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参数资料
| 型号： | LM3102TLX-1 |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PBGA28 |
| 封装： | USMD-28 |
| 文件页数： | 6/20页 |
| 文件大小： | 623K |
| 代理商： | LM3102TLX-1 |
Applications Information
EXTERNAL COMPONENTS
The following guidelines can be used to select external com-
ponents.
R
FB1 and RFB2 : These resistors should be chosen from stan-
dard values in the range of 1.0 k
to 10 k, satisfying the
following ratio:
R
FB1/RFB2 = (VOUT/0.8V) - 1
(7)
For V
OUT = 0.8V, the FB pin can be connected to the output
directly with a pre-load resistor drawing more than 20 A. It is
because the converter operation needs a minimum inductor
current ripple to maintain good regulation when no load is
connected.
R
ON: Equation (2) can be used to select R
ON if a desired op-
erating frequency is selected. But the minimum value of
R
ON is determined by the minimum on-time. It can be calcu-
lated as follows:
(8)
If R
ON calculated from (2) is smaller than the minimum value
determined in (8), a lower frequency should be selected to re-
calculate R
ON by (2). Alternatively, VIN(MAX) can also be limited
in order to keep the frequency unchanged. The relationship
of V
On the other hand, the minimum off-time of 260 ns can limit
the maximum duty ratio. Larger R
ON should be selected in any
application requiring large duty ratio.
30021329
FIGURE 5. Maximum V
IN for selected RON
L:
The main parameter affected by the inductor is the ampli-
tude of inductor current ripple (I
LR). Once ILR is selected, L can
be determined by:
(9)
where V
IN is the maximum input voltage and fSW is determined
from (2).
If the output current I
OUT is determined, by assuming that
I
OUT = IL, the higher and lower peak of ILR can be determined.
Beware that the higher peak of I
LR should not be larger than
the saturation current of the inductor and current limits of the
main and synchronous MOSFETs. Also, the lower peak of
I
LR must be positive if CCM operation is required.
30021331
FIGURE 6. Inductor selection for V
OUT = 3.3V
30021332
FIGURE 7. Inductor selection for V
OUT = 0.8V
various V
OUT and RON. For small RON, according to (8), VIN is
limited. Some curves are therefore limited as shown in the
figures.
C
VCC: The capacitor on the V
CC output provides not only noise
filtering and stability, but also prevents false triggering of the
V
CC UVLO at the main MOSFET on/off transitions. CVCC
should be no smaller than 680 nF for stability, and should be
a good quality, low ESR, ceramic capacitor.
C
OUT and COUT3: C
OUT should generally be no smaller than
10 F. Experimentation is usually necessary to determine the
minimum value for C
OUT, as the nature of the load may require
a larger value. A load which creates significant transients re-
quires a larger C
OUT than a fixed load.
C
OUT3 is a small value ceramic capacitor located close to the
LM3102 to further suppress high frequency noise at V
OUT. A
100 nF capacitor is recommended.
www.national.com
14
LM3102
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