Datasheet

LTC4359
8
4359fa
For more information www.linear.com/LTC4359
APPLICATIONS INFORMATION
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
, R
DS(ON)
, the maximum drain-source voltage,
BV
DSS
, and the gate threshold voltage V
GS(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
V
GS(MAX)
during input shorts.
The maximum allowable drain-source voltage, BV
DSS
, 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 BV
DSS
> OUT + |V
IN
|.
The MOSFET’s on-resistance, R
DS(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:
R
DS(ON)
<
Forward Vo ltage Drop
I
LOAD
The resulting power dissipation is
P
d
= (I
LOAD
)
2
• R
DS(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 sig-
nal, 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 2.6µA (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 V
SS
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,
V
SS
, 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.
Figure 2. Forward Voltage Drop Comparison
Between MOSFET and Schottky Diode
VOLTAGE (V)
0
CURRENT (A)
10
15
0.5
4359 F02
5
0
0.20.1
0.3
0.4
20
MOSFET
(BSC028N06NS)
SCHOTTKY DIODE
(SBG2040CT)