Maxim Integrated MAX6877 User Manual

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Keywords: sequencing, sequencer, monitor, reset, voltage tracking, voltage tracker

USER GUIDE 3918

User's Guide for the MAX6877 Power -Supply
Tracker/Sequencer

By: Eric Schlaepfer, Applications Engineer
Sep 20, 2006

Abstract: This application note is a step - by -step User's Guide for designing with the MAX6877 voltage
tracker and sequencer. The note explains how to make suitable component calculations.

Figure 1. The basic circuit for the MAX6877.

MAX6877 Overview

Tracks or sequences up to three voltages
Closed- loop tracking control with programmable slew rate
Program thresholds with resistors
Program slew rate and timing delays with capacitors
Fault output and power-good output

Description

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The MAX6877 voltage tracker/sequencer can track or sequence up to three voltages. Use the
TRACK/SEQ pin to select tracking or sequencing. The device will either track or sequence the three
voltages, but it cannot track and sequence simultaneously. Power - up begins when EN/active- low UV is
asserted and all three input voltages are above their thresholds. In the example circuit in Figure 1 ,
EN/active- low UV is connected to the 12V bus, so the circuit will turn on when the bus exceeds its
threshold and turn off when it falls under its threshold.

To create a basic design with the MAX6877, such as the one in Figure 1, follow these steps:

1. Select the input voltage thresholds for IN1, IN2, and IN3. The input voltages must exceed these
thresholds for power-up to begin. Use the following formula to set the resistor- divider values for
IN1. (For IN2 or IN3, replace R

with R3, or R5 and R2 with R4 or R6.)

1

2. Select the EN/active- low UV threshold using the following formula. EN/active -low UV can also be
used as a logic-level input to control power up. If not used, connect EN/active - low UV to ABP.

3. Set the voltage slew rate. (This is the rate at which the voltages at the outputs OUT1, OUT2, OUT3
ramp up and down.) Select capacitor C

by using the following formula:

SLEW

where C

is in farads, and SR is in V/s. C

SLEW

must be in the range of 100pF < C

SLEW

SLEW

<

1nF.

4. In sequencing-mode only, the voltages come up one after another, separated by a time delay which
is set by capacitor C

where C

is in farads, and t

DELAY

thresholds are exceeded to the start of sequencing or tracking. C
circuit, in which case t

using the following formula:

DELAY

is in seconds. t

DELAY

becomes the default 200µs.

DELAY

is also the delay time from when all

DELAY

can be safely left out of the

DELAY

5. Pick the MOSFETs for each channel. Ensure that the MAX6877 gate-drive circuit can drive the
gate capacitance of the MOSFET.

Multiple MAX6877 parts can be connected together by connecting the power- good output of the first
device to the enable input (EN/active-low UV) of the next device. For more information, see the data
sheet.

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