LTC3735 Datasheet, PDF (24/32 Pages) Linear Technology – 2-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3735
APPLICATIONS INFORMATION
Design Example
As a design example, assume V_IN = 12V (nominal), V_IN
= 21V (max), V_OUT = 1.5V, I_MAX = 35A, and f = 350kHz
(each phase).
The inductance value is chosen first based on a 40% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. The minimum
inductance for 40% ripple current is:
L
≥
V_OUT
f • ∆I
•


1–
V_OUT
V_IN


=
350kHz
•
1.5V
(40%
•
17.5A)
•


1–
1.5V
21V


=
0.57µH
Using L = 0.6µH, a common “off-the-shelf” value results
in 38%ripple current. The peak inductor current will be
the maximum DC current plus one half of the ripple cur-
rent, or 21A.
Tie the FREQSET pin to 1.2V, resistively divided down from
PV_CC to have 350kHz operation for each phase.
The minimum on-time also occurs at maximum input
voltage:
t_ON(MIN)
=
V_OUT
V_IN • f
=
1.5V
21V • 350kHz
=
204ns
which is larger than 150ns, the typical minimum on time
of the LTC3735.
R_SENSE1 and R_SENSE2 can be calculated by using a con-
servative maximum sense voltage threshold of 40mV and
taking into account of the peak current:
R_SENSE
=
40mV
21A
=
0.002Ω
The power loss dissipated by the top MOSFET can be cal-
culated with equations 3 and 7. Using a Fairchild FDS7760
as an example: R_DS(ON) = 8mΩ, Q_G = 55nC at 5V V_GS, C_RSS
= 307pF, V_TH(MIN) = 1V. At maximum input voltage with
T_J(estimated) = 85°C at an elevated ambient temperature:
( ) ^P_TOP
=
1.5V
21V
•


35A
2


2
•
1+ 0.005 • (85°C – 25°C)
•
0.008Ω
+
21V
2
• 17.5A
2
•
350kHz
•
307pF
•
2Ω
•


5V
1
–
1V
+
1
1V


=
1.26W
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
•


35A
2


2
•
(^{1+ 0.005 •}(^{85°C – 25°C})) ^{• 0.008Ω}
= 2.95W
Therefore it is necessary to have two FDS7760s in parallel
to split the power loss.
A short-circuit to ground will result in a folded back cur-
rent of about:
I_SC
=
25mV
0.002Ω
+
1
2
•


200ns • 21V
0.6µH


=
16A
The worst-case power dissipation by the bottom MOSFET
under short-circuit conditions is:
1 _{– 200ns}
P_BOT
=
350kHz
1
•(16A)² •
350kHz
(^{1+ 0.005 •}(^{85°C – 25°C})) ^{• 0.008Ω}
= 2.48W
which is less than normal, full load conditions.
The nominal duty cycle of this application is equation 1:
DC
=
1.5V
12V
= 12.5%
3735fa
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