Datasheet

P
SW(fsw)
+
ǒ
V
IN
I
O
t
SW
Ǔ
f
SW
+ 55 V 5 A 20 ns 130 kHz + 0.715 W
P
COND
+ 1.2
2
0.12
(
1 ) 0.007
(
150 * 25
))
+ 0.324 W
I
RMS
+ I
O
d
Ǹ
+ 5 0.0588
Ǹ
+ 1.2 A
DI + I
O
2 0.2 + 5 2 0.2 + 2.0 A
f
SW
+
0.0588
400 ns
+ 147 kHz
1
T
SW
+ f
SW
+
ȧ
ȧ
ȧ
ȧ
ȡ
Ȣ
ǒ
V
O(min)
V
IN(max)
Ǔ
T
ON
ȧ
ȧ
ȧ
ȧ
ȣ
Ȥ
V
O(min)
V
IN(max)
+
t
ON
T
SW
or
d
MIN
+
V
O(min)
V
IN(max)
+ 0.0588 d
MAX
+
V
O(max)
V
IN(min)
++ 0.187
TPS40060
TPS40061
SLUS543F –DECEMBER 2002–REVISED JUNE 2013 www.ti.com
DESIGN EXAMPLE
• Input voltage: 18 V
DC
to 55 V
DC
• Output voltage: 3.3 V ±2%
• Output current: 5 A (maximum, steady-state), 7 A (surge, 10-ms duration, 10% duty cycle maximum)
• Output ripple: 33 mV
P-P
at 5 A
• Output load response: 0.3 V => 10% to 90% step load change
• Operating temperature: –40°C to 85°C
• f
SW
= 130 kHz
1. Calculate maximum and minimum duty cycles
(42)
2. Select switching frequency
The switching frequency is based on the minimum duty cycle ratio and the propagation delay of the current limit
comparator. In order to maintain current limit capability, the on time of the upper MOSFET, t
ON
, must be greater
than 330 ns (see Electrical Characteristics table). Therefore
(43)
(44)
Using 400 ns to provide margin,
(45)
Since the oscillator can vary by 10%, decrease f
SW
, by 10%
f
SW
= 0.9 × 147 kHz = 130 kHz
and therefore choose a frequency of 130 kHz.
3. Select ΔI
In this case ΔI is chosen so that the converter enters discontinuous mode at 20% of nominal load.
(46)
4. Calculate the high-side MOSFET power losses
Power losses in the high-side MOSFET (Si9407AGY) at 55-V
IN
where switching losses dominate can be
calculated from Equation 46 through Equation 49.
(47)
substituting Equation 47 into Equation 29 yields
(48)
and from Equation 31, the switching losses can be determined.
(49)
The MOSFET junction temperature can be found by substituting Equation 33 into Equation 32
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