MAX6877 データシートの表示（PDF） - Maxim Integrated

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MAX6877

Dual-/Triple-Voltage, Power-Supply Trackers/Sequencers/Supervisors

Maxim Integrated 

Dual-/Triple-Voltage, Power-Supply

Trackers/Sequencers/Supervisors

Detailed Description

The MAX6877/MAX6878/MAX6879 multivoltage power

trackers/sequencers/supervisors monitor up to three

system voltages and provide proper power-up and

power-down control for systems requiring voltage

tracking or sequencing. These devices ensure con-

trolled voltage tracking with a specified range or

sequencing in the proper order as system power sup-

plies are enabled. The MAX6877/MAX6878/MAX6879

generate all required voltages and timing to control up

to three external n-channel pass FETs for the

OUT1/OUT2/OUT3 supply voltages (see the Selector

Guide for different features of each device.)

The MAX6877/MAX6878/MAX6879 feature adjustable

undervoltage thresholds for each input supply. When

all the voltages are above these adjusted thresholds,

the devices turn on the external n-channel MOSFETs to

either sequence or track the voltages to the system.

During the voltage-tracking mode, the voltage at the

GATE of each MOSFET is increased to slowly bring up

all supplies at a controlled slew rate. The voltage at the

source (output) of each MOSFET is internally compared

to a control ramp to maintain a low differential between

each monitored supply. Tracking is dynamically adjust-

ed to force all outputs to track within 125mV of the ref-

erence ramp. If for any reason any supplies fail to track

within ±250mV of the reference ramp, the FAULT out-

put is asserted, the power-up mode is terminated, and

all outputs are quickly powered off. In sequencing

mode, the outputs are turned on one after the other,

OUT1 first and OUT3 last. The MAX6877/MAX6878/

MAX6879 feature an autoretry or latch-off mode with

capacitor-adjusted timing.

These devices also provide a controlled power-down

(tracking mode) when the system shuts off in an orderly

manner. When an unexpected fault occurs, the outputs

are all pulled down simultaneously with an internal

100Ω pulldown to help discharge capacitive loads at

the MOSFET’s source.

The MAX6877/MAX6878/MAX6879 feature independent

internal charge pumps to fully enhance the external

FETs for low-voltage drops at highpass currents. The

MAX6877/MAX6878 also feature a power-good output

with a selectable timeout period that can be used for

system reset.

The MAX6877/MAX6878/MAX6879 monitor up to three

voltages. Devices may be configured to exclude any

IN_. To disable the tracking or sequencing operation of

any IN_, connect the IN_ to ground (or leave uncon-

nected) and connect SET_ to a voltage greater than

0.5V. The channel exclusion feature adds more flexibili-

ty to the device in a variety of different applications. As

an example, the MAX6877 can track or sequence two

voltages using IN1 and IN2 while IN3 is left disabled.

Powering the

MAX6877/MAX6878/MAX6879

These devices derive power from either the IN1, IN2, or

IN3 voltage inputs or VCC (see the Functional Diagram).

VCC or one of the IN_ inputs must be at least +2.7V to

ensure full device operation.

The highest input voltage on IN1/IN2/IN3 or VCC sup-

plies power to the devices. Internal hysteresis ensures

that the supply input that initially powers these devices

continues to power the MAX6877/MAX6878/MAX6879

when multiple input voltages are within 100mV (typ) of

each other.

ABP

ABP powers the analog circuitry. Bypass ABP to GND

with a 1µF ceramic capacitor installed as close to the

device as possible. ABP takes the highest voltage of

IN_ or VCC. Do not use ABP to provide power to exter-

nal circuitry. ABP maintains the device supply voltage

during rapid power-down conditions.

Tracking and Sequencing Modes

(TRK/SEQ)

To enable the power-up/power-down voltage-tracking

operation, drive TRK/SEQ low (connect TRK/SEQ to

GND). To enable power-up sequencing and power-

down tracking functions, drive TRK/SEQ high (connect

TRK/SEQ to ABP) or leave it unconnected. TRK/SEQ is

internally pulled to ABP through a 10µA current source

(see Figures 1 and 3).

Tracking

To operate in tracking mode, connect TRK/SEQ to

GND. When VEN/UV > 1.25V and all SET_ inputs are

above the internal SET_ threshold (0.5V), the tracking

process is initiated. The MAX6877/MAX6878/MAX6879

generate an internal reference ramp voltage that drives

the control loops for the tracked voltages. The tracking

functionality is monitored with a comparator control

block for each output (see the Functional Diagram).

The comparators monitor each OUT_ voltage with

respect to the common reference ramp voltage to

ensure the OUT_ voltages stay within 125mV of the ref-

erence ramp, monitor each tracked output voltage with

respect to its source input voltage, and monitor each

output voltage with respect to GND during power-

up/retry cycles. If for any reason any supplies fail to

track within ±250mV of the reference ramp, the FAULT

output is asserted, the power-up mode is terminated,

and all outputs are quickly powered off.

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