LINEAR TECHNOLOGY LTC4355 Technical data

L DESIGN IDEAS

4358

B530C

CURRENT (A)

VOLTAGE DROP (mV)

5000

6

0

100 200 300 400

1

2

4

3

5

DIODE (B530C)

FET (LTC4358)

CURRENT (A)

0

0

POWER DISSIPATION (W)

0.5

1.0

1.5

2.0

2.5

213 4 5 6

DIODE (B530C)

POWER SAVED

FET (LTC4358)

LTC4358

GND

IN DRAIN

V

DD

OUT

VIN = 12V

V

OUT

TO

5A LOAD

Ideal Diode Betters a Schottky by a
Factor of Four in Power and Space
Consumption

Introduction

High availability systems often use
parallel power supplies or battery
feeds to achieve redundancy and
enhance system reliability. Tradition­ally, Schottky ORing diodes are used
to connect these supplies at the point
of load and prevent backfeeding into
a faulty power supply. Unfortunately,
the forward voltage drop of these diodes
reduces the available supply voltage
and dissipates significant power at
high currents—costly heat sinks and
elaborate layouts are needed to keep
the diodes cool.

When power dissipation is a
concern, the Schottky diode can be
replaced with a MOSFET-based ideal
diode. This reduces the voltage drop
and power dissipation, thereby reduc­ing the complexity, size and cost of the
thermal layout and increasing system
efficiency. The LTC4355, LTC4357

Figure 1. No external components are needed for a 12V/5A ideal diode.

With one-fourth the

dissipated power, system

efficiency is increased and

PCB layout is simplified—no

need for costly and bulky

heat sinks.

by Meilissa Lum

and LTC4358 enable MOSFET-based
ideal diode solutions for various ap­plications—the choice depends on the
current and operating voltage of the
application. Table 1 compares these
devices.

Ideal Diode Easier to Use
Than a Schottky

Of particular interest is the LTC4358,
which includes an internal 20mΩ

Figure 2. The LTC4358 ideal diode takes on a 5A B530C Schottky diode. The LTC4358 easily wins in voltage drop, power loss and package size.

Table 1. Comparison of ideal diode parts

Part Number Description Operating Voltage Configuration Package

LTC4355

LTC4357

LTC4358 Ideal Diode

38

38

Positive Voltage Diode-OR

Controller and Monitor

Single Positive Voltage

Ideal Diode Controller

9V–80V,

100V Abs Max

9V–80V,

100V Abs Max

9V–26.5V,

28V Abs Max

Dual, External MOSFETs DFN14 (4mm × 3mm), SO16

Single, External MOSFET DFN6 (2mm × 3mm), MSOP8

5A Internal MOSFET DFN14 (4mm × 3mm), TSSOP16

Linear Technology Magazine • June 2008

DESIGN IDEAS L

V

IN

GND

V

OU T

DIODE CURRENT (A)

3.5

AREA (INCH

2

)

4.5

10

0.1

1

4.0 6.56.0

5.5

5.0

3.0

85oC 70oC 25oC

TA =

50oC

Authors can be contacted

at [phone]

MOSFET as the pass element. No ex­ternal components are required. The
IN pins are the source of the MOSFET
and act like the anode of a diode, while
the drain behaves as the cathode,
as shown for a 12V/5A application
in Figure 1. When power is first ap­plied, the load current initially flows
through the MOSFET’s body diode.
The MOSFET’s gate is enhanced and
turned on to maintain a 25mV forward
voltage drop. If the load current causes
more than 25mV of voltage drop, the
MOSFET is driven fully on, and the
forward drop equals R

DS(ON)

• I

LOAD

. If
the load current reverses, as may occur
during an input short, the LTC4358
responds by turning off the internal
MOSFET in less than 0.5µs.

Power Saved Versus
Schottky Diode

Compared to a B530C Schottky di­ode in the SMC package, not only is
the LTC4358’s DE14 (4mm × 3mm)
package one-fourth the size, the volt­age drop and power dissipation are
also considerably less as shown in
Figure 2. The reduced voltage drop

of the ideal diode also increases the
voltage at the load, which reduces the
capacitance required to hold up the
output during supply disruptions. The

Not only is the LTC4358’s

DE14 (4mm × 3mm) package

one-fourth the size, the

voltage drop and power

dissipation are also

considerably less than
a Schottky. The reduced
voltage drop of the ideal
diode also increases the

voltage at the load, which

reduces the capacitance

required to hold up the

output during supply

disruptions.

power dissipated at 5A in the Schottky
is 2W versus 0.5W for the LTC4358.
With one-fourth the power dissipated,
system efficiency is increased and PCB

Figure 4. Maximum diode current vs PCB area

layout is simplified—no need for costly
and bulky heat sinks.

PCB Layout

As described above, with only one­fourth as much power dissipation as
a Schottky, thermal layout with the
LTC4358 is much easier. Most of the
heat escapes the part through the
DRAIN/exposed pad, while some exits
through the IN pins. Maximizing the
copper of these connections increases
the allowable maximum current.
Figure 3 shows an optimal layout for
a 1" × 1" single sided PCB with the
DFN package. Copper connected to
the exposed pad above and below the
LTC4358 helps remove heat from the
package. If you are using a two-sided
PCB, use vias under the LTC4358 to
transfer heat to copper on the bot­tom of the PCB, thus increasing the
maximum current by 10%. Use Figure
4 to determine the amount of copper
area needed for a specified current
and ambient temperature.

Linear Technology Magazine • June 2008

Figure 3. DFN layout considerations for 1" × 1" single sided PCB

Conclusion

The LTC4358 is a MOSFET -based
ideal diode that can directly replace
a 5A Schottky diode in 9V to 26.5V
applications. The LTC4358 betters a
Schottky by a factor of four on volt­age drop, power loss and package
size, thus significantly shrinking the
thermal layout and improving overall
performance. Also, simple optimiza­tion the PCB layout increases the
maximum current—no heat sinks
required.

L

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