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TPS40075 POWER DISSIPATION
P
D
+ Q
g
V
DR
f
SW
(Wattsńdriver)
(12)
P
T
+
ǒ
2 P
D
V
DR
) I
Q
Ǔ
V
IN
(Watts)
(13)
P
T
+
ǒ
2 Q
g
f
SW
) I
Q
Ǔ
V
IN
(Watts)
(14)
P
T
+
T
J
* T
A
q
JA
(Watts)
(15)
f
SW
+
ǒ
ƪ
ǒ
T
J
*T
A
Ǔ
ǒ
q
JA
V
IN
Ǔ
ƫ* I
Q
Ǔ
ǒ
2 Q
g
Ǔ
(Hz)
(16)
BOOST DIODE
TPS40075
SLUS676A – MAY 2006 – REVISED SEPTEMBER 2007
Automatic startup sequencing can be accomplished by connecting the PGD pin of a master supply based on the
TPS40075 to the SS pin of a slave supply. The master comes up first and release the salve SS pin to allow the
slave to come up. Controlled shutdown of sequenced supplies can be accomplished by either pulling the SS pin
of the master below the shutdown threshold and letting the PGD pin pull the slave SS pin down, or by pulling
down the SS pins of all supplies simultaneously.
The power dissipation in the TPS40075 is largely dependent on the MOSFET driver currents and the input
voltage. The driver current is proportional to the total gate charge, Qg, of the external MOSFETs. Driver power
(neglecting external gate resistance) can be calculated from:
where
• V
DR
is the driver output voltage
The total power dissipation in the TPS40075, assuming the same MOSFET is selected for both the high-side and
synchronous rectifier is described in Equation 13 .
or
where
• I
Q
is the quiescent operating current (neglecting drivers)
The maximum power capability of the TPS40075 PowerPAD package is dependent on the layout as well as air
flow. The thermal impedance from junction to air ambient assuming 2-oz. copper trace and thermal pad with
solder and no air flow is θ
JA
= 60 ° C/W
The maximum allowable package power dissipation is related to ambient temperature by Equation 15 .
Substituting Equation 15 into Equation 14 and solving for f
SW
yields the maximum operating frequency for the
TPS4007x. The result is described in Equation 16 .
The TPS40075 has internal diodes to charge the boost capacitor connected from SW to BOOST. The drop
across this diode is rather large at 1.4-V nominal at room temperature resulting in the drive voltage to the
high-side MOSFET being reduced by this amount from the DBP voltage. If this drop is too large for a particular
application, an external diode may be connected from DBP (anode) to BOOST (cathode). This provides
significantly improved gate drive for the high-side MOSFET, especially at lower input voltages.
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