MAX793RCSE データシートの表示（PDF） - Maxim Integrated

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MAX793RCSE

3.0V/3.3V Adjustable Microprocessor Supervisory Circuits

Maxim Integrated 

3.0V/3.3V Adjustable Microprocessor

Supervisory Circuits

VCC

VCC

BATT

3.6V

25Ω EQUIVALENT

SOURCE IMPEDANCE

MAX793

MAX794

MAX795

50Ω CABLE

CE IN CE OUT

50Ω

50Ω

50pF

CL*

GND

*CL INCLUDES LOAD CAPACITANCE AND SCOPE PROBE CAPACITANCE.

Figure 9. CE Propagation Delay Test Circuit

10). If there is no easy access to the unregulated sup-

ply, the LOWLINE output can be used to generate an

NMI to the µP (see LOWLINE Output section).

LOWLINE Output (MAX793/MAX794)

The low-line comparator monitors VCC with a threshold

voltage typically 45mV above the reset threshold (10mV

of hysteresis) for the MAX793, and 15mV above RESET

IN (4mV of hysteresis) for the MAX794. For normal

operation (VCC above the reset threshold), LOWLINE is

pulled to VCC. Use LOWLINE to provide an NMI to the

µP when power begins to fall.

In most battery-operated portable systems, reserve

energy in the battery provides ample time to complete

the shutdown routine once the low-line warning is

encountered and before reset asserts. If the system

must also contend with a more rapid VCC fall time, such

as when the main battery is disconnected or a high-

side switch is opened during normal operation, use

capacitance on the VCC line to provide time to execute

the shutdown routine (Figure 11).

First, calculate the worst-case time required for the sys-

tem to perform its shutdown routine. Then, with the worst-

case shutdown time, the worst-case load current, and the

minimum low-line to reset threshold (VLR min), calculate

the amount of capacitance required to allow the shut-

down routine to complete before reset is asserted:

CHOLD > ILOAD x tSHDN / VLR

where ILOAD is the current being drained from the

capacitor, VLR is the low-line to reset threshold differ-

ence (VLL - VRST), and tSHDN is the time required for

the system to complete an orderly shutdown routine.

Power-Fail Comparator (MAX793/MAX794)

The MAX793/MAX794’s PFI input is compared to an

internal reference. If PFI is less than the power-fail

threshold (VPFT), PFO goes low. The power-fail com-

parator is intended for use as an undervoltage detector

to signal a failing power supply (Figure 12). However,

the comparator does not need to be dedicated to this

function because it is completely separate from the rest

of the circuitry.

UNREGULATED

SUPPLY

3.0V OR 3.3V

REGULATOR

VCC

R1

MAX793

MAX794

PFI

PFO

R2

GND

TO µP NMI

3.0V OR 3.3V

REGULATOR

VCC

CHOLD

LOWLINE

MAX793

MAX794

CHOLD > ILOAD x tSHDN

VLR

GND

TO µP NMI

Figure 10. Using the Power-Fail Comparator to Generate

Power-Fail Warning

Figure 11. Using LOWLINE to Provide Power-Fail Warning

to the µP

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