Calculate the discharge time, t
1
, using the following
equation:
where t
1
is in ms, V
OV
is the adjusted overvoltage
threshold in volts, I
LOAD
is the external load current in
mA, and I
GATEPD
is the 63mA (typ) internal pulldown
current of GATE. C
SOURCE
is the value of the capacitor
connected between the source of the MOSFET and
PGND in µF.
GATE Delay Time (t
2
)
When SOURCE falls 4% below the overvoltage threshold
voltage, the internal current sink is disabled and the
internal charge pump begins recharging the external
GATE voltage. Due to the external load, the SOURCE
voltage continues to drop until the gate of the MOSFET is
recharged. The time needed to recharge GATE and re-
enhance the external MOSFET is approximately:
where t
2
is in µs, C
iss
is the input capacitance of the
MOSFET in pF, and V
GS(TH)
is the gate-to-source thresh-
old voltage of the MOSFET in volts. V
F
is the 0.7V (typ)
internal clamp diode forward voltage of the MOSFET in
volts, and I
GATE
is the charge-pump current 45µA (typ).
Any external capacitance between GATE and PGND will
add up to C
iss
.
During t
2
, the SOURCE capacitance, C
SOURCE
, loses
charge through the output load. The voltage across
C
SOURCE
decreases by ΔV
2
until the MOSFET reaches
its V
GS(TH)
threshold. Approximate ΔV
2
using the fol-
lowing formula:
SOURCE Output Charge Time (t
3
)
Once the GATE voltage exceeds the gate-to-source thresh-
old, V
GS(TH)
, of the external MOSFET, the MOSFET turns
on and the charge through the internal charge pump with
respect to the drain potential, Q
G
, determines the slope of
the output-voltage rise. The time required for the SOURCE
voltage to rise again to the overvoltage threshold is:
where ΔV
SOURCE
= (V
OV
x 0.04) + ΔV
2
in volts, and
C
rss
is the MOSFET’s reverse transfer capacitance in
pF. Any external capacitance between GATE and
PGND adds up to C
rss
.
Power Dissipation/Junction Temperature
During normal operation, the MAX15008/MAX15010
has two main sources of internal power dissipation: the
LDO and the voltage tracker.
Calculate the power dissipation due to the LDO as:
P
LDO
= (V
IN
- V
OUT_LDO
) x I
OUT_LDO
where V
IN
is the LDO input supply voltage in volts,
V
OUT_LDO
is the output voltage of the LDO in volts, and
I
OUT_LDO
is the LDO total load current in mA.
Calculate power dissipation due to the tracker as:
P
TRK
= (V
TRACK
- V
OUT_TRK
) x I
OUT_TRK
where V
TRACK
is the tracker input supply voltage in
volts, V
OUT_TRK
is the output voltage of the tracker in
volts, and I
OUT_TRK
is the tracker load current in mA.
The total power dissipation P
DISS
in mW as:
P
DISS
= P
LDO
+ P
TRK
For prolonged exposure to overvoltage events, use the
V
IN
and V
TRACK
voltages expected during overvoltage
conditions. Under these circumstances the corre-
sponding internal power dissipation contribution, P
OVP
,
calculated in the
Overvoltage-Limiter Mode Switching
Frequency
section should also be included in the total
power dissipation, P
DISS
.
For a given ambient temperature, T
A
, calculate the
junction temperature, T
J
, as follows:
T
J
= T
A
+ P
DISS
x θ
JA
where T
J
and T
A
are in °C and θ
JA
is the junction-to-
ambient thermal resistance in °C/W as listed in the
Absolute Maximum Ratings
section.
The junction temperature should never exceed +150°C
during normal operation.
Thermal Protection
When the junction temperature exceeds T
J
= +160°C,
the MAX15008/MAX15010 shut down to allow the
device to cool. When the junction temperature drops to
+140°C, the thermal sensor turns all enabled blocks
on again, resulting in a cycled output during continu-
ous thermal-overload conditions. Thermal protection
protects the MAX15008/MAX15010 from excessive
power dissipation. For continuous operation, do not
exceed the absolute maximum junction temperature
rating of +150°C.
