LTC1628-PG View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1628-PG' PDF : 28 Pages View PDF
LTC3734
APPLICATIO S I FOR ATIO
voltage with TJ(estimated) = 85°C at an elevated ambient
temperature:
The worst-case power dissipation by the bottom MOSFET
under short-circuit conditions is:
( ) PTOP
=
1.5V
21V
• 20A2
•
1+ 0.005 • (85°C – 25°C)
•
0.008Ω + 21V2 • 20A • 350kHz • 307pF •
2
2Ω
•
⎛⎝⎜
5V
1
–
1V
+
1
1V
⎞⎠⎟
=
1.48W
Equation 4 gives the worst-case power loss dissipated by
the bottom MOSFET (assuming FDS7760 and TJ = 85°C
again):
PBOT
=
21V – 1.5V
21V
• 20A2
•
( ( )) 1+ 0.005 • 85°C – 25°C • 0.008Ω
= 3.86W
Therefore, it is necessary to have two FDS7760s in parallel
to split the power loss for both the top and bottom
MOSFETS.
A short-circuit to ground will result in a folded back current
of about:
ISC
=
25mV
0.002Ω
+
1
2
⎛
• ⎝⎜
200ns • 21V⎞
0.5μH ⎠⎟
=
16.7A
1 – 200ns
PBOT
=
350kHz
1
•16.7A2 •
350kHz
( ( )) 1+ 0.005 • 85°C – 25°C • 0.008Ω
= 2.7W
which is less than normal, full load conditions.
The RMS input ripple current will be:
IINRMS = 20A/2 = 10A
An input capacitor(s) with a 10A RMS current rating is
required.
The output capacitor ripple current is calculated. The
output ripple will be highest at the maximum input voltage:
ΔIOUT(MAX)
=
1.5V
350kHz • 0.5μH
⎛
• ⎝⎜1−
1.5⎞
2.1⎠⎟
=
8AP-P
Assuming the ESR of output capacitor(s) is 5mΩ, the
output ripple voltage is:
( ) ΔVOUT
⎛
≈ 8AP-P⎝⎜⎜5mΩ +
1
8 • 350kHz • 4 • 270μF
⎞
⎠⎟⎟
= 42.6mVP-P
3734f
23
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