LTC3735EUHF Linear Technology, LTC3735EUHF Datasheet - Page 13

• Manufacturer Part Number: LTC3735EUHF
• Description: IC CTRLR DC/DC 2PH HI EFF 38-QFN
• Manufacturer: Linear Technology

Specifications of LTC3735EUHF

• Applications: Controller, Intel Mobile CPU
• Number Of Outputs: 1
• Voltage - Output: 0.7 ~ 1.71 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 38-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Input: -

APPLICATIO S I FOR ATIO
The inductor value has a direct effect on ripple current.
The inductor ripple current ΔI
inductance or frequency and increases with higher V
where f is the individual output stage operating frequency.
In a 2-phase converter, the net ripple current seen by the
output capacitor is much smaller than the individual
inductor ripple currents due to ripple cancellation. The
details on how to calculate the net output ripple current
can be found in Linear Technology Application Note 77.
Figure 3 shows the net ripple current seen by the output
capacitors for 1- and 2-phase configurations. The output
ripple current is plotted for a fixed output voltage as the
duty factor is varied between 10% and 90% on the x-axis.
The graph can be used in place of tedious calculations,
simplifying the design process.
Accepting larger values of ΔI
inductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is
ΔI
Remember, the maximum ΔI
L
ΔI
= 0.4(I
L
=
V
Figure 3. Normalized Output Ripple Current
vs Duty Factor [I
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
OUT
fL
OUT
0
0.1
)/2, where I
⎛
⎜
⎝
0.2
1
−
U
0.3
DUTY FACTOR (V
V
V
OUT
IN
0.4
RMS
⎞
⎟
⎠
U
OUT
0.5
≈ 0.3 (ΔI
L
OUT
L
0.6
L
is the total load current.
occurs at the maximum
, decreases with higher
/V
allows the use of low
IN
0.7
W
O(P–P)
)
1-PHASE
2-PHASE
0.8
3735 F03
)]
0.9
U
IN
:
input voltage. The individual inductor ripple currents are
determined by the frequency, inductance, input and out-
put voltages.
Inductor Core Selection
Once the values for L1 and L2 are known, the type of
inductor must be selected. High efficiency converters
generally cannot afford the core loss found in low cost
powdered iron cores, forcing the use of more expensive
ferrite, molypermalloy, or Kool Mμ
loss is independent of core size for a fixed inductor value,
but it is very dependent on inductor type selected. As
inductance increases, core losses go down. Unfortu-
nately, increased inductance requires more turns of wire
and therefore copper losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
A variety of inductors designed for high current, low
voltage applications are available from manufacturers
such as Sumida, Coilcraft, Coiltronics, Toko and Pana-
sonic.
Power MOSFET, D1 and D2 Selection
Two external power MOSFETs must be selected for each
output stage with the LTC3735: one N-channel MOSFET
for the top (main) switch, and one N-channel MOSFET for
the bottom (synchronous) switch.
The peak-to-peak drive levels are set by the PV
This voltage typically ranges from 4.5V to 7V. Conse-
quently, logic-level threshold MOSFETs must be used in
most applications. Pay close attention to the BV
fication for the MOSFETs as well; most of the logic-level
MOSFETs are limited to 30V or less.
Kool Mμ is a registered trademark of Magnetics, Inc.
®
cores. Actual core
LTC3735
CC
DSS
voltage.
13
speci-
3735f