LTC4359CDCB-TRMPBF データシートの表示（PDF） - Linear Technology

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LTC4359CDCB-TRMPBF

Ideal Diode Controller with Reverse Input Protection

Linear Technology 

LTC4359

Applications Information

20

MOSFET

(BSC028N06NS)

15

10

SCHOTTKY DIODE

(SBG2040CT)

5

0

0

0.1

0.2

0.3

0.4

0.5

VOLTAGE (V)

4359 F02

Figure 2. Forward Voltage Drop Comparison

Between MOSFET and Schottky Diode

It is important to note that the SHDN pin, while disabling

the LTC4359 and reducing its current consumption to

9µA, does not disconnect the load from the input since

Q1’s body diode is ever-present. A second MOSFET is

required for load switching applications.

MOSFET Selection

All load current passes through an external MOSFET,

Q1. The important characteristics of the MOSFET are on-

resistance, RDS(ON), the maximum drain-source voltage,

BVDSS, and the gate threshold voltage VGS(TH).

Gate drive is compatible with 4.5V logic-level MOSFETs

over the entire operating range of 4V to 80V. In applications

above 8V, standard 10V threshold MOSFETs may be used.

An internal clamp limits the gate drive to 15V maximum

between the GATE and SOURCE pins. For 24V and higher

applications, an external Zener clamp (D4) must be added

between GATE and SOURCE to not exceed the MOSFET’s

VGS(MAX) during input shorts.

The maximum allowable drain-source voltage, BVDSS, must

be higher than the power supply voltage. If the input is

grounded, the full supply voltage will appear across the

MOSFET. If the input is reversed, and the output is held

up by a charged capacitor, battery or power supply, the

sum of the input and output voltages will appear across

the MOSFET and BVDSS > OUT + |VIN |.

The MOSFET’s on-resistance, RDS(ON), directly affects

the forward voltage drop and power dissipation. Desired

forward voltage drop should be less than that of a diode

for reduced power dissipation; 100mV is a good starting

point. Choose a MOSFET which has:

RDS(ON)

<

Forward Voltage

ILOAD

Drop

The resulting power dissipation is

Pd = (ILOAD)2 • RDS(ON)

Shutdown Mode

In shutdown, the LTC4359 pulls GATE low to SOURCE,

turning off the MOSFET and reducing its current consump-

tion to 9µA. Shutdown does not interrupt forward current

flow, a path is still present through Q1’s body diode, as

shown in Figure 1. A second MOSFET is needed to block

the forward path; see the section Load Switching and

Inrush Control. When enabled the LTC4359 operates as

an ideal diode. If shutdown is not needed, connect SHDN

to IN. SHDN may be driven with a 3.3V or 5V logic signal,

or with an open drain or collector. To assert SHDN low,

the pull down must sink at least 5µA at 500mV. To enable

the part, SHDN must be pulled up to at least 2V. If SHDN

is driven with an open drain, open collector or switch

contact, an internal pull up current of 2µA (1µA minimum)

asserts SHDN high and enables the LTC4359. If leakage

from SHDN to ground cannot be maintained at less than

100nA, add a pull up resistor to >2V to assure turn on.

The self-driven open circuit voltage is limited internally

to 2.5V. When floating the impedance is high and SHDN

is subject to capacitive coupling from nearby clock lines

or traces exhibiting high dV/dt. Bypass SHDN to VSS with

10nF to eliminate injection. Figure 3a is the simplest way

to control the shutdown pin. Since the control signal

ground is different from the SHDN pin reference, VSS, there

could be momentary glitches on SHDN during transients.

Figures 3b and 3c are alternative solutions that level shift

the control signal and eliminate glitches.

4359f

8

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