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參數(shù)資料
| 型號： | LTC1779CS6 |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.5 A SWITCHING REGULATOR, 550 kHz SWITCHING FREQ-MAX, PDSO6 |
| 封裝： | PLASTIC, SOT-23, 6 PIN |
| 文件頁數(shù)： | 9/12頁 |
| 文件大?。?/td> | 170K |
| 代理商： | LTC1779CS6 |
6
LTC1779
The basic LTC1779 application circuit is shown in Figure
1. External component selection is driven by the load
requirement and begins with the selection of L1 and
RSENSE (= R1). Next, the output diode D1 is selected
followed by CIN (= C1)and COUT(= C2).
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
With the current comparator monitoring the voltage devel-
oped across RSENSE, the threshold of the comparator
determines the inductor’s peak current. The output cur-
rent the LTC1779 can provide is given by:
IM
R
I
OUT
SENSE
RIPPLE
=
+
012
22
.
where IRIPPLE is the inductor peak-to-peak ripple current
(see Inductor Value Calculation section) and M is as
defined in the previous section and in Figure 3.
A reasonable starting point for setting ripple current is
IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it
becomes:
R
M
I
SENSE
OUT
=
10
– 2
for Duty Cycle < 40%
where IOUT is in amps.
However, for operation that is above 40% duty cycle, slope
compensation effect has to be taken into consideration to
select the appropriate value to provide the required amount
of current. Using Figure 2, the value of RSENSE is:
R
M
I
SF
SENSE
OUT
=
10
100
2
–
where SF is the slope factor from Figure 2 in percent.
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. The inductor’s peak-to-peak ripple current is given
by:
I
VV
fL
VV
RIPPLE
IN
OUT
D
IN
D
=
()
+
(Refer to Functional Diagram)
OPERATIO
U
The variable M is the ratio of the total switch current to that
portion of the switch current that flows through RSENSE. M
is a function of both RSENSE and ROUT of the internal power
switch, which in turn, is a strong function of supply
voltage. For values of M refer to Figure 3. In order to
guarantee the desired IPK over the full range of supply
voltage, the minimum value of M, corresponding to the
minimum supply voltage seen in the application, should
be chosen. Note that the selection of RSENSE, and hence
the resulting M, is an iterative process. For most applica-
tions, a value of RSENSE between 0 and 20 will be
chosen.
Figure 3. M vs Supply Voltage
APPLICATIO S I FOR ATIO
WU
UU
SUPPLY VOLTAGE (V)
0
M
(mA/mA)
60
55
50
45
40
35
30
25
20
8
1779 F03
246
10
7
135
9
RSENSE = 0
RSENSE = 2
RSENSE = 18.2
IOUT = 100mA
RSENSE = 14
RSENSE = 10
RSENSE = 6.2
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