ltc1772b Linear Technology Corporation, ltc1772b Datasheet - Page 7

• Manufacturer Part Number: ltc1772b
• Description: Constant Frequency Current Mode Step-down Dc/dc Controller In Sot-23
• Manufacturer: Linear Technology Corporation

APPLICATIONS
where f is the operating frequency. Accepting larger values
of I
higher output voltage ripple and greater core losses. A
reasonable starting point for setting ripple current is
I
occurs at the maximum input voltage.
The ripple current is normally set such that the inductor
current is continuous down to approximately 1/4 of maxi-
mum load current. This results in:
This implies a minimum inductance of:
A smaller value than L
however, the inductor current transitioning from continu-
ous to discontinuous will occur at a higher load current.
Power MOSFET Selection
An external P-channel power MOSFET must be selected
for use with the LTC1772B. The main selection criteria for
the power MOSFET are the threshold voltage V
the “on” resistance R
C
Since the LTC1772B is designed for operation down to low
input voltages, a logic 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 LTC1772B is less
than the absolute maximum V
The required minimum R
erned by its allowable power dissipation. For applications
that may operate the LTC1772B in dropout, i.e., 100%
duty cycle, at its worst case the required R
by:
RIPPLE
RSS
(Use V
I
L
RIPPLE
RIPPLE
MIN
and total gate charge.
= 0.4(I
IN(MAX)
allows the use of low inductances, but results in
f
V
IN
R
R
OUT(MAX)
0 03
SENSE
0 03
SENSE
GS
.
.
V
= V
OUT
= 2.5V) is required for applications that
U
IN
DS(ON)
). Remember, the maximum I
)
MIN
V
INFORMATION
V
OUT
DS(ON)
U
IN
, reverse transfer capacitance
could be used in the circuit;
GS
V
V
D
of the MOSFET is gov-
rating, typically 8V.
D
W
DS(ON)
GS(TH)
U
is given
DS(ON)
RIPPLE
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 LTC1772B is in continuous mode, the
R
where DC is the maximum operating duty cycle of the
LTC1772B.
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:
DS(ON)
R
R
I
V
D
F
DS ON
DS ON
(
(
P
I
is governed by:
SC MAX
V
is the allowable power dissipation and p is the
)
)
IN
V
DC
P
(
IN
D
100
PEAK
DC I
V
%
)
V
OUT
D
at close to 100% duty cycle. Therefore,
OUT
I
P
I
OUT MAX
OUT
P
2
1
(
DS(ON)
P
p
)
P
2
1
. (1 + p) is generally
IN
LTC1772B
p
approaches V
DS(ON)
7
OUT
vs