LTC3734 Datasheet, PDF (23/28 Pages) Linear Technology – Single-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3734
APPLICATIO S I FOR ATIO
voltage with T_J(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 T_J = 85°C
again):
P_BOT
=
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:
I_SC
=
25mV
0.002Ω
+
1
2
⎛
• _⎝⎜
200ns • 21V^⎞
0.5μH ⎠⎟
=
16.7A
1 _{– 200ns}
P_BOT
=
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:
I_INRMS = 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:
ΔI_OUT(MAX)
=
1.5V
350kHz • 0.5μH
⎛
• _⎝⎜1−
1.5^⎞
2.1⎠⎟
=
8A_P-P
Assuming the ESR of output capacitor(s) is 5mΩ, the
output ripple voltage is:
( ) ^ΔVOUT
⎛
≈ 8A_{P-P⎝⎜⎜}5mΩ +
1
8 • 350kHz • 4 • 270μF
⎞
⎠⎟⎟
= 42.6mV_P-P
3734f
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