MAX5079 datasheet(13/18 Pages) MAXIM

13 / 18 page

ORing MOSFET Controller with

Ultra-Fast 200ns Turn-Off

______________________________________________________________________________________

VIN = 5V or VIN = 12V. For VIN

≤ 8V, the gate drive is 5V

above VIN and for VIN > 8V, the gate drive is 7V above

VIN. Lower gate drive means faster turn-off during faults,

while higher gatedrive means lower RDSON.

A fast and slow comparator monitor the voltage from IN

to BUS. When this voltage crosses the negative fast- or

slow-comparator threshold voltage for the blanking time

duration, GATE is pulled low by an internal 2A current

sink. Both comparators have an adjustable threshold

voltage. GATE is pulled low if any of the following con-

ditions are met.

1) VUVLO < 0.6V.

2) VAUXIN < 2.25V and VIN < 2.25V.

3) VOVI

≥ 0.6V.

4) VIN

≤ (VBUS - VFTH) or VIN ≤ (VBUS - VSTH) and

(VGATE - VIN)

≥ 1.8V.

When the above conditions are not true and VIN

≤

VBUS, GATE is shorted to IN. To insure that the external

MOSFET is quickly turned off, given the above condi-

tions, the GATE pulldown circuitry is powered by either

VIN, VAUXIN, or VBUS as long as any one is greater

then 2.75V.

Fast Comparator (FTH)

The fast comparator has a 50ns blanking time to avoid

unintentional turn-off of the ORing MOSFET during fast

transients. Additionally, the fast-comparator reverse

voltage threshold (VFTH) is programmable to suit the

need of an individual application. Higher VFTH thresh-

old allows for a larger glitch at BUS during a fault, but

improves the noise immunity. Lower VFTH reduces

glitches at BUS during a fault, however, with lower VFTH

spikes at BUS or glitches at IN can be read as faults,

unintentionally turning off the ORing MOSFET. Program

VFTH by connecting a resistor from FTH to GND. Adjust

VFTH to optimize the system performance using the fol-

lowing equation:

VFTH can be chosen from 24mV to 400mV. Connect

FTH to GND to choose the default 24mV threshold.

Slow Comparator (STH)

The MAX5079 includes a slow comparator to provide

glitch immunity during the hot insertion or removal of

paralleled power supplies. During the hot insertion,

BUS can see voltage spikes. These spikes can be

interpreted as a reverse voltage across the ORing

MOSFET. The amplitude of the spikes is proportional to

the load step seen by the parallel power supply while

the duration of the spikes depends on the loop

response of the load share and PWM controller.

The slow comparator has a programmable reverse volt-

age threshold (VSTH) as well as a programmable blank-

ing time (tSTH). An internal transconductance amplifier

converts the IN to BUS differential voltage to a current

and applies it to a parallel combination of resistor and

capacitor (RSTH and CSTH) from STH to GND. The

reverse threshold voltage (VSTH) for the slow compara-

tor is adjusted through RSTH. Use the following equa-

tion to calculate the RSTH for a required VSTH.

where GM_STH = 0.17mS.

The internal 500k

Ω resistance from the output of the

transconductance to GND can change the actual VSTH

if RSTH is above 50k

Ω. In this case, see the Typical

Operating Circuit to select RSTH. Once RSTH is chosen,

the blanking time can be adjusted by CSTH. The delay

time is:

where tSBL = 0.9ms and is the default blanking time

generated by an internal digital delay. Leaving STH

floating results in a 12mV threshold voltage and a 0.9ms

blanking time. VOD (overdrive) is the difference between

actual reverse voltage (VBUS - VIN) and VSTH threshold.

Overvoltage Protection Latch (OVI/OVP)

OVI is the negative input to the overvoltage compara-

tor. The positive input of this comparator is connected

to the internal 0.6V reference and an open-drain output

is provided at OVP. The overvoltage sensing for over-

voltage protection is done at either IN or BUS. OVP

DELAY

STH

SBL

=×

× −

−





















ln 1

VmV x G

STH

M STH

= ()

−

VmV

FTH

− 24

667

MAX5079 Datasheet(PDF) 13 Page - Maxim Integrated Products

MAX5079 Datasheet(HTML) 13 Page - Maxim Integrated Products

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Datasheet Download

Link URL