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參數(shù)資料
| 型號(hào)： | LTC3772ETS8#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 650 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 1 MM HEIGHT, PLASTIC, MO-193, SOT-23, 8 PIN |
| 文件頁數(shù)： | 3/20頁 |
| 文件大小： | 248K |
| 代理商： | LTC3772ETS8#PBF |
11
LTC3772
3772f
APPLICATIO S I FOR ATIO
WU
UU
Inductor Core Selection
Once the inductance value is determined, the type of induc-
tor must be selected. Actual core loss is independent of core
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 loss 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 in-
ductor ripple current and consequent output voltage ripple.
Do not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price vs size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko and Sumida.
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 VIN approaches VOUT 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
IPEAK at close to 100% duty cycle. Therefore, it is impor-
tant to adequately specify the diode peak current and av-
erage power dissipation so as not to exceed the diode
ratings.
duty cycle–at its worst case the required RDS(ON) is given
by:
R
P
I
DS ON DC
P
OUT MAX
P
()(
%)
()
() (
)
=
+
100
2 1 δ
where PP is the allowable power dissipation and δP is the
temperature dependency of RDS(ON). (1 + δP) is generally
given for a MOSFET in the form of a normalized RDS(ON) vs
temperature curve, but δP = 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 LTC3772 is in continuous mode, the RDS(ON)
is governed by:
R
P
DC I
DS ON
P
OUT
P
()
(
)
+
2 1 δ
where DC is the maximum operating duty cycle of the
LTC3772.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies permit the use
of a smaller inductor for the same amount of inductor ripple
current. However, this is at the expense of efficiency due
to an increase in MOSFET gate charge losses.
The inductance value also has a direct effect on ripple
current. The ripple current, IRIPPLE, decreases with higher
inductance or frequency and increases with higher VIN or
VOUT.Theinductor’speak-to-peakripplecurrentisgivenby:
I
VV
fL
VV
RIPPLE
IN
OUT
D
IN
D
=
+
()
where f is the operating frequency. Accepting larger values
of IRIPPLE 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 IRIPPLE
= 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE occurs
at the maximum input voltage.
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