Datasheet

T
J
+
ǒ
P
COND
) P
SW
Ǔ
q
JA
) T
A
+
(
0.129 ) 1.152
)
40 ) 85 + 136
O
C
P
SW(fsw)
+
ǒ
V
IN
I
O
t
SW
Ǔ
f
SW
+ 24 V 8 A 20 ns 300 kHz + 1.152 W
P
COND
+ 2.93
2
0.008
(
1 ) 0.007
(
150 * 25
))
+ 0.129 W
I
RMS
+ I
O
d
Ǹ
+ 8 0.135
Ǹ
+ 2.93 A
DI + I
O
2 0.2 + 8 2 0.2 + 3.2 A
f
SW
+
0.135
400 ns
+ 337 kHz
( )
( )
( )
( )
O min
IN max
O min
ON
SW
SW SW ON
IN max
V
V
V
t
1
or f
V t t t
æ ö
æ ö
ç ÷
ç ÷
ç ÷
ç ÷
æ ö
æ ö
è ø
ç ÷
ç ÷
= = =
ç ÷
ç ÷
ç ÷
è ø
è ø
ç ÷
ç ÷
è ø
( )
( )
( )
( )
O min O max
MIN MAX
IN max IN min
V V
3.234 3.366
D 0.135 D 0.337
V 24 V 10
= = = = = =
TPS40054
TPS40055
TPS40057
SLUS593H –DECEMBER 2003–REVISED JULY 2012 www.ti.com
DESIGN EXAMPLE
• Input voltage: 10 Vdc to 24 Vdc
• Output voltage: 3.3 V ±2% (3.234 ≤ V
O
≤ 3.366)
• Output current: 8 A (maximum, steady state), 10 A (surge, 10 ms duration, 10% duty cycle maximum)
• Output ripple: 33 mV
PP
at 8 A
• Output load response: 0.3 V ≥ 10% to 90% step load change, from 1 A to 7 A
• Operating temperature: -40°C to 85°C
• f
SW
= 300 kHz
1. Calculate maximum and minimum duty cycles
(47)
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 300 ns (see Electrical Characteristics Table ). Therefore:
(48)
Using 400 ns to provide margin,
(49)
Since the oscillator can vary by 10%, decrease f
SW
, by 10%
f
SW
= 0.9 × 337 kHz = 303 kHz
and therefore choose a frequency of 300 kHz.
3. Select ΔI
In this case ΔI is chosen so that the converter enters discontinuous mode at 20% of nominal load.
(50)
4. Calculate the high-side MOSFET power losses
Power losses in the high-side MOSFET (Si7860DP) at 24-V
IN
where switching losses dominate can be calculated
from Equation 51.
(51)
Substituting Equation 34 into Equation 33 yields
(52)
and from Equation 35, the switching losses can be determined.
(53)
The MOSFET junction temperature can be found by substituting Equation 52 and Equation 53 into Equation 36:
(54)
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