Datasheet LM2681M6X, LM2681M6 Datasheet (NSC)

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LM2681 Switched Capacitor Voltage Converter
LM2681 Switched Capacitor Voltage Converter
March 1999
General Description
The LM2681 CMOS charge-pump voltage converter oper­ates asa voltage doubler for an input voltage in the range of +2.5V to +5.5V. Two low cost capacitors and a diode (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.
The LM2681 operates at 160 kHz oscillator frequency to re­duce output resistance and voltage ripple. With an operating current of only550 µA (operating efficiency greaterthan 90 with most loads) the LM2681 provides ideal performance for battery powered systems. The device is in SOT-23-6 pack­age.
Basic Application Circuits
Voltage Doubler
Splitting Vinin Half
Features
n Doubles or Splits Input Supply Voltage n SOT23-6 Package n 15Typical Output Impedance n 90%Typical Conversion Efficiency at 20 mA
Applications
n Cellular Phones n Pagers n PDAs
%
n Operational Amplifier Power Suppliers n Interface Power Suppliers n Handheld Instruments
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© 1999 National Semiconductor Corporation DS100965 www.national.com
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required, please contact theNational Semiconductor Sales Office/ Distributors for availability and specifications.
Supply Voltage (V+ to GND, or GND to OUT) 5.8V V+ and OUT Continuous Output Current 30 mA Output Short-Circuit Duration to GND (Note 2) 1 sec. Continuous Power
Dissipation (T
=
A
25˚C)(Note 3)
600 mW
T
(Note 3) 150˚C
JMax
θ
(Note 3) 210˚C/W
JA
Operating Junction Temperature
−40˚ to 85˚C
Range Storage Temperature Range −65˚C to +150˚C Lead Temp. (Soldering, 10 seconds) 300˚C ESD Rating 2kV
Electrical Characteristics
Limits in standard typeface are for T less otherwise specified: V+=5V, C
Symbol Parameter Condition Min Typ Max Units
V+ Supply Voltage 2.5 5.5 V I
Q
I
L
R
SW
R
OUT
f
OSC
f
SW
P
EFF
V
OEFF
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: OUTmay be shorted to GND for onesecond without damage. However, shorting OUT to V+ may damage the device and should be avoided.Also, for tem­peratures 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 ambient temperature, and θ
Note 4: Inthetestcircuit, capacitors C voltage and efficiency.
Note 5: Specifiedoutputresistance includes internal switch resistance and capacitor ESR. See thedetails in the application information for positive voltage doubler. Note 6: The output switches operate at one half of the oscillator frequency, f
Supply Current No Load 550 1000 µA Output Current 20 mA Sum of the R
internal MOSFET switches
ds(on)
Output Resistance (Note 5) I Oscillator Frequency (Note 6) 80 160 kHz Switching Frequency (Note 6) 40 80 kHz Power Efficiency RL(1.0k) between GND and
Voltage Conversion Efficiency No Load 99 99.96
is the junction-to-ambient thermal resistance of the specified package.
JA
=
25˚C, and limits in boldface type apply over the full operating temperature range. Un-
J
=
=
C
3.3 µF. (Note 4)
1
2
of the four
and C2are 3.3 µF, 0.3maximum ESR capacitors. CapacitorswithhigherESRwillincreaseoutput resistance, reduce output
1
=
I
20 mA 8 16
L
=
20 mA 15 40
L
OUT
=
I
20 mA to GND 90
L
DMax
OSC
=
=
(T
JMax−TA
2f
SW
)/θJA, where T
.
86 93
is the maximum junction temperature, TAis the
JMax
%
%
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Test Circuit
FIGURE 1. LM2681 Test Circuit
Typical Performance Characteristics
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(Circuit of Figure 1, V+=5V unless otherwise specified)
Supply Current vs Supply Voltage
Output Source Resistance vs Supply Voltage
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Supply Current vs Temperature
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Output Source Resistance vs Temperature
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Typical Performance Characteristics (Circuit of Figure 1, V+
specified) (Continued)
=
5V unless otherwise
Output Voltage Drop vs Load Current
Oscillator Frequency vs Supply Voltage
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Efficiency vs Load Current
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Oscillator Frequency vs Temperature
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Connection Diagram
6-Lead SOT (M6)
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Actual Size
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Top View With Package Marking
Ordering Information
Order Number Package
Number
LM2681M6 MA06A S10A (Note 7) Tape and Reel (250 units/rail)
LM2681M6X MA06A S10A (Note 7) Tape and Reel (3000 units/rail)
Note 7: The first letter Sidentifies the part as a switched capacitor converter. The next two numbers are the device number. The fourth letter Aindicates the grade. Only one grade is available. Larger quantity reels are available upon request.
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Package
Marking
Supplied as
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Pin Description
Pin Name Function
Voltage Doubler Voltage Split
1 V+ Power supply positive voltage input Positive voltage output 2 GND Power supply ground input Same as doubler 3 CAP− Connect this pin to the negative terminal of the
charge-pump capacitor 4 GND Power supply ground input Same as doubler 5 OUT Positive voltage output Power supply positive voltage
6 CAP+ Connect this pin to the positive terminal of the
charge-pump capacitor
Same as doubler
input Same as doubler
Circuit Description
The LM2681 contains four large CMOS switches which are switched in a sequence to double the input supply voltage. Energy transfer and storageare provided byexternal capaci­tors. Figure 2 illustrates the voltage conversion scheme. When S age V+. During this time interval, switches S open. In the next time interval, S same time, S age V+ and the voltage across C age when there is no load. The output voltage drop when a
and S4are closed, C1charges to the supply volt-
2
and S4are open; at the
and S3are closed, the sum of the input volt-
1
2
gives the 2V+ output volt-
1
and S3are
1
load is added is determined by the parasitic resistance (R
of the MOSFET switchesand the ESR ofthe capacitors)
s(on)
and the charge transfer loss between capacitors. Details will be discussed in the following application information section.
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FIGURE 2. Voltage Doubling Principle
Application Information
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 ofthis circuit canbe approximated
by an ideal voltage source in series with a resistance. The voltage source equals 2V+. The output resistance R function of the ON resistance of the internal MOSFET switches, the oscillator frequency, the capacitance and ESR of C
and C2. Since the switching current charging and dis-
1
charging C effect of the ESR of the pumping capacitor C plied by four in the output resistance. The output capacitor C
2
is approximately twice as the output current, the
1
1
is charging and discharging at a current approximately
is a
out
will be multi-
equal to the output current, therefore, its ESR only counts once in the output resistance. A good approximation of R is:
where RSWis 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
-
d
:
2
High capacitance, low ESR capacitors can reduce both the output reslistance and the voltage ripple.
The Schottky diode D nal oscillator circuit uses theOUT pin and the GNDpin. Volt-
is only needed for start-up. The inter-
1
age across OUT andGND must be largerthan 1.8V to insure the operation of the oscillator. During start-up, D 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 Schottkydi­ode D
should have enough current carrying capability to
1
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 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.
Split V+ in Half
Another interesting application shown in the Basic Applica­tion Circuits is using the LM2681 as a precision voltage di­vider. . This circuit can be derived from the voltage doubler by switching the input andoutput connections. In the voltage divider,the input voltage applies across the OUTpin and the GND pin (which arethe power rails forthe internal oscillator), therefore no start-up diode is needed. Also, since the off-voltage across each switchequals V can be raised to +11V.
is used to
1
/2, the input voltage
in
out
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Application Information (Continued)
Capacitor Selection
As discussed in the output resistance and ripple voltage are dependent on the capacitance and ESR values of the external capacitors. The output voltage dropis the loadcurrent times the output resis­tance, and the power efficiency is
Positive Voltage Doubler
section, the
Where IQ(V+) is the quiescent power loss of the IC device,
2
and I
R
is the conversion loss associated with the switch
L
out
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 age ripple, and the converter efficiency. Low ESR capacitors
outRout
(Table 1) are recommended to maximize efficiency, reduce the output voltage drop and voltage ripple.
Low ESR Capacitor Manufacturers
Manufacturer Phone Capacitor Type
Nichicon Corp. (708)-843-7500 PL & PF series, through-hole aluminum electrolytic AVX Corp. (803)-448-9411 TPS series, surface-mount tantalum Sprague (207)-324-4140 593D, 594D, 595D series, surface-mount tantalum Sanyo (619)-661-6835 OS-CON series, through-hole aluminum electrolytic Murata (800)-831-9172 Ceramic chip capacitors Taiyo Yuden (800)-348-2496 Ceramic chip capacitors Tokin (408)-432-8020 Ceramic chip capacitors
Other Applications
Paralleling Devices
Any number ofLM2681s can beparalleled to reduce the out­put resistance. Each device must have its own pumping ca­pacitor C shown in Figure 3. The composite output resistance is:
, while only one outputcapacitor C
1
is needed as
out
), the output volt-
FIGURE 3. Lowering Output Resistance by Paralleling Devices
Cascading Devices
Cascading the LM2681sis an easyway to produce a greater voltage (A two-stage cascade circuit is shown in Figure 4).
The effective output resistance is equal to the weighted sum of each individual device:
=
R
1.5R
out
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out_1+Rout_2
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Note that, the increasing of the number of cascading stages is pracitically limited since it significantly reduces the effi­ciency, increases the output resistnace and output voltage ripple.
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Other Applications (Continued)
FIGURE 4. Increasing Output Voltage by Cascading Devices
Regulating V
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.
A different output voltage is possible by use of LP2980-3.3, LP2980-3.0, or LP2980-adj.
OUT
2V
2V
in_min
in_max
>
V
out_min+Vdrop_max
<
V
out_max+Vdrop_min
Note that, the following conditions must be satisfied simulta­neously for worst case design:
(LP2980) + I
(LP2980) + I
out_maxxRout_max
out_minxRout_min
(LM2681)
(LM2681)
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FIGURE 5. Generate a Regulated +5V from +3V Input Voltage
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Physical Dimensions inches (millimeters) unless otherwise noted
LM2681 Switched Capacitor Voltage Converter
6-Lead Small Outline Package (M6)
NS Package Number MA06A
For Order Numbers, refer to the table in the Ordering Informationsection of this document.
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