MAX6499 Maxim Integrated Products, MAX6499 Datasheet - Page 11

• Manufacturer Part Number: MAX6499
• Description: Overvoltage-Protection Switches/Limiter Controllers
• Manufacturer: Maxim Integrated Products

• The overvoltage waveform period (t
• The power dissipated across the package (P
During an initial overvoltage occurrence, the discharge
time (∆t
discharge time is approximately:
where V
current, and I
current.
Upon OUT falling below the threshold point, the
MAX6495/MAX6496/MAX6499s’ charge-pump current
must recover and begins recharging the external GATE
voltage. The time needed to recharge GATE from -V
the MOSFET’s gate threshold voltage is:
where C
V
the internal clamp (from OUTFB to GATE) diode’s for-
ward voltage (1.5V, typ) and I
current (100µA typ).
During ∆t
The voltage across C
MOSFET reaches its V
approximated using the following formula:
Figure 5. MAX6495/MAX6496/MAX6499 Timing
GS(TH)
GATE
OUTFB
Limiter Controllers with an External MOSFET
1
OV
is the MOSFET’s gate threshold voltage, V
) of C
2
ISS
∆t
, C
is the overvoltage threshold, I
1
∆t
OUT
is the MOSFET’s input capacitance,
GATEPD
2
OUT
=
=
C
loses charge through the output load.
______________________________________________________________________________________
, caused by I
OUT
C
ISS
72V, Overvoltage-Protection Switches/
is the GATE’s 100mA pulldown
OUT
GS(TH)
∆t
(
I
V
OUT
1
GS TH
V
(∆V
OV
(
∆t
I
GATE
∆t
GATE
OV
+
2
2
threshold and can be
OUT
)
×
) decreases until the
I
GATEPD
+
∆t
0 95
is the charge-pump
3
.
V
OV
and I
D
)
OUT
)
GATEPD
is the load
DISS
. The
)
D
D
to
is
Once the MOSFET V
output-voltage rise is determined by the MOSFET Q
charge through the internal charge pump with respect
to the drain potential. The new rise time needed to
reach a new overvoltage event can be calculated using
the following formula:
where Q
The total period of the overvoltage waveform can be
summed up as follows:
The MAX6495/MAX6496/MAX6499 dissipate the most
power during an overvoltage event when I
maximum power dissipation can be approximated
using the following equation:
The die-temperature increase is related to θ
and 8.5°C/W for the MAX6495/MAX6496/MAX6499,
respectively) of the package when mounted correctly
with a strong thermal contact to the circuit board. The
MAX6495/MAX6496/MAX6499 thermal shutdown is
governed by the equation:
Based on these calculations, the parameters of the
MOSFET, the overvoltage threshold, the output load
current, and the output capacitors are external vari-
ables affecting the junction temperature. If these para-
meters are fixed, the junction temperature can also be
affected by increasing ∆t
is on. By increasing the capacitance at the GATE pin,
∆t
required to charge up this additional capacitance
(75µA gate current). As a result, ∆t
3
increases as it increases the amount of time
P
DISS
GD
T
J
=
= T
is the gate-to-drain charge.
∆
V
A
t
OV
3
∆t
∆
+ P
V
≅
OV =
2
×
DISS
GS(TH)
Q
V
=
0 975
GS
GD
.
∆t
I
3
(θ
OUT
1
, which is the time the switch
JC
is obtained, the slope of the
+ ∆t
∆
I
×
GATE
V
+θ
C
OUT
2
I
GATEPD
∆
OUT
CA
+ ∆t
www.DataSheet4U.com
t
2
OV
) < +170°C
3
increases, there-
×
OUT
JC
∆
∆
t
OV
t
(8.3°C/W
= 0. The
1
11
g