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參數(shù)資料
| 型號： | LTC1435IG#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | High Efficiency Low Noise Synchronous Step-Down Switching Regulator |
| 中文描述： | 2 A SWITCHING CONTROLLER, 400 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 0.209 INCH, PLASTIC, SSOP-16 |
| 文件頁數(shù)： | 19/20頁 |
| 文件大小： | 338K |
| 代理商： | LTC1435IG#PBF |
8
LTC1435
APPLICATIONS INFORMATION
WU
U
The basic LTC1435 application circuit is shown in Figure
1, High Efficiency Step-Down Converter. External compo-
nent selection is driven by the load requirement and
begins with the selection of RSENSE. Once RSENSE is
known, COSC and L can be chosen. Next, the power
MOSFETs and D1 are selected. Finally, CIN and COUT are
selected. The circuit shown in Figure 1 can be configured
for operation up to an input voltage of 28V (limited by the
external MOSFETs).
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
The LTC1435 current comparator has a maximum thresh-
old of 150mV/RSENSE and an input common mode range
of SGND to INTVCC. The current comparator threshold
sets the peak of the inductor current, yielding a maximum
average output current IMAX equal to the peak value less
half the peak-to-peak ripple current
IL.
Allowing a margin for variations in the LTC1435 and
external component values yields:
R
mV
I
SENSE
MAX
=
100
The LTC1435 works well with values of RSENSE from
0.005
to 0.2.
COSC Selection for Operating Frequency
The LTC1435 uses a constant frequency architecture with
the frequency determined by an external oscillator capaci-
tor COSC. Each time the topside MOSFET turns on, the
voltage COSC is reset to ground. During the on-time, COSC
is charged by a fixed current. When the voltage on the
capacitor reaches 1.19V, COSC is reset to ground. The
process then repeats.
The value of COSC is calculated from the desired operating
frequency:
CpF
OSC ()
–
=
1.37(10 )
Frequency (kHz)
4
11
A graph for selecting COSC vs frequency is given in Figure
2. As the operating frequency is increased the gate charge
OPERATING FREQUENCY (kHz)
C
OSC
VALUE
(pF)
300
250
200
150
100
50
0
100
200
300
400
LTC1435 F02
500
0
Figure 2. Timing Capacitor Value
losses will be higher, reducing efficiency (see Efficiency
Considerations). The maximum recommended switching
frequency is 400kHz.
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
IL decreases with higher induc-
tance or frequency and increases with higher VIN or VOUT:
I
fL
V
L
OUT
IN
= ()( )
1
1–
Accepting larger values of
IL 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
IL = 0.4(IMAX). Remember, the
maximum
IL occurs at the maximum input voltage.
The inductor value also has an effect on low current
operation. The transition to low current operation begins
when the inductor current reaches zero while the bottom
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