LINEAR TECHNOLOGY LTC4352 Technical data

L DESIGN FEATURES

LOAD CURRENT (A)

0

0

POWER DISSIPATION (W)

2.5

2.0

1.5

1.0

0.5

4.0

3.0

3.5

2

4 6 8 10

DIODE (SBG1025L)

MOSFET (Si7336ADP)

POWER

SAVED

V

CC

UV

OV

REV

LTC4352

STATUS

FAULT

MOSFET ON
STATUS

OPTIONAL

0.1µF

TO LOAD

2.9V TO 18V

Si4438DY

FAULT

0.1µF

CPO V

IN

SOURCE GATE

GND

OUT

VINV

CC

V

CC

UV

OV

0.5V

REV

1V

–

+

LDO

GND

LOGIC

CHARGE

PUMP

OPEN

MOSFET

DETECT

MOSFET

ON

DETECT

STATUS

FAULT

10µA

10µA

100µA

0.1µF

0.1µF

25mV

OUTCPOGATESOURCE

AMP

Si7336ADP

SUPPLY

INPUT

OUTPUT
TO LOAD

V

IN

V

CC

V

CC

+

–

+

–

+
–

+

0V to 18V Ideal Diode Controller
Saves Watts and Space over Schottky

Introduction

Schottky diodes are used in a variety
of ways to implement multisource
power systems. For instance, high
availability electronic systems, such
as network and storage servers, use
power Schottky diode-OR circuits to
realize a redundant power system.
Diode-ORing is also used in systems
that have alternate power sources,
such as an AC wall adapter and a
backup battery feed. Power diodes
can be combined with capacitors to
hold up a load voltage during an in­put brownout. In this case, the power
diodes are placed in series with the
input voltage, with the capacitors on
the load side of the diode. While the
capacitors provide power, the reverse­biased diode isolates the load from the
sagging input.

Schottky diodes suffice for these
applications when currents are below
a few amperes, but for higher cur­rents, the excess power dissipated in
the diode due to its forward voltage
drop demands a better solution. For
instance, 5A flowing through a diode
with a 0.5V drop wastes 2.5W within
the diode. This heat must be dissipated
with dedicated copper area on the
PCB or heat sinks bolted to the diode,
both of which take significant space.
The diode’s forward drop also makes

by Pinkesh Sachdev

Figure 1. The LTC4352 controlling an N-Channel MOSFET replaces a power diode and associated
heat sink to save power, PCB area, and voltage drop. Also shown: the small PCB footprint of the
ideal diode circuit using a 3mm × 3mm DFN-12 packaged LTC4352 and SO-8 size MOSFET.

it impractical for low voltage applica­tions. This problem calls out for an
ideal diode with a zero forward voltage
drop to save power and space.

The LTC4352 ideal diode controller
in tandem with an N-channel MOSFET
creates a near -ideal diode for use with
0V to 18V input supplies. Figure 1 il­lustrates the simplicity of this solution.
This ideal diode circuit can replace a

power Schottky diode to create a highly
efficient power ORing or supply holdup
application. Figure 2 shows the power
savings of the ideal diode circuit over
a Schottky diode. 3.5W is saved at
10A, and the saving increases with
load current. With its fast dynamic
response, the controller excels in low
voltage diode-OR applications which
are more sensitive to voltage droop.

Figure 2. As load current increases, so do the
power savings gained from using an ideal diode
(LTC4352 + Si7336ADP) instead of a power
Schottky diode (SBG1025L).

24

Figure 3. Simplified internals of the LTC4352

Linear Technology Magazine • September 2008

DESIGN FEATURES L

FORWARD VOLTAGE (V)

0.025

0

CURRENT (A)

CONSTANT

R

DS(ON)

0.5

CONSTANT

VOLTAGE

SCHOTTKY
DIODE

LTC4352

25mV

R

DS(ON)

TIME (5µs/DIV)

V

IN1

V

LOAD

V

IN1

V

IN2

V

IN2

VOLTAGE

(2V/DIV)

V

CC

UV

OV

REV

LTC4352

STATUS

FAULT

0.1µF

V

IN1

3.5V

Q1

Si4438DY

Q3

Si4438DY

0.1µF

CPO V

IN

SOURCE GATE

GND

OUT

V

CC

UV

OV

REV

LTC4352

STATUS

FAULT

0.1µF

C

L

100µF

V

IN2

3.3V
I

L

8A

0.1µF

CPO V

IN

SOURCE GATE

GND

OUT

TIME (5µs/DIV)

$V

GATE1

$V

GATE

= V

GATE

– V

SOURCE

$V

GATE2

VOLTAGE

(2V/DIV)

What Makes It Ideal?

The LTC4352 monitors the differential
voltage across the MOSFET source
(the “anode”) and drain (the “cathode”)
terminals. The MOSFET has an intrin­sic source-to-drain body diode which
conducts the load current at initial
power-up. When the input voltage is
higher than the output, the MOSFET is
turned on, resulting in a forward volt­age drop of I
can be suitably chosen to provide an
easy 10x reduction over a Schottky
diode’s voltage drop. When the input
drops below the output, the MOSFET
is turned off, thus emulating the be­havior of a reverse biased diode.

An inferior ideal diode control tech­nique monitors the voltage across the
MOSFET with a hysteretic comparator.
For example, the MOSFET could be
turned on whenever the input to out­put voltage exceeds 25mV. However,
choosing the lower turn-off threshold
can be tricky. Setting it to a positive
forward voltage drop, say 5mV, causes
the MOSFET to be turned off and
on repeatedly at light load currents.
Setting it to a negative value, such as
–5mV, allows DC reverse current.

LOAD

• R

DS(ON)

. The R

DS(ON)

Figure 4. The forward I-V characteristic of the
LTC4352 ideal diode vs a Schottky diode.

The LTC4352 implements a linear
control method to avoid the problems
of the comparator-based technique.
It servos the gate of the MOSFET to
maintain the forward voltage drop
across the MOSFET at 25mV (AMP
of Figure 3). At light load currents,
the gate of the MOSFET is slightly
above its threshold voltage to cre­ate a resistance of 25mV/I

LOAD

. As
the load current increases, the gate
voltage rises to reduce the MOSFET
resistance. Ultimately, at large load
currents, the MOSFET gate is driven
fully on, and the forward voltage drop

rises linearly with load current as I

• R

. Figure 4 shows the resulting

DS(ON)

LOAD

ideal diode I-V characteristic.

In a reverse voltage condition, the
gate is servoed low to completely turn
off the MOSFET, thus avoiding DC
reverse current. The linear method
also provides a smooth switchover
of currents for slowly crossing input
supplies in diode-OR applications. In
fact, depending on MOSFET and trace
impedances, the input supplies share
the load current when their voltages
are nearly equal.

Fast Switch Control

Most ideal diode circuits suffer slower
transient response compared to con­ventional diodes. The LTC4352, on
the other hand, responds quickly to
changes in the input to output volt­age. A powerful driver turns off the
MOSFET to protect the input supply
and board traces from large reverse
currents. Similarly, the driver turns
on the switch rapidly to limit voltage
droop during supply switchover in
diode-OR applications.

Figure 5 shows a fast switchover
event occurring in a 3.3V ideal diode-

Linear Technology Magazine • September 2008

a. Ideal diode-OR of 3.5V and 3.3V input supply. b. Supply switchover from V

on V

shows minimal disturbance on load voltage.

IN1

Figure 5. Ideal diode-OR fast switchover

IN1

to V

due to short-circuit

IN2

25