LP3470 datasheet(9/22 Pages) TI1

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LP3470

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SNVS003G – JUNE 1999 – REVISED APRIL 2016

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Feature Description (continued)

Figure 9. Reset Output Timing Diagram

7.3.3 Pullup Resistor Selection

The Reset output structure of the LP3470 is a simple open-drain N-channel MOSFET switch. A pullup resistor

(R1) must be connected to VCC.

R1 must be large enough to limit the current through the output MOSFET (Q1) below 10 mA. A resistor value of

more than 680

Ω ensures this. R1 must also be small enough to ensure a logic high while supplying all the

leakage current through the Reset pin. A resistor value of less than 68 k

Ω satisfies this condition. A typical pullup

resistor value of 20 k

Ω is sufficient in most applications.

7.3.4 Negative-Going VCC Transients

The LP3470 is relatively immune to short duration negative-going VCC transients (glitches). The Typical

Characteristics show the maximum transient duration versus negative transient amplitude (see Figure 6), for

which reset pulses are not generated. This graph shows the maximum pulse width a negative-going VCC

transient may typically have without causing a reset pulse to be issued. As the transient amplitude increases (in

other words, goes farther below the reset threshold), the maximum allowable pulse width decreases. A 0.1-µF

bypass capacitor mounted close to VCC provides additional transient immunity.

7.4 Device Functional Modes

7.4.1 Reset Output Low

When the VCC supply voltage is below a threshold (VRTH) voltage minus a hysteresis (VHYST) voltage, the Reset

pin will output logic low. Reset is ensured to be a logic low for VCC > 0.5 V.

7.4.2 Reset Output High

When the VCC supply voltage exceeds the reset threshold, the Reset is kept asserted for a time period (tRP)

programmed by an external capacitor (C1); after this interval Reset goes to logic high.

LP3470 Datasheet(PDF) 9 Page - Texas Instruments