Datasheet
SLVS612 − APRIL 2006
18
www.ti.com
APPLICATION INFORMATION
The 2-multiplier is used because the body-diode conducts twice each cycle (once on the rising edge and once
on the falling edge). The reverse recovery losses are due to the time it takes for the body diode to recovery from
a forward bias to a reverse blocking state. The reverse recovery losses are described in equation (33).
P
RR
+ 0.5 Q
RR
V
IN
f
SW
(Watts)
where:
D Q
RR
is the reverse recovery charge of the body diode
The total synchronous rectifier MOSFET power dissipation is described in equation (34).
P
SR
+ P
DC
) P
RR
) P
COND
(Watts)
TPS40056 POWER DISSIPATION
The power dissipation in the TPS40056 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, refer to [2] can be calculated from equation (35).
P
D
+ Q
g
V
DR
f
SW
(Watts)
And the total power dissipation in the TPS40056, assuming the same MOSFET is selected for both the high-side
and synchronous rectifier is described in equation (36).
P
T
+
ǒ
2 P
D
V
DR
) I
Q
Ǔ
V
IN
(Watts)
or
P
T
+
ǒ
2 Q
g
f
SW
) I
Q
Ǔ
V
IN
(Watts)
where:
D I
Q
is the quiescent operating current (neglecting drivers)
The maximum power capability of the device’s PowerPad package is dependent on the layout as well as air flow.
The thermal impedance from junction to air, assuming 2 oz. copper trace and thermal pad with solder and no
air flow.
θ
JA
= 36.51°C/W
The maximum allowable package power dissipation is related to ambient temperature by equation (29).
Substituting equation (29) into equation (37) and solving for f
SW
yields the maximum operating frequency for
the TPS4005x. The result is described in equation (38).
f
SW
+
ǒ
ƪ
ǒ
T
J
*T
A
Ǔ
ǒ
q
JA
V
DD
Ǔ
ƫ
* I
Q
Ǔ
ǒ
2 Q
g
Ǔ
(Hz)
(33)
(34)
(35)
(36)
(37)
(38)