參數(shù)資料
型號(hào)： LTC1439IGW
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 DUAL 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)
文件大小： 596K
代理商： LTC1439IGW
12
LTC1438/LTC1439
APPLICATIO
S I
FOR
ATIO
U
W
U
U
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
synchronizable applications, choose C
OSC
corresponding
to a frequency approximately 30% below your center
frequency. (See Phase-Locked Loop and Frequency
Sychronization).
OPERATING FREQUENCY (kHz)
C
O
300
250
200
150
100
50
0
100
200
300
400
LTC1435 F02
500
0
V
PLLLPF
= 0V
Figure 2. Timing Capacitor Value
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 increases with higher V
IN
or V
OUT
:
I
f LV
V
V
L
OUT
OUT
IN
=
1
1
–
The basic LTC1439 application circuit is shown in Fig-
ure 1. External component selection is driven by the load
requirement and begins with the selection of R
SENSE
. Once
R
SENSE
is known, C
OSC
and L can be chosen. Next, the
power MOSFETs and D1 are selected. Finally, C
IN
and C
OUT
are selected. The circuit shown in Figure 1 can be config-
ured for operation up to an input voltage of 28V (limited by
the external MOSFETs).
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
The LTC1438/LTC1439 current comparator has a maxi-
mum threshold of 150mV/R
SENSE
and an input common
mode range of SGND to INTV
CC
. The current comparator
threshold sets the peak of the inductor current, yielding a
maximum average output current I
MAX
equal to the peak
value less half the peak-to-peak ripple current,
I
L
.
Allowing some margin for variations in the LTC1438/
LTC1439 and external component values yield:
R
mV
I
SENSE
MAX
=
100
The LTC1438/LTC1439 work well with values of R
SENSE
from 0.005
to 0.2
.
C
OSC
Selection for Operating Frequency
The LTC1438/LTC1439 use a constant frequency archi-
tecture 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
)(LTC1439 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):
C
pF
OSC
(
)
.
(
)
=
1 37 10
Frequency (kHz)
11
4
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