Datasheet

LTC4366-1/LTC4366-2
436612fd
For more information www.linear.com/LTC4366
15
applicaTions inForMaTion
has exceeded V
REG
. Since the OUT pin voltage is at least
2.55V larger than V
SS
it exceeds the specified maximum.
Choosing the match point (with supply at the maximum)
sufficiently below V
REG
(by at least 2.55V), allows C1 to
charge up in time to protect the load from overvoltage.
In reality having V
SS
pin voltage 7V below V
REG
provides
required margin for charging C1.
V
SS(MATCH)(MAX)
= V
REG
– 7V
Increasing R
SS
increases the match point, so determin-
ing the maximum R
SS
value while still protecting from
overvoltage is useful. Using I
RIN
= I
RSS
:
R
SS
= R
IN
•
V
RSS
V
RIN
Using:
V
RSS
= V
SS(MATCH)(MAX)
= V
REG
– 7V
V
RIN
= V
IN
– V
Z(VDD)
– V
RSS
Substituting:
R
SS(MAX)
=
R
IN
• V
REG
– 7V
( )
V
IN(MAX)
– 12V – V
REG
– 7V
( )
R
SS(MAX)
=
R
IN
• V
REG
– 7V
( )
V
IN(MAX)
– 5V – V
REG
If we guarantee that R
SS
< R
SS(MAX)
then the following
is true:
V
SS(MATCH)
< V
SS(MATCH)(MAX)
C1 bypasses the charge pump, and requires at least a
0.22µF. The size of C1 needs limits also. The gate capaci-
tor (C
G
) dictates the maximum output capacitor C1
(MAX)
that will charge to the 2.55V UVLO1 threshold (V
UVLO1
)
before the OUT voltage exceeds the overvoltage threshold.
C1
(MAX )
=
–C
G
• R
SS
+R
IN
( )
V
REG
– V
SS(MATCH )
( )
I
G
•R
SS
•R
IN
•In 1–
2 • V
UVLO1
V
REG
– V
SS(MATCH )
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
In most cases:
C1
(MAX)
= 10 • C
G
to 100 • C
G
GATE Capacitor, C
G
The gate capacitor is used for three functions. First, C
G
absorbs charge from the gate-to-drain capacitance of
the MOSFET during overvoltage transients. Second, the
capacitor also acts as a compensation element for the
overvoltage regulation amplifier. The minimum value for
C
G
to guarantee stability is 2nF. Finally, C
G
sets the slew
rate of the GATE and OUT pins. The voltage at the GATE pin
rises at a slope equal to 20µA/C
G
. This slope determines
the charging current into the load capacitor.
I
INRUSH
=
C
LOAD
C
G
•I
G
The voltage rating for C
G
must be greater than the regula-
tion voltage (V
REG
).
MOSFET Selection
The LTC4366 drives an N-channel MOSFET to conduct
the load current. The important features of the MOSFET
are on-resistance, R
DS(ON)
, the maximum drain-source
voltage, V
(BR)DSS
, the threshold voltage, and the SOA.
The maximum allowable drain-source voltage must be
higher than the supply voltage. If the output is shorted
to ground or during an overvoltage event, the full supply
voltage will appear across the MOSFET.
The threshold voltage of the MOSFET is used in the mini
-
mum supply start-up calculation. For applications with
supplies less than 12V, a logic-level MOSFET is required.
Above 12V a standard threshold N-channel MOSFET is
sufficient.
The SOA of the MOSFET must encompass all fault condi
-
tions. In normal operation the pass transistor is fully on,
dissipating very little power
. But during overvoltage faults,
the GATE pin is servoed to regulate the output voltage
through the MOSFET. Large current and high voltage drop
across the MOSFET can coexist in these cases. The SOA
curves of the MOSFET must be considered carefully along
with the selection of the fault timer capacitor.