LTC1772 Linear Technology, LTC1772 Datasheet - Page 7

• Manufacturer Part Number: LTC1772
• Description: Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23
• Manufacturer: Linear Technology

APPLICATIONS
or Kool Mu
size for a fixed inductor value, but it 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 losses and are
preferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent 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 manu-
facturer is Kool Mu. Toroids are very space efficient,
especially when you can use several layers of wire.
Because they generally lack a bobbin, mounting is more
difficult. However, new designs for surface mount that do
not increase the height significantly are available.
Power MOSFET Selection
An external P-channel power MOSFET must be selected
for use with the LTC1772. The main selection criteria for
the power MOSFET are the threshold voltage V
the “on” resistance RDS(ON), reverse transfer capaci-
tance C
Since the LTC1772 is designed for operation down to low
input voltages, a sublogic level threshold MOSFET (R
guaranteed at V
work close to this voltage. When these MOSFETs are used,
make sure that the input supply to the LTC1772 is less than
the absolute maximum V
The required minimum R
erned by its allowable power dissipation. For applications
that may operate the LTC1772 in dropout, i.e., 100% duty
cycle, at its worst case the required R
R
DS ON
(
RSS
)
DC
®
and total gate charge.
cores. Actual core loss is independent of core
100
GS
%
= 2.5V) is required for applications that
U
I
OUT MAX
INFORMATION
GS
DS(ON)
U
(
rating, typically 8V.
P
)
P
2
of the MOSFET is gov-
1
W
DS(ON)
p
is given by:
GS(TH)
U
DS(ON)
and
where P
temperature dependency of R
given for a MOSFET in the form of a normalized R
temperature curve, but p = 0.005/ C can be used as an
approximation for low voltage MOSFETs.
In applications where the maximum duty cycle is less than
100% and the LTC1772 is in continuous mode, the R
is governed by:
where DC is the maximum operating duty cycle of the
LTC1772.
Output Diode Selection
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the
diode conducts most of the time. As V
the diode conducts only a small fraction of the time. The
most stressful condition for the diode is when the output
is short-circuited. Under this condition the diode must
safely handle I
it is important to adequately specify the diode peak current
and average power dissipation so as not to exceed the
diode ratings.
Under normal load conditions, the average current con-
ducted by the diode is:
The allowable forward voltage drop in the diode is calcu-
lated from the maximum short-circuit current as:
where P
determined by efficiency and/or thermal requirements.
Kool Mu is a registered trademark of Magnetics, Inc.
R
I
V
D
F
DS ON
(
P
I
D
SC MAX
V
is the allowable power dissipation and p is the
)
is the allowable power dissipation and will be
IN
V
P
(
IN
D
PEAK
DC I
V
)
V
OUT
D
at close to 100% duty cycle. Therefore,
OUT
P
I
OUT
P
2
1
DS(ON)
p
. (1 + p) is generally
IN
approaches V
LTC1772
DS(ON)
DS(ON)
7
OUT
vs