LTC1709EG-7 Linear Technology, LTC1709EG-7 Datasheet - Page 13

• Manufacturer Part Number: LTC1709EG-7
• Description: IC SW REG STEP-DOWN SYNC 36-SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1709EG-7

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 1.3 ~ 3.5 V
• Current - Output: 3A
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

APPLICATIO S I FOR ATIO
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 Application Note 77.
Figure 3 shows the net ripple current seen by the output
capacitors for the 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 output ripple current is normalized against the
inductor ripple current at zero duty factor. The graph can
be used in place of tedious calculations, simplifying the
design process.
Accepting larger values of I
inductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is
Remember, the maximum I
input voltage. The individual inductor ripple currents are
determined by the inductor, input and output 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
I
L
= 0.4(I
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
0
0.1
)/2, where I
0.2
U
0.3
DUTY FACTOR (V
RMS
0.4
U
OUT
0.5
0.3 ( I
L
L
OUT
0.6
is the total load current.
occurs at the maximum
allows the use of low
/V
IN
0.7
O(P–P)
W
1-PHASE
2-PHASE
)
0.8
17097 F03
)]
0.9
U
ferrite, molypermalloy, or Kool M
loss is independent of core size for a fixed inductor value,
but it is very dependent on inductance selected. As induc-
tance increases, core losses go down. Unfortunately,
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!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive
than ferrite. A reasonable compromise from the same
manufacturer is Kool M . Toroids are very space effi-
cient, especially when you can use several layers of wire.
Because they lack a bobbin, mounting is more difficult.
However, designs for surface mount are available which
do not increase the height significantly.
Power MOSFET, D1 and D2 Selection
Two external power MOSFETs must be selected for each
output stage with the LTC1709-7: 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 INTV
voltage. This voltage is typically 5V during start-up
(see EXTV
threshold MOSFETs must be used in most applications.
The only exception is if low input voltage is expected
(V
(V
BV
logic-level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
input voltage and maximum output current. When the
LTC1709-7 is operating in continuous mode the duty
Kool M is a registered trademark of Magnetics, Inc.
GS(TH)
IN
DSS
< 5V); then, sublogic-level threshold MOSFETs
specification for the MOSFETs as well; most of the
< 1V) should be used. Pay close attention to the
CC
DS(ON)
Pin Connection). Consequently, logic-level
, reverse transfer capacitance C
®
LTC1709-7
cores. Actual core
13
RSS
CC
,