參數(shù)資料
型號(hào)： LTC1539IGW
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： Dual High Efficiency, Low Noise, Synchronous Step-Down Switching Regulators
中文描述： 2 A SWITCHING CONTROLLER, 400 kHz SWITCHING FREQ-MAX, PDSO36
封裝： 0.300 INCH, PLASTIC, SSOP-36
文件頁(yè)數(shù)： 12/32頁(yè)
文件大?。?/td> 451K
代理商： LTC1539IGW
12
LTC 1538-AUX/LTC 1539
APPLICATIO
S I
FOR
ATIO
U
Allowing some margin for variations in the LTC1538-AUX/
LTC1539 and external component values yield:
mV
I
MAX
The LTC1538-AUX/LTC1539 work well with values of
R
SENSE
from 0.005
to 0.2
.
W
U
U
R
SENSE
=
100
C
OSC
Selection for Operating Frequency
The LTC1538-AUX/LTC1539 use a constant frequency
architecture with the frequency determined by an external
oscillator capacitor on C
OSC
. Each time the topside MOSFET
turns on, the voltage on C
OSC
is reset to ground. During the
on-time, C
OSC
is charged by a fixed current plus an
additional current which is proportional to the output
voltage of the phase detector (V
PLLLPF
)(LTC1539 only).
When the voltage on the capacitor reaches 1.19V, C
OSC
is
reset to ground. The process then repeats.
The value of C
OSC
is calculated from the desired operating
frequency. Assuming the phase-locked loop has no exter-
nal oscillator input (V
PLLLPF
= 0V):
(
A graph for selecting C
OSC
vs frequency is given in Figure
2. As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations). The maximum recommended switching
frequency is 400kHz. When using Figure 2 for
C
pF
Frequency kHz
OSC
(
)
.
±
=
)
)
137 10
11
4
synchronizable applications, choose C
OSC
corresponding
to a frequency approximately 30% below your center
frequency. (See Phase-Locked Loop and Frequency
Sychronization).
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate charge losses. In addition to this basic trade
off, the effect of inductor value on ripple current and low
current operation must also be considered.
The inductor value has a direct effect on ripple current. The
inductor ripple current
I
L
decreases with higher induc-
tance or frequency and generally increases with higher V
IN
or V
OUT
:
V
V
IN
f LV
I
L
OUT
OUT
=
1
1
±
Accepting larger values of
I
L
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
L
= 0.4(I
MAX
). Remember, the
maximum
I
L
occurs at the maximum input voltage.
The inductor value also has an effect on low current
operation. The transition to low current operation begins
OPERATING FREQUENCY (kHz)
C
O
300
250
200
150
100
50
0
100
200
300
400
LTC1538 F02
500
0
V
PLLLPF
= 0V
Figure 2. Timing Capacitor Value
OPERATING FREQUENCY (kHz)
0
0
I
μ
H
10
20
30
40
60
50
100
150
200
1538 F03
250
300
50
V
OUT
= 5.0V
V
OUT
= 3.3V
V
OUT
= 2.5V
Figure 3. Recommended Inductor Values
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