LTC1771
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
SENSE
. Once R
SENSE
is known, L can be chosen. Next, the
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
IN
is selected for its
ability to handle the required RMS input current and C
OUT
is chosen with low enough ESR to meet the output voltage
ripple and transient specifications.
R
SENSE
Selection
R
SENSE
is chosen based on the required output current.
The LTC1771 current comparator has a maximum thresh-
old of 140mV/R
SENSE
. The current comparator threshold
sets the peak inductor current, yielding a maximum aver-
age output current I
MAX
equal to the peak less half the
peak-to-peak ripple current
∆I
L
. For best performance
when Burst Mode operation is enabled, choose
∆I
L
equal
to 35% of peak current. Allowing a margin for variations in
the LTC1771 and external components gives the following
equation for choosing R
SENSE
:
R
SENSE
= 100mV/I
MAX
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:
I
PEAK
≅
V –V
0.14
+
0.5
IN OUT
R
SENSE
L(
µ
H)
1/ 2
Inductor Value Selection
Once R
SENSE
is known, the inductor value can be deter-
mined. The inductance value has a direct effect on ripple
current. The ripple current decreases with higher induc-
tance and increases with higher V
OUT
. The ripple current
during continuous mode operation is set by the off-time
and inductance to be:
V
+
V
∆
I
L(CONT)
=
t
OFF
OUT D
L
Kool Mµ is a registered trademark of Magnetics, Inc.
6
U
where t
OFF
= 3.5µs. However, the ripple current at low
loads during Burst Mode operation is:
∆I
L(BURST)
≈
35% of I
PEAK
≈
0.05/R
SENSE
For best efficiency when Burst Mode operation is enabled,
choose:
∆I
L(CONT)
≤ ∆I
L(BURST)
so that the inductor current is continuous during the burst
periods. This sets a minimum inductor value of:
L
MIN
= (70µH)(V
OUT
+ V
D
)(R
SENSE
)
When burst is disabled, ripple currents less than
∆I
L(BURST)
can be achieved by choosing L > L
MIN
. Lower ripple
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 Mµ
®
cores. Actual core loss is
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 Mµ. 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.
W
U U
LINER [ LINEAR TECHNOLOGY ]
