Datasheet

LTC4366-1/LTC4366-2
436612fd
For more information www.linear.com/LTC4366
18
Step 5: Determine C
G
, C1
(MAX)
, Check R
SS
The gate capacitor (C
G
) determines the gate slew rate and
therefore the slew rate of the OUT pin since the output
voltage follows the GATE pin. The voltage at the GATE pin
rises with a slope equal to 7.5µA/C
G
at startup and 20µA/C
G
when the charge pump is on. Limiting this slope will limit
the inrush current charging the load capacitance where:
I
INRUSH
=
C
LOAD
C
G
•I
G
In this example we choose C
G
to be 10nF which limits the
inrush current to be 660mA for a 330µF C
LOAD
.
C1 is used as a bypass capacitor for the circuitry between
the OUT and V
SS
pins. C1 also stabilizes the shunt regula-
tor that clamps the voltage between these pins where the
minimum value for regulator stability is 0.22µF. An even
greater 0.47µF value is desired for C1 to protect the OUT
to V
SS
circuitry from transients on the OUT pin.
The startup into an overvoltage creates an upper bound-
ary on the value of C1. The value of C
G
, R
SS
and R
VIN
determines a maximum C1 that will reach UVLO1 and
power the regulation amplifier before the OUT pin voltage
exceeds the overvoltage threshold. If our desired value
for C1 (0.47µF) exceeds the maximum allowed C1 then
a smaller R
SS
must be used to iterate a new solution for
C1
(MAX)
. We start with calculating V
SS(MATCH)
:
V
SS(MATCH)
=
R
SS
R
SS
+R
VIN
• V
IN
– V
Z(VDD)
( )
If we use the worst-case 1% maximum value for R
SS
(51.6k) and minimum value for R
VIN
(291k):
V
SS(MATCH)
= 35.8V
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)
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
Use the worst-case maximum gate current of 11µA instead
of the typical 7.5µA and the worst-case minimum UVLO1
applicaTions inForMaTion
threshold, 2.75V:
C1
(MAX )
=
–10nF • 51.6k
+
291k
( )
43V – 35.8V
( )
11µA • 51.6k • 291k •In 1–
2 • 2.75V
43V −35.8V
⎡
⎣
⎢
⎤
⎦
⎥
or
C1
(MAX)
= 0.1µF
This limit on C1 does not meet the shunt regulator stability
requirements (C1 > 0.22µF).
If we desire a larger value of C1 then a lower size of R
SS
is required. A lower value for R
SS
is 48.7k, which calls
out an R
IN
value of 309k and a max C1 value of 0.27µF.
The next lower value of 46.4k with R
VIN
of 324k, results
in the worst-case maximum C1 value of 0.49µF. A larger
C1 increases circuit immunity to transients in exchange
for slightly higher current. Therefore, a selection of com
-
ponents that allow a 0.47µF C1 is recommended.
The lowered R
SS
value of 46.4k now considers the toler-
ances of all the components that set the C1 ramp rate to
guarantee it charges to the 2.55V UVLO1 threshold before
the OUT voltage exceeds the overvoltage threshold.
Step 6: Determine R
FB1
, R
FB2
The feedback resistors, R
FB1
and R
FB2
, are chosen to
regulate the overvoltage at 43V. One way to quickly choose
these resistors is to assign 100µA or 1.2V across a 12.4k
R
FB1
. R
FB2
would need to drop the remainder of the regu-
lated voltage. Dividing this remainder by 100µA yields the
value for R
FB2
. In this example R
FB2
drops 41.8V. When
divided by 100µA it results in a 422k value.
Step 7: Determine C
T
, R1
During an overvoltage the power dissipated in the MOS-
FET is dependent on the load current and the difference
between the supply and
regulated voltages. It is necessar
y
to keep the device power in a safe range. In the power
MOSFET data sheets there is a maximum safe operating
curve displaying current versus drain to source voltage
for a fixed pulsed time. Other pulsed time data from DC
to 10µs are plotted on the one graph. The different lines
of operation generally follow a constant power squared