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参数资料
| 型号： | LM3351MWC |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHED CAPACITOR CONVERTER, 275 kHz SWITCHING FREQ-MAX, UUC |
| 封装： | WAFER |
| 文件页数： | 7/11页 |
| 文件大小： | 316K |
| 代理商： | LM3351MWC |
Detailed Operation (Continued)
STEP-UP APPLICATIONS NEED AN EXTRA DIODE
The LM3351 is biased from pin V
high. Thus for step-up appli-
cations, an external Schottky diode (D1) is needed to supply
power to V
high during startup (See Figure 1). Note that during
shutdown, this diode will provide a DC path from V
in to Vout.
The load may therefore continue to draw current from the in-
put voltage source. This Schottky diode is not required for
step-down applications (See
Figure 2).
SHUTDOWN MODE
When ENABLE is a logic low (ground), the LM3351 enters a
low power shutdown mode. In this mode, all circuitry is dis-
abled and therefore, all switching action stops. During shut-
down, the current consumption drops to 250 nA (typical).
When ENABLE is a logic high, (i.e. 3.3V for step-up mode
and 5.0V for step-down mode), the LM3351 returns to nor-
mal operation.
Application Information
CAPACITOR SELECTION
The LM3351 requires four capacitors: an input bypass ca-
pacitor (connected between V
in and ground), an output hold
capacitor (connected between V
out and ground), and two
sampling capacitors (C1 and C2 in
Figures 1, 2). 1.0 F
(±20%) ceramic chip type capacitors are recommended for
all four capacitors. The usable operating frequency should
be greater than 5 MHz for all capacitors.
The output hold capacitor value determines the output ripple.
Increasing the value of the hold capacitor decreases the
ripple. The value of this capacitor (C
out) can be calculated
(approximately) based on the output ripple (
V
out) require-
ments from:
where I
load is the load current and fO is the oscillator fre-
quency.
In order to ensure superior performance over the entire oper-
ating temperature range, capacitors made of X7R dielectric
material are suggested. However, capacitors made of other
dielectric materials that still meet the ±20% specification
over the entire temperature range can also be used.
PRECAUTIONS
The LM3351 is not short circuit protected.
LM3351
www.national.com
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