LTC3735 LINER [Linear Technology], LTC3735 Datasheet - Page 24

• Manufacturer Part Number: LTC3735
• Description: 2-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs
• Manufacturer: LINER [Linear Technology]

LTC3735
APPLICATIONS INFORMATION
Design Example
As a design example, assume V
= 21V (max), V
(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:
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
The minimum on-time also occurs at maximum input
voltage:
which is larger than 150ns, the typical minimum on time
of the LTC3735.
R
servative maximum sense voltage threshold of 40mV and
taking into account of the peak current:
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
= 307pF , V
T
24
J
SENSE1
(estimated) = 85°C at an elevated ambient temperature:
CC
L ≥
R
t
ON(MIN)
SENSE
to have 350kHz operation for each phase.
V
f • ∆I
OUT
and R
TH(MIN)
=
=
• 1–
40mV

 
V
V
21A
SENSE2
IN
OUT
OUT
DS(ON)
• f
= 1V. At maximum input voltage with
V
V
OUT
= 0.002Ω
= 1.5V, I
=
IN
can be calculated by using a con-
21V • 350kHz
= 8mΩ, Q

 
=
1.5V
350kHz • 40% • 17.5A
MAX

 
1–
G
IN
= 35A, and f = 350kHz
1.5V
21V
= 55nC at 5V V
= 12V (nominal), V
= 204ns
(
1.5V

 
= 0.57µH
GS
, C
)
•
RSS
IN
Equation 4 gives the worst-case power loss dissipated
by the bottom MOSFET (assuming FDS7760 and T
85°C again):
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:
The worst-case power dissipation by the bottom MOSFET
under short-circuit conditions is:
which is less than normal, full load conditions.
The nominal duty cycle of this application is equation 1:
P
I
P
P
DC =
TOP
SC
BOT
BOT
=
=
= 2.48W
=
=
1.5V
= 2.95W
0.002Ω
12V
25mV
1.5V
0.008Ω +
2Ω •
21V
21V – 1.5V
(
(
350kHz
1+ 0.005 • 85°C – 25°C
1+ 0.005 • 85°C – 25°C
21V
1
= 12.5%

 
350kHz
•
5V – 1V

 
+
35A
1
– 200ns
2
1
2
1
21V
•
•
(
(

 

 

 
2
200ns • 21V
2
+
35A
• 1+ 0.005 • 85°C – 25°C
• 17.5A
2
2
1V
(
0.6µH
1
• 16A
(

 

 
2
= 1.26W
•
• 350kHz • 307pF •
)
2
)
)

 
)
)
•
• 0.008Ω
• 0.008Ω
= 16A
(
)
3735fa
)
J
•
=