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參數資料
| 型號： | LM3351MMX/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHED CAPACITOR CONVERTER, 275 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | MINI, SO-8 |
| 文件頁數： | 8/10頁 |
| 文件大?。?/td> | 298K |
| 代理商： | LM3351MMX/NOPB |
Detailed Operation
OPERATING PRINCIPLE
The LM3351 is a charge-pump voltage converter that pro-
vides a voltage conversion ratio of 3/2 in step-up mode and
a conversion ratio of 2/3 in the step-down mode. Thus it can
be used in the step-down mode to provide a 3.3V output
from a regulated 5V input or in the step-up mode to provide
a 5V output from a regulated 3.3V input. Other values of
input voltages can be used as long as they are within the
limits.
The LM3351 contains an array of CMOS switches which are
operated in a certain sequence to provide the step-up or
step-down of the input supply. An internal RC oscillator
provides the timing signals.
Energy transfer and storage are provided by four inexpen-
sive ceramic capacitors. The selection of these capacitors is
explained in the Capacitor Selection section under Applica-
tion Information.
STEP-UP APPLICATIONS NEED AN EXTRA DIODE
The LM3351 is biased from pin V
high. Thus for step-up
applications, 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 input 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
disabled and therefore, all switching action stops. During
shutdown, 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
(±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
operating temperature range, capacitors made of X7R di-
electric material are suggested. However, capacitors made
of other dielectric materials that still meet the ±20% specifi-
cation over the entire temperature range can also be used.
PRECAUTIONS
The LM3351 is not short circuit protected.
Typical Application Circuits
10014612
FIGURE 1. Step-Up Converter
LM3351
www.national.com
7
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