MAX6745(2003) Просмотр технического описания (PDF) - Maxim Integrated
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MAX6745 Datasheet PDF : 16 Pages
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Low-Power Dual-/Triple-Voltage SC70 µP
Supervisory Circuits
A) VIN IS POSITIVE
VEXT_TH
R1
R2
MAX6742
MAX6745
( ) VEXT_TH = VTH-PFI
R1 + R2
R2
PFI
PFO
GND
B) VIN IS NEGATIVE
VCC
R1
R2
VEXT_TH
MAX6742
MAX6745
( )( ) VEXT_TH = R2 (VTH-PFI)
1+1
R1 R2
-
VCC
R1
VPFI = 488mV
PFI
PFO
GND
Figure 4. Using Power-Fail Input to Monitor an Additional
Power Supply
Adding Hysteresis to the
Power-Fail Comparator
The power-fail comparator has a typical input hystere-
sis of 2.5mV. This is sufficient for most applications in
which a power-supply line is being monitored through
an external voltage-divider. If additional noise margin is
desired, connect a resistor between PFO and PFI, as
shown in Figure 6. Select the values of R1, R2, and R3
such that PFI sees VTH-PFI (488mV) when VEXT falls to
its power-fail trip point (VFAIL) and when VEXT rises to
its power-good trip point (VGOOD). The hysteresis win-
dow extends between the specified VFAIL and VGOOD
thresholds. R3 adds the additional hysteresis by sink-
ing current from the R1/R2 divider network when the
PFO output is logic low and sourcing current into the
network when PFO is logic high. R3 is typically an order
of magnitude greater than R1 or R2.
The current through R2 should be at least 1µA to ensure
that the 10nA (max) PFI input current does not significant-
ly shift the trip points. Therefore, for most applications:
R2 < VTH-PFI / 1mA < 0.488V / 1mA < 488kΩ
PFO is an open-drain output requiring an external
pullup resistor, R4. Select R4 to be less than 1% of R3.
VGOOD = DESIRED VEXT GOOD VOLTAGE THRESHOLD
VFAIL = DESIRED VEXT FAIL VOLTAGE THRESHOLD
VPU = VPULLUP (FOR OPEN-DRAIN PFO)
R2 = 488kΩ (FOR >1µA R2 CURRENT)
( ) VTH- PFI(VGOOD - VFAIL
( ) VGOOD - VTH−PFI −
R1 = R2
VPU
VTH - PFI
R3 = (R1 x VPU) / (VGOOD - VFAIL)
R4 ≤ 0.01 x R3
Power Sequencing Applications
Many dual-voltage processors/ASICs require specific
power-up/power-down sequences for the I/O and core
supplies.
VCC1 VCC2
RESET TO OTHER SYSTEM COMPONENTS
MAX6737
MAX6739
MAX6743
VCC2 MAX6744 RESET
MAX6745
VCC1
4.7kΩ
µP
RESET
GND
GND
VEXT
R1
R2
R3
MAX6742
MAX6745
VPULLUP
VIN
PFO
R4
VGOOD
VFAIL
PFI
PFO
VGOOD = DESIRED VEXT GOOD VOLTAGE THRESHOLD
VFAIL = DESIRED VEXT FAIL VOLTAGE THRESHOLD
VOH = VCC1 (FOR PUSH-PULL PFO)
R2 = 200kΩ (FOR > 2.5µA R2 CURRENT)
GND
R1 = R2 ((VGOOD - VTH-PFI) - (VTH-PFI)(VGOOD - VFAIL) / VPU) / VTH-PFI
R3 = (R1 x VOH) / (VGOOD - VFAIL)
Figure 5. Interfacing to µPs with Bidirectional Reset I/O
Figure 6. Adding Hysteresis to Power Fail for Push-Pull PFO
12 ______________________________________________________________________________________
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