LM2681M6X [NSC]
Switched Capacitor Voltage Converter; 开关电容电压转换器
| 型号： | LM2681M6X |
| 厂家： | 
National Semiconductor |
| 描述： | Switched Capacitor Voltage Converter |
| 文件： | 总9页 (文件大小：199K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
March 1999
LM2681
Switched Capacitor Voltage Converter
General Description
The LM2681 CMOS charge-pump voltage converter oper-
ates as a voltage doubler for an input voltage in the range of
Features
n Doubles or Splits Input Supply Voltage
n SOT23-6 Package
+2.5V to +5.5V. Two low cost capacitors and
a diode
n 15Ω Typical Output Impedance
n 90% Typical Conversion Efficiency at 20 mA
(needed during start-up) is used in this circuit to provide up
to 20 mA of output current. The LM2681 can also work as a
voltage divider to split a voltage in the range of +1.8V to
+11V in half.
Applications
n Cellular Phones
n Pagers
The LM2681 operates at 160 kHz oscillator frequency to re-
duce output resistance and voltage ripple. With an operating
current of only 550 µA (operating efficiency greater than 90%
with most loads) the LM2681 provides ideal performance for
battery powered systems. The device is in SOT-23-6 pack-
age.
n PDAs
n Operational Amplifier Power Suppliers
n Interface Power Suppliers
n Handheld Instruments
Basic Application Circuits
Voltage Doubler
DS100965-1
Splitting Vin in Half
DS100965-2
© 1999 National Semiconductor Corporation
DS100965
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
T
JMax(Note 3)
150˚C
210˚C/W
θJA (Note 3)
Operating Junction Temperature
Range
−40˚ to 85˚C
Storage Temperature Range
Lead Temp. (Soldering, 10 seconds)
ESD Rating
−65˚C to +150˚C
300˚C
Supply Voltage (V+ to GND, or GND to OUT)
V+ and OUT Continuous Output Current
Output Short-Circuit Duration to GND (Note 2)
Continuous Power
5.8V
30 mA
2kV
1 sec.
600 mW
=
Dissipation (TA 25˚C)(Note 3)
Electrical Characteristics
=
Limits in standard typeface are for TJ 25˚C, and limits in boldface type apply over the full operating temperature range. Un-
=
=
=
less otherwise specified: V+ 5V, C1 C2 3.3 µF. (Note 4)
Symbol
V+
Parameter
Supply Voltage
Condition
Min
2.5
Typ
Max
5.5
Units
V
IQ
Supply Current
Output Current
No Load
550
1000
µA
IL
20
mA
RSW
Sum of the Rds(on)of the four
internal MOSFET switches
=
IL 20 mA
8
16
40
Ω
=
ROUT
fOSC
fSW
Output Resistance (Note 5)
Oscillator Frequency
Switching Frequency
Power Efficiency
IL 20 mA
15
160
80
Ω
(Note 6)
(Note 6)
80
40
kHz
kHz
PEFF
RL (1.0k) between GND and
OUT
86
93
%
=
IL 20 mA to GND
90
VOEFF
Voltage Conversion Efficiency
No Load
99
99.96
%
Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device
beyond its rated operating conditions.
Note 2: OUT may be shorted to GND for one second without damage. However, shorting OUT to V+ may damage the device and should be avoided. Also, for tem-
peratures above 85˚C, OUT must not be shorted to GND or V+, or device may be damaged.
=
Note 3: The maximum allowable power dissipation is calculated by using P
DMax
(T
JMax
− T )/θ , where T
JA
is the maximum junction temperature, T is the
JMax A
A
ambient temperature, and θ is the junction-to-ambient thermal resistance of the specified package.
JA
Note 4: In the test circuit, capacitors C and C are 3.3 µF, 0.3Ω maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce output
1
2
voltage and efficiency.
Note 5: Specified output resistance includes internal switch resistance and capacitor ESR. See the details in the application information for positive voltage doubler.
=
Note 6: The output switches operate at one half of the oscillator frequency, f
2f
.
SW
OSC
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2
Test Circuit
DS100965-3
FIGURE 1. LM2681 Test Circuit
=
Typical Performance Characteristics (Circuit of Figure 1, V+ 5V unless otherwise specified)
Supply Current vs
Supply Voltage
Supply Current vs
Temperature
DS100965-4
DS100965-5
Output Source
Resistance vs Supply
Voltage
Output Source
Resistance vs
Temperature
DS100965-6
DS100965-7
3
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=
Typical Performance Characteristics (Circuit of Figure 1, V+ 5V unless otherwise
specified) (Continued)
Output Voltage Drop
Efficiency vs
vs Load Current
Load Current
DS100965-9
DS100965-8
Oscillator Frequency vs
Supply Voltage
Oscillator Frequency vs
Temperature
DS100965-10
DS100965-11
Connection Diagram
6-Lead SOT (M6)
DS100965-22
Actual Size
DS100965-13
Top View With Package Marking
Ordering Information
Order Number
Package
Number
MA06A
MA06A
Package
Supplied as
Marking
LM2681M6
S10A (Note 7)
S10A (Note 7)
Tape and Reel (250 units/rail)
Tape and Reel (3000 units/rail)
LM2681M6X
Note 7: The first letter ″S″ identifies the part as a switched capacitor converter. The next two numbers are the device number. The fourth letter ″A″ indicates the
grade. Only one grade is available. Larger quantity reels are available upon request.
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4
Pin Description
Pin
Name
Function
Voltage Doubler
Power supply positive voltage input
Power supply ground input
Voltage Split
Positive voltage output
Same as doubler
1
2
3
V+
GND
CAP−
Connect this pin to the negative terminal of the
charge-pump capacitor
Same as doubler
4
5
GND
OUT
Power supply ground input
Positive voltage output
Same as doubler
Power supply positive voltage
input
6
CAP+
Connect this pin to the positive terminal of the
charge-pump capacitor
Same as doubler
equal to the output current, therefore, its ESR only counts
once in the output resistance. A good approximation of Rout
is:
Circuit Description
The LM2681 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 S2 and S4 are closed, C1 charges to the supply volt-
age V+. During this time interval, switches S1 and S3 are
open. In the next time interval, S2 and S4 are open; at the
same time, S1 and S3 are closed, the sum of the input volt-
age V+ and the voltage across C1 gives the 2V+ output volt-
age when there is no load. The output voltage drop when a
where RSW 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 C2:
load is added is determined by the parasitic resistance (Rd
-
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.
High capacitance, low ESR capacitors can reduce both the
output reslistance and the voltage ripple.
The Schottky diode D1 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, D1 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 D1 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.
DS100965-14
FIGURE 2. Voltage Doubling Principle
Application Information
Split V+ in Half
Another interesting application shown in the Basic Applica-
tion Circuits is using the LM2681 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 Vin/2, the input voltage
can be raised to +11V.
Positive Voltage Doubler
The main application of the LM2681 is to double the input
voltage. The range of the input supply voltage is 2.5V 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 Rout is a
function of the ON resistance of the internal MOSFET
switches, the oscillator frequency, the capacitance and ESR
of C1 and C2. Since the switching current charging and dis-
charging C1 is approximately twice as the output current, the
effect of the ESR of the pumping capacitor C1 will be multi-
plied by four in the output resistance. The output capacitor
C2 is charging and discharging at a current approximately
5
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Where IQ(V+) is the quiescent power loss of the IC device,
and IL Rout is the conversion loss associated with the switch
Application Information (Continued)
2
Capacitor Selection
on-resistance, the two external capacitors and their ESRs.
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
The selection of capacitors is based on the specifications of
the dropout voltage (which equals Iout Rout), 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.
Low ESR Capacitor Manufacturers
Manufacturer
Phone
Capacitor Type
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
Nichicon Corp.
(708)-843-7500
(803)-448-9411
(207)-324-4140
(619)-661-6835
(800)-831-9172
(800)-348-2496
(408)-432-8020
AVX Corp.
Sprague
Sanyo
Murata
Taiyo Yuden
Tokin
Ceramic chip capacitors
Ceramic chip capacitors
Other Applications
Paralleling Devices
Any number of LM2681s can be paralleled to reduce the out-
put resistance. Each device must have its own pumping ca-
pacitor C1, while only one output capacitor Cout is needed as
shown in Figure 3. The composite output resistance is:
DS100965-19
FIGURE 3. Lowering Output Resistance by Paralleling Devices
Cascading Devices
Cascading the LM2681s is an easy way to produce a greater
voltage (A two-stage cascade circuit is shown in Figure 4).
Note that, the increasing of the number of cascading stages
is pracitically limited since it significantly reduces the effi-
ciency, increases the output resistnace and output voltage
ripple.
The effective output resistance is equal to the weighted sum
of each individual device:
=
Rout 1.5Rout_1 + Rout_2
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6
Other Applications (Continued)
DS100965-20
FIGURE 4. Increasing Output Voltage by Cascading Devices
Regulating VOUT
It is possible to regulate the output of the LM2681 by use of
a low dropout regulator (such as LP2980-5.0). The whole
converter is depicted in Figure 5.
Note that, the following conditions must be satisfied simulta-
neously for worst case design:
A different output voltage is possible by use of LP2980-3.3,
LP2980-3.0, or LP2980-adj.
>
2Vin_min Vout_min +Vdrop_max (LP2980) + Iout_max x Rout_max (LM2681)
<
2Vin_max Vout_max +Vdrop_min (LP2980) + Iout_min x Rout_min (LM2681)
DS100965-21
FIGURE 5. Generate a Regulated +5V from +3V Input Voltage
7
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Physical Dimensions inches (millimeters) unless otherwise noted
6-Lead Small Outline Package (M6)
NS Package Number MA06A
For Order Numbers, refer to the table in the ″Ordering Information″ section of this document.
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