LTC4417_15 Datasheet, PDF (12/32 Pages) Linear Technology – Prioritized PowerPat Controller

LTC4417
Applications Information
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.
OV
OV_HYS
UV_HYS
UV
REDUCED
OPERATING
WINDOW
V1
UV1 FAULT
OV/UV
WINDOW
OV1 FAULT
V1 VALID
4417 F02
Figure 2. OV and UV Thresholds and Hysteresis Voltage
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
12
V1 INPUT
SUPPLY
R10
R12
UV1
R9
R11
OV1
R8
R7
UV1
R6
V1
R3
UV1
+ ^UV
1V –
I_HYS/8
UV1
VALID
LTC4417 _VOUT
VALID1
R_P
R5
OV1
R4
R2 _VLDO
256ms
TIMER
M1
T-RESISTIVE
CONNECTION
DUAL-
RESISTIVE
CONNECTION
OPTIONAL INDEPENDENT
HYSTERESIS
OV1
R1
GND
I_HYS/8
1V + ^OV
–
OV1
VALID
Figure 3. LTC4417 External Hysteresis
HYS
4417 F03
I_HYS
R_HYS
124k
TO
1.24M
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
=
(^OV/
UV
R_TOP
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 • ⎡⎣OVHYS – IOVUV(HYS) • (R9 +R10)⎤⎦}
I_OVUV(HYS) • (R8 +R9 +R10)
(4)
R12 = ^{(R8 +R9) • ⎡⎣UVHYS – IOVUV(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.
4417f