LTC1771IS8 Linear Technology, LTC1771IS8 Datasheet - Page 7

• Manufacturer Part Number: LTC1771IS8
• Description: IC CTRLR DC/DC STEPDOWN HE 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1771IS8

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 18 V
• Current - Output: 5A
• Voltage - Input: 2.8 ~ 18 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

APPLICATIO S I FOR ATIO
The basic LTC1771 application circuit is shown in Figure
1 on the first page. External component selection is driven
by the load requirement and begins with the selection of
R
MOSFET and D1 are selected. The inductor is chosen
based largely on the desired amount of ripple current and
for Burst Mode operation. Finally C
ability to handle the required RMS input current and C
is chosen with low enough ESR to meet the output voltage
ripple and transient specifications.
R
R
The LTC1771 current comparator has a maximum thresh-
old of 140mV/R
sets the peak inductor current, yielding a maximum aver-
age output current I
peak-to-peak ripple current I
when Burst Mode operation is enabled, choose I
to 35% of peak current. Allowing a margin for variations in
the LTC1771 and external components gives the following
equation for choosing R
At higher supply voltages, the peak currents may be
slightly higher due to overshoot from current comparator
delay and can be predicted from the second term in the
following equation:
Inductor Value Selection
Once R
mined. The inductance value has a direct effect on ripple
current. The ripple current decreases with higher induc-
tance and increases with higher V
during continuous mode operation is set by the off-time
and inductance to be:
Kool M is a registered trademark of Magnetics, Inc.
SENSE
SENSE
SENSE
R
I
PEAK
SENSE
I
L CONT
(
. Once R
SENSE
Selection
is chosen based on the required output current.
= 100mV/I
R
)
0 14
SENSE
is known, the inductor value can be deter-
.
SENSE
t
SENSE
OFF
U
MAX
MAX
is known, L can be chosen. Next, the
0 5
. The current comparator threshold
V
.
OUT
SENSE
equal to the peak less half the
U
L
V
IN
L H
V
:
(
–
D
L
V
. For best performance
OUT
OUT
)
W
IN
. The ripple current
is selected for its
1 2
/
U
L
equal
OUT
where t
loads during Burst Mode operation is:
For best efficiency when Burst Mode operation is enabled,
choose:
so that the inductor current is continuous during the burst
periods. This sets a minimum inductor value of:
When burst is disabled, ripple currents less than I
can be achieved by choosing L > L
current reduces output voltage ripple and core losses, but
too low of ripple current will adversely effect efficiency.
Inductor Core Selection
Once the value of L is 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 M
independent of core size for a fixed inductor value, but is
very dependent on inductance selected. As inductance
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 increase in 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 manu-
facturer is Kool M . Toroids are space efficient, especially
when you can use several layers of wire. Because they
generally lack a bobbin, mounting is more difficult. How-
ever, designs for surface mount are available that do not
increase the height significantly.
L
MIN
I
I
L(BURST)
L(CONT)
OFF
= (75 H)(V
= 3.5 s. However, the ripple current at low
35% of I
I
L(BURST)
OUT
+ V
PEAK
D
®
)(R
cores. Actual core loss is
SENSE
0.05/R
)
MIN
SENSE
. Lower ripple
LTC1771
L(BURST)
7