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參數(shù)資料
| 型號： | LTC1772B |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23 |
| 中文描述： | 恒定頻率電流模式降壓DC / DC控制器，采用SOT - 23 |
| 文件頁數(shù)： | 7/12頁 |
| 文件大?。?/td> | 196K |
| 代理商： | LTC1772B |
7
LTC1772B
APPLICATIO
S I
FOR
ATIO
U
where f is the operating frequency. Accepting larger values
of I
RIPPLE
allows the use of low inductances, but results in
higher output voltage ripple and greater core losses. A
reasonable starting point for setting ripple current is
I
RIPPLE
= 0.4(I
OUT(MAX)
). Remember, the maximum I
RIPPLE
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:
W
U
U
I
R
RIPPLE
SENSE
≤
0 03
.
This implies a minimum inductance of:
L
V
V
f
R
V
V
V
V
MIN
IN
OUT
SENSE
OUT
D
IN
D
=
0 03
.
+
+
(Use V
IN(MAX)
= V
IN
)
A smaller value than L
MIN
could be used in the circuit;
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
GS(TH)
and
the “on” resistance R
DS(ON)
, reverse transfer capacitance
C
RSS
and total gate charge.
Since the LTC1772B is designed for operation down to low
input voltages, a logic level threshold MOSFET (R
DS(ON)
guaranteed at V
GS
= 2.5V) is required for applications that
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
GS
rating, typically 8V.
The required minimum R
DS(ON)
of the MOSFET is gov-
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
DS(ON)
is given
by:
R
P
I
p
DS ON
(
P
)
OUT MAX
(
DC
)
)
%
=
=
(
+
(
)
100
2
1
δ
where P
P
is the allowable power dissipation and
δ
p is the
temperature dependency of R
DS(ON)
. (1 +
δ
p) is generally
given for a MOSFET in the form of a normalized R
DS(ON)
vs
temperature curve, but
δ
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
DS(ON)
is governed by:
R
P
DC I
(
p
DS ON
(
P
2
1
δ
OUT
)
+
(
)
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
IN
approaches V
OUT
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
PEAK
at close to 100% duty cycle. Therefore,
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:
I
V
V
V
V
I
D
IN
OUT
IN
D
OUT
=
+
The allowable forward voltage drop in the diode is calcu-
lated from the maximum short-circuit current as:
V
P
I
F
D
SC MAX
(
≈
)
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