LTC4371CMS#TRPBF Datasheet LTC4371CMS#PBF, LTC4371CDD#PBF, LTC4371CMS#TRPBF | P7-P9

LTC4371

4371f

For more information www.linear.com/LTC4371

operaTion

The LTC4371 controls N-channel MOSFETs to emulate two

ideal diodes (see Block Diagram). By sensing the MOSFET’s

source-to-drain voltage drop, amplifiers AMPA and AMPB

control the gate of their respective external MOSFET to act

as an ideal diode with a 15mV forward (∆V

) drop. With

low load currents, the amplifier regulates the MOSFET gate

near its threshold to maintain a forward drop of 15mV. As

load current increases, the gate voltage is driven higher

to maintain a drop of 15mV. For very large load currents

where the MOSFET gate is driven fully on, the forward drop

rises linearly with current according to R

DS(ON)

• I

LOAD

. If

the forward drop is less than 15mV, or if ∆V

reverses,

the amplifier turns the MOSFET off and the load current

transfers to the other channel.

When the power supply voltages are nearly equal, this

regulation technique ensures that the load current is

smoothly shared between the supplies without oscilla

tion. The current balance depends on the R

DS(ON)

of the

MOSFETs and the output resistance of the supplies.

In the case of supply failure, such as supply V

, while

conducting most or all of the load current is shorted to

return, a large reverse current flows from return through

M1 to any load capacitance and through M2 to supplyV

AMPA detects the current reversal and turns off M1 in less

than 220ns. Fast turn-off prevents reverse current from

rising to a damaging level.

The remaining supply V

delivers load current through

the body diode of M2, until the gate is driven on. With

700mV forward drop across M2, AMPB responds quickly

and drives the gate with 5mA pull-up current, limiting

the body diode conduction time to under 100μs. This

minimizes power dissipation arising from switchover and

is especially important in 60Hz AC applications. As the

forward drop reduces, a weaker output stage takes over

and regulates the forward drop, within the limitations of

DS(ON)

, to 15mV.

The LTC4371 can be powered in –4.5V to –16V applica-

tions by connecting V

directly to the power supply

return. In higher voltage applications or to guard against

input transients, V

and V

can be connected together

and powered from return through a bias resistor, R

. For

repetitive 5mA gate pull-up current, V

can be driven

by a buffer biased by V

. The V

pin is shunt regulated to

12.4V with respect to V

with 50μA minimum bias, and

is capable of sinking up to 10mA.

The LTC4371 is designed to withstand high voltage tran

sients exceeding ±300V, such as those experienced during

lightning-induced surges and input supply short circuit

events, without damage. 130V internal clamps protect

drain pins DA and DB against positive spikes. External

resistors R

and R

are necessary to limit the peak

clamp current to less than 10mA.

In an application circuit, negative spikes are clamped by

the MOSFET’s body diode to V

OUT

, such that the drain

pin never sees more than –700mV with respect to V

. A

safely clamped negative transient on one input manifests

itself as a positive transient on the second input and as

an increased voltage from RTN to V

OUT

. The bias resistor,

, limits the current into the V

shunt regulator to less

than 10mA.

A Fault Detection circuit monitors MOSFET ∆V

; FAULTB

pulls low if ∆V

of either channel exceeds 200mV while

the gate is driven fully on. This is an indication of an open

circuit MOSFET and can be configured for fuse monitor

ing by moving the drain pin connection to the input side

of the fuse.

LTC4371

4371f

For more information www.linear.com/LTC4371

applicaTions inForMaTion

Figure 1. –36V to –72V/25A Ideal Diode-OR Controller

Figure 2. Simplest Solution: V

Connected Directly to V

4371 F01

LTC4371

DA DB GA GB SA

SB V

2.2μF

20k

IPT020N10N3

–36V TO

–72V

–36V TO

–72V

OUT

25A LOAD

GREEN LED

30k

33k

RTN

FAULTB

LTC4371

2.2μF

RT N

OUT

4371 F02

High availability systems employ parallel connected power

supplies or battery feeds to achieve redundancy and

enhance system reliability. Schottky diodes are a popular

means of ORing these supplies together at the point of load.

The chief disadvantage of Schottky diodes is their sig

nificant forward voltage drop and resulting power and

efficiency loss. This drop reduces the available supply

voltage and dissipates significant power

. The

LTC4371

solves these problems by using an N-channel MOSFET

as a low loss pass element to emulate the behavior of a

diode (see Figure1).

The MOSFET is turned on when power passes in the

forward direction (positive current flow from source to

drain), allowing for a low voltage drop from load to sup

ply. In the reverse direction, the MOSFET is turned off to

block current flow. By these means, the MOSFET is made

to approach the function and per

formance of an ideal

diode. The MOSFET voltage drop, ∆V

, is sensed by the

DA and SA, or DB and SB pins.

Powering V

The LTC4371 is fundamentally a low voltage device op-

erating over a range of 4.5V to 16V at the V

pin, with

respect to V

. The gate amplifiers are powered from the

pin and pull-up to within 300mV of V

In low voltage applications such as –5V or –12V, the V

pin can be powered directly from return, with V

con-

nected to V

OUT

An internal 12.4V shunt regulator at the V

pin provides

a means of operating the LTC4371 from higher voltage

supplies. It regulates over a range of 50μA to 10mA. In the

simplest configuration shown in Figure2, V

is connected

directly to V

and biased by resistor R

from the return.

A 2.2μF decoupling capacitor is required to stabilize the

shunt regulator, and to momentarily provide the 5mA

fast pull-up current at the gate pins as needed.

Bias resistor R

is chosen to bias the shunt regulator

and provide the maximum V

current at the expected

minimum input voltage according to:

IN(MIN)

– V

Z(MIN)

DD(MAX)

+ 50µA

(1)

Maximum bias resistor dissipation is calculated from:

D(RZ)

IN(MAX)

– V

Z(MIN)

)

(2)

The maximum shunt regulator current must not exceed

10mA such that:

IN(MAX)

– V

Z(MIN)

10mA

(3)

In –48V applications a single 1206 size 30kΩ resistor is

adequate to power the LTC4371. In the application shown

in Figure 1, at 100V (a commonly specified maximum

transient condition) peak dissipation in R

just exceeds

250mW, while the maximum V

current is slightly less

than 3mA.

Dissipation rises in certain applications so that a larger

package or multiple series units are necessary to imple

ment R

. Examples include AC applications where the

gate drivers demand additional current to supply repetitive

LTC4371

4371f

For more information www.linear.com/LTC4371

applicaTions inForMaTion

Figure 4. MOSFET Cascode for High Voltage >250V

Applications with 5mA Gate Pull-Up Current

Figure 3. V

Connected to V

with NPN for Repetitive

5mA Gate Pull-Up Current

LTC4371

0.1μF

RTN

OUT

4371 F03

4371 F04

LTC4371

0.1μF

10Ω

RTN

OUT

2N3904

BSP125 (600V)

pulses from the 5mA fast pull-up, applications where the

input operating voltage exceeds 72V and applications with

a wide range of input voltage, particularly those where the

minimum input voltage approaches the operating voltage

of the LTC4371. A wide input voltage range may also result

in a situation where the maximum V

current calculated

in Equation 3 exceeds 10mA. For these cases an NPN

transistor can be used to buffer the shunt regulator and

power V

, as shown in Figure3. Equation 1 becomes:

IN(MIN)

– V

Z(MIN)

50µA +

DD(MAX)

(4)

(or R

) may be split into multiple segments in order

to achieve the desired standoff voltage or dissipation.

Whereas 1206 size resistors are commonly rated for 200V

working and 400V peak, pad spacing and circuit board de

sign rules may limit the working rating to as little as 100V.

In Figure

3, the voltage drop and power dissipation of Q1

may be augmented by the use of one or more resistors

in series with the collector. The same applies for M1 in

Figure4.

For all applications R

(or R

+ R

) must limit the maxi-

mum V

current to less than 10mA, as calculated using

Equation 3. If voltage transients are anticipated, V

IN(MAX)

becomes the peak transient voltage. Transient require-

ments may force the use of Figure3 or Figure4 instead

of Figure

The peak V

current may also be reduced

to less than 10mA by filtering, e.g. split R

(or R

) into

two equal parts and connect a bypass capacitor from the

central node to V

Strong Gate Pull-Up

For fast turn-on, a strong 5mA driver pulls up on the gate

when the MOSFET forward drop (∆V

) is large. In simple

shunt-regulated applications such as shown in Figure2,

the bias resistor R

may be incapable of supplying 5mA.

In this case, a 2.2µF bypass capacitor is required to

momentarily provide the strong pull-up current to fully

charge the MOSFET gate. In normal operation the 5mA

drive is not a DC condition, as it flows only long enough

to deliver gate charge to the MOSFET. The amount of

where 50μA represents the minimum V

shunt regulator

operating current and β is Q1’s DC current gain.

The maximum power dissipation in R

and the maximum

current are calculated from Equations 2 and 3. Dissipa-

tion in emitter follower Q1 is given by:

D(Q1)

IN(MAX)

– V

Z(MIN)

)•I

DD(MAX)

(5)

In buffered applications, bypass V

with a 100nF capacitor

to V

. Bypassing V

is unnecessary.

For applications at very high voltages, beyond 300V, small

high voltage MOSFETs are more readily available than

bipolar devices and the circuit of Figure4 is preferred.

, calculated using Equation 4, is split into two parts,

and R

. R

is sized to produce a 3V drop when

operating at V

IN(MIN)

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P22

LTC4371CMS#TRPBF

Mfr. #:

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Manufacturer:

Analog Devices Inc.

Description:

Power Management Specialized - PMIC 2x Neg V Ideal Diode-OR Cntr & Mon

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LTC4371CMS#TRPBF

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