ISL6441EVAL1Z datasheet(8/30 Pages) INTERSIL

8 / 30 page

FN9131.7

October 6, 2011

High Voltage Pass and Clamp

A high voltage pass and clamping circuit prevents the high

output voltage from damaging the comparators in case of

quick drop in VIN. The comparators are running from the 5V

supply between HVREF and VIN. These devices are rated

for 5V and will be damaged if VOUT is allowed to be present

(as the output is powered from other parallel supplies), and

does not fall when VIN is falling. For example, if VIN falls to

30V, VOUT remains at 48V and the differential Voltage

between the “-” and “+” terminals of the comparator would be

18V, exceeding the rating of the devices and causing

permanent damage to the IC.

Fault Detection Block

The fault detection block has two monitoring circuits (refer to

Figure 2):

1. Gate monitoring detects when the GATE < VIN + 0.37V

2. VOUT monitoring detects when VIN - 0.41V > VOUT

These two outputs are ORed, inverted, level shifted, and

delayed using an internal filter (tFLT)

The following failures can be detected by the fault detection

circuitry:

1. ORing FET off due to dead short in the sourcing supply,

leading to VIN < VOUT

2. Shorted terminals of the ORing FET

3. Blown fuse in the power path of the sourcing supply

4. Open Gate terminal

5. HVREF UV

The FAULT pin is not latched off and the pull down will shut

off as soon as the fault is removed and the pin becomes high

impedance. Typically, an external pull-up resistor is

connected to an external voltage source (for example 5V,

3.3V) to pull the pin high, an LED can be used to indicate the

presence of a fault.

Application Considerations

ORing MOSFET Selection

Using an ORing MOSFET instead of an ORing diode results

in increased overall power system efficiency as losses

across the ORing elements are reduced. The use of ORing

MOSFETs becomes more important at higher current levels,

as power loss across the traditionally used ORing diode is

very high. The high power dissipation across these diodes

requires special thermal design precautions such as heat

sinks and forced airflow.

For example, in a 48V, 40A (1+1) redundant system with

current sharing, using a Schottky diode as the ORing

(auctioneering) device (see Figure 3), the forward voltage

drop is in the 0.4V to 0.7V range. Let us assume it is 0.5V,

power loss across each diode is as shown in Equation 4:

Total power loss across the two ORing diodes is 20W.

If a 5m

Ω single MOSFET per feed is used, the power loss

across each MOSFET is as shown in Equation 5:

Total power loss across the two ORing MOSFETs is 4W.

In case of failure of current sharing scheme, or failure of

DC/DC #1, the full load will be supplied by DC/DC #2. ORing

MOSFET M2 or ORing Diode D2 will be conducting the full

FIGURE 2. FAULT DETECTION BLOCK

0.41V

0.37V

LEVEL SHIFT

DELAY

120µs

FAULT

GATE

VIN

VOUT

Ploss D1

()

Ploss D2

()

IOUT

---------------

⋅

20A 0.5V

⋅

10W

(EQ. 4)

FIGURE 3. 1 + 1 REDUNDANT SYSTEM WITH DIODE ORing

+IN

-IN

+OUT

-S

-OUT

DC/DC

(Note 11)

Figure 14

Rpb1

CIN1

100µF

Cd1

220nF

Ccs1

1nF

DC/DC

Rpb2

Ccs2

1nF

36VDC TO 75 VDC

INPUT BUS 2

INPUT BUS 1

PRIMARY GROUND

+OUT1 = 48V

36VDC TO 75 VDC

VOUT

(40A)

0.5V@ 20A

SECONDARY

+OUT2 = 48V

(Note 11)

CIN2

100µF

Cd2

220nF

GROUND

+IN

-IN

+OUT

-S

-OUT

Figure 14

Ploss M1

()

Ploss M2

()

IOUT

---------------

⎝⎠

⎛⎞

rDS ON

()

⋅

Ploss M1

()

20A

()

2 5mΩ

⋅

(EQ. 5)

ISL6144

ISL6441EVAL1Z Datasheet(PDF) 8 Page - Intersil Corporation

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