LM2681M6X National Semiconductor, LM2681M6X Datasheet - Page 5

• Manufacturer Part Number: LM2681M6X
• Description: IC CONVERTER SWITCH CAP SOT23-6
• Manufacturer: National Semiconductor
• Type: Switched Capacitor (Charge Pump), Divider, Doublerr

Specifications of LM2681M6X

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.8 ~ 11 V
• Current - Output: 20mA
• Frequency - Switching: 80kHz
• Voltage - Input: 2.5 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: SOT-23-6
• Power - Output: 600mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: LM2681M6XTR

Pin Description
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 S
voltage V+. During this time interval, switches S
open. In the next time interval, S
same time, S
voltage V+ and the voltage across C
voltage when there is no load. The output voltage drop when
a load is added is determined by the parasitic resistance
(R
tors) and the charge transfer loss between capacitors. De-
tails will be discussed in the following application information
section.
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 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
function of the ON resistance of the internal MOSFET
switches, the oscillator frequency, the capacitance and ESR
of C
discharging C
the effect of the ESR of the pumping capacitor C
multiplied by four in the output resistance. The output ca-
pacitor C
ds(on)
Pin
1
2
3
4
5
6
1
and C
of the MOSFET switches and the ESR of the capaci-
2
2
FIGURE 2. Voltage Doubling Principle
and S
is charging and discharging at a current approxi-
2
1
1
. Since the switching current charging and
is approximately twice as the output current,
and S
4
Name
CAP−
CAP+
GND
GND
OUT
V+
are closed, C
3
are closed, the sum of the input
Power supply positive voltage input
Power supply ground input
Connect this pin to the negative terminal of the
charge-pump capacitor
Power supply ground input
Positive voltage output
Connect this pin to the positive terminal of the
charge-pump capacitor
2
1
and S
charges to the supply
1
gives the 2V+ output
4
are open; at the
1
and S
Voltage Doubler
1
out
will be
3
10096514
is a
are
5
mately equal to the output current, therefore, its ESR only
counts once in the output resistance. A good approximation
of R
where R
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
High capacitance, low ESR capacitors can reduce both the
output reslistance and the voltage ripple.
The Schottky diode D
internal oscillator circuit uses the OUT pin and the GND pin.
Voltage across OUT and GND must be larger than 1.8V to
insure the operation of the oscillator. During start-up, D
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 diode D
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 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 Applica-
tion Circuits is using the LM2681 as a precision voltage
divider. . 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
can be raised to +11V.
out
is:
Function
SW
2
is the sum of the ON resistance of the internal
:
1
should have enough current carrying
1
Positive voltage output
Same as doubler
Same as doubler
Same as doubler
Power supply positive voltage input
Same as doubler
is only needed for start-up. The
Voltage Split
in
/2, the input voltage
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1
is