤ I
OVUV(HYS)
≤ 500nA
Figure 3. LTC4417 External Hysteresis
Independent OV and UV hysteresis values are available
by separating the single string resistive dividers R1, R2
and R3, shown in Figure3, into two resistive strings, R4-
R5 and R6-R7. In such a configuration, the top resistor
defines the amount of hysteresis and the bottom resistor
defines the threshold. Use Equations (2) and (3) to cal-
culate the values.
R
TOP
=
HYST
I
OVUV(HYS)
(2)
where HYST is the desired hysteresis voltage at V1.
R
BOTTOM
=
R
TOP
OV/ UV Threshold
( )
– 1
(3)
When large independent hysteresis voltages are required,
a resistive T structure can be used to define hysteresis
values, also shown in Figure 3. After the desired OV and
UV thresholds are set with resistors R8 through R10, R11
and R12 are calculated using:
R11=
R8 • OV
HYS
–I
OVUV(HYS)
•(R9+R10)
⎡
⎣
⎤
⎦
I
OVUV(HYS)
•(R8+R9+R10)
(4)
R12 =
(R8+R9) • UV
HYS
–I
OVUV(HYS)
•R10
⎡
⎣
⎤
⎦
I
OVUV(HYS)
•(R8+R9+R10)
(5)
where OV
HYS
, UV
HYS
are the desired OV and UV hysteresis
voltage magnitudes at V1 through V3, and I
OVUV(HYS)
is
the programmed hysteresis current.