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參數(shù)資料
| 型號： | LM2665M6 |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Switched Capacitor Voltage Converter |
| 中文描述： | SWITCHED CAPACITOR CONVERTER, 160 kHz SWITCHING FREQ-MAX, PDSO6 |
| 封裝： | SOT-23, 6 PIN |
| 文件頁數(shù)： | 6/8頁 |
| 文件大小： | 210K |
| 代理商： | LM2665M6 |
Application Information
(Continued)
voltage source equals 2V+. The output resistance R
is a
function of the ON resistance of the internal MOSFET
switches, the oscillator frequency, the capacitance and ESR
of C
and C
2
. Since the switching current charging and dis-
charging C
is approximately twice as the output current, the
effect of the ESR of the pumping capacitor C
will be multi-
plied by four in the output resistance. The output capacitor
C
is charging and discharging at a current approximately
equal to the output current, therefore, its ESR only counts
once in the output resistance. A good approximation of R
out
is:
where R
is the sum of the ON resistance of the internal
MOSFET switches shown in Figure 2.
The peak-to-peak output voltage ripple is determined by the
oscillator frequency, the capacitance and ESR of the output
capacitor C
2
:
High capacitance, low ESR capacitors can reduce both the
output resistance and the voltage ripple.
The Schottky diode D
is only needed for start-up. The inter-
nal oscillator circuit uses the OUT pin and the GND pin. Volt-
age across OUT and GND must be larger than 1.8V to insure
the operation of the oscillator. During start-up, D
is used to
charge up the voltage at the OUT pin to start the oscillator;
also, it protects the device from turning-on its own parasitic
diode and potentially latching-up. Therefore, the Schottky di-
ode D
should have enough current carrying capability to
charge the output capacitor at start-up, as well as a low for-
ward voltage to prevent the internal parasitic diode from
turning-on. A Schottky diode like 1N5817 can be used for
most applications. If the input voltage ramp is less than 10V/
ms, a smaller Schottky diode like MBR0520LT1 can be used
to reduce the circuit size.
Low ESR Capacitor Manufacturers
Split V+ in Half
Another interesting application shown in the Basic Applica-
tion Circuits is using the LM2665 as a precision voltage di-
vider. . This circuit can be derived from the voltage doubler
by switching the input and output connections. In the voltage
divider, the input voltage applies across the OUT pin and the
GND pin (which are the power rails for the internal oscillator),
therefore no start-up diode is needed. Also, since the
off-voltage across each switch equals V
in
/2, the input voltage
can be raised to +11V.
Shutdown Mode
A shutdown (SD) pin is available to disable the device and
reduce the quiescent current to 1 μA. In normal operating
mode, the SD pin is connected to ground. The device can be
brought into the shutdown mode by applying to the SD pin a
voltage greater than 40% of the V+ pin voltage.
Capacitor Selection
As discussed in the Positive Voltage Doubler section, the
output resistance and ripple voltage are dependent on the
capacitance and ESR values of the external capacitors. The
output voltage drop is the load current times the output resis-
tance, and the power efficiency is
Where I
Q
(V+) is the quiescent power loss of the IC device,
and I
L2
R
is the conversion loss associated with the switch
on-resistance, the two external capacitors and their ESRs.
The selection of capacitors is based on the specifications of
the dropout voltage (which equals I
R
), the output volt-
age ripple, and the converter efficiency. Low ESR capacitors
(Table 1) are recommended to maximize efficiency, reduce
the output voltage drop and voltage ripple.
Manufacturer
Nichicon Corp.
AVX Corp.
Sprague
Sanyo
Murata
Taiyo Yuden
Tokin
Phone
Capacitor Type
(708)-843-7500
(803)-448-9411
(207)-324-4140
(619)-661-6835
(800)-831-9172
(800)-348-2496
(408)-432-8020
PL & PF series, through-hole aluminum electrolytic
TPS series, surface-mount tantalum
593D, 594D, 595D series, surface-mount tantalum
OS-CON series, through-hole aluminum electrolytic
Ceramic chip capacitors
Ceramic chip capacitors
Ceramic chip capacitors
Other Applications
Paralleling Devices
Any number of LM2665s can be paralleled to reduce the out-
put resistance. Each device must have its own pumping ca-
pacitor C
, while only one output capacitor C
is needed as
shown in Figure 3. The composite output resistance is:
L
www.national.com
6
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