Datasheet

C
O
+
2.9 m
ǒ
(
8 A
)
2
*
(
1 A
)
2
Ǔ
ǒ
(
3.3
)
2
*
(
3.0
)
2
Ǔ
+ 97 mF
( )
( )
KFF IN(min) T
R V 3.48 58.14 R 1340 72.8k use71.5k= - ´ ´ + = W \ W
R
T
+
ǒ
1
f
SW
17.82 10
*6
* 17
Ǔ
kW + 170 kW N use 169 kW
L +
(
24 * 3.3 V
)
3.3 V
24 V 3.2 A 300 kHz
+ 2.96 mH
T
J
+ P
SR
q
JA
) T
A
+
(
1.322
)
40 ) 85 + 139
o
C
P
SR
+ P
RR
) P
COND
) P
DC
+ 0.108 ) 0.83 ) 0.384 + 1.322 W
P
RR
+ 0.5 Q
RR
V
IN
f
SW
+ 0.5 30 nC 24 V 300 kHz + 0.108 W
P
DC
+ 2 I
O
V
FD
t
DELAY
f
SW
+ 2 8.0 A 0.8 V 100 ns 300 kHz + 0.384
( )
( )
2
COND
P 7.44 0.008 1 0.007 150 25 0.83 W= ´ ´ + ´ - =
I
RMS
+ I
O
1 * d
Ǹ
+ 8 1 * 0.135
Ǹ
+ 7.44 A
RMS
TPS40054
TPS40055
TPS40057
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SLUS593H DECEMBER 2003REVISED JULY 2012
5. Calculate synchronous rectifier losses
The synchronous rectifier MOSFET has two (2) loss components, conduction, and diode reverse recovery
losses. The conduction losses are due to I
RMS
losses as well as body diode conduction losses during the dead
time associated with the anti-cross conduction delay.
The I
RMS
current through the synchronous rectifier from Equation 37:
(55)
The synchronous MOSFET conduction loss from Equation 33 is:
(56)
The body diode conduction loss from Equation 38 is:
(57)
The body diode reverse recovery loss from Equation 39 is:
(58)
The total power dissipated in the synchronous rectifier MOSFET from Equation 40 is:
(59)
The junction temperature of the synchronous rectifier at 85°C is:
(60)
In typical applications, paralleling the synchronous rectifier MOSFET with a Schottky rectifier increases the
overall converter efficiency by approximately 2% due to the lower power dissipation during the body diode
conduction and reverse recovery periods.
6. Calculate the inductor value
The inductor value is calculated from Equation 5.
(61)
A 2.9-µH Coev DXM1306-2R9 or 2.6-µH Panasonic ETQ-P6F2R9LFA can be used.
7. Setting the switching frequency
The clock frequency is set with a resistor (R
T
) from the RT pin to ground. The value of R
T
can be found from
Equation 1, with f
SW
in kHz.
(62)
8. Programming the ramp generator circuit
The PWM ramp is programmed through a resistor (R
KFF
) from the KFF pin to V
IN
. The ramp generator also
controls the input UVLO voltage. For an undervoltage level of 10 V, R
KFF
can be calculated from Equation 2:
(63)
9. Calculating the output capacitance (C
O
)
In this example the output capacitance is determined by the load response requirement of ΔV = 0.3 V for a 1-A
to 8-A step load. C
O
can be calculated using Equation 11:
(64)
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