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參數(shù)資料
| 型號(hào)： | LM2767M5 |
| 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： | 0.03 A SWITCHED CAPACITOR CONVERTER, 25 kHz SWITCHING FREQ-MAX, PDSO5 |
| 封裝： | SOT-23, 5 PIN |
| 文件頁(yè)數(shù)： | 6/9頁(yè) |
| 文件大?。?/td> | 194K |
| 代理商： | LM2767M5 |
Circuit Description
The LM2767 contains four large CMOS switches which are
switched in a sequence to double the input supply voltage.
Energy transfer and storage are provided by external capaci-
tors. Figure 2 illustrates the voltage conversion scheme.
When S
2
and S
4
are closed, C
1
charges to the supply volt-
age V+. During this time interval, switches S
and S
are
open. In the next time interval, S
and S
are open; at the
same time, S
and S
are closed, the sum of the input volt-
age V+ and the voltage across C
gives the 2V+ output volt-
age when there is no load. The output voltage drop when a
load is added is determined by the parasitic resistance (R
d-
s(on)
of the MOSFET switches and the ESR of the capacitors)
and the charge transfer loss between capacitors. Details will
be discussed in the following application information section.
Application Information
Positive Voltage Doubler
The main application of the LM2767 is to double the input
voltage. The range of the input supply voltage is 1.8V to
5.5V.
The output characteristics of this circuit can be approximated
by an ideal voltage source in series with a resistance. The
voltage source equals 2V+. The output resistance R
is a
function of the ON resistance of the internal MOSFET
switches, the oscillator frequency, and the capacitance and
ESR of C
and C
. Since the switching current charging and
discharging C
is approximately twice 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
2
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 resistances of the internal
MOSFET switches shown in Figure 2. R
SW
is typically 4.5
for the LM2767.
The peak-to-peak output voltage ripple is determined by the
oscillator frequency as well as 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 to protect the device
from turning-on its own parasitic diode and potentially
latching-up. During start-up, D
will also quickly charge up
the output capacitor to V
minus the diode drop thereby de-
creasing the start-up time. Therefore, the Schottky diode D
1
should have enough current carrying capability to charge the
output capacitor at start-up, as well as a low forward voltage
to prevent the internal parasitic diode from turning-on. A
Schottky diode like 1N5817 can be used for most applica-
tions. If the input voltage ramp is less than 10V/ms, a smaller
Schottky diode like MBR0520LT1 can be used to reduce the
circuit size.
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 allowable voltage
droop (which equals I
R
), and the desired output volt-
age ripple. Low ESR capacitors (Table 1) are recommended
to maximize efficiency, reduce the output voltage drop and
voltage ripple.
TABLE 1. Low ESR Capacitor Manufacturers
Manufacturer
Nichicon Corp.
Phone
Website
Capacitor Type
(847)-843-7500
www.nichicon.com
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
AVX Corp.
Sprague
Sanyo
(843)-448-9411
(207)-324-4140
(619)-661-6835
www.avxcorp.com
www.vishay.com
www.sanyovideo.com
Murata
Taiyo Yuden
Tokin
(800)-831-9172
(800)-348-2496
(408)-432-8020
www.murata.com
www.t-yuden.com
www.tokin.com
DS101274-14
FIGURE 2. Voltage Doubling Principle
L
www.national.com
6
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