LM2765M6 National Semiconductor, LM2765M6 Datasheet - Page 6

• Manufacturer Part Number: LM2765M6
• Manufacturer: National Semiconductor
• Type: Step Upr

Specifications of LM2765M6

• Operating Supply Voltage (min): 1.8V
• Operating Supply Voltage (max): 5.5V
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Package Type: SOT-23
• Pin Count: 6
• Mounting: Surface Mount
• Output Voltage: 3.6 to 11V
• Output Current: 20mA
• Lead Free Status / RoHS Status: Not Compliant

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Circuit Description
The LM2765 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 LM2765 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
function of the ON resistance of the internal MOSFET
switches, the oscillator frequency, and 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
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. R
the LM2765.
ds(on)
out
is:
of the MOSFET switches and the ESR of the capaci-
SW
2
2
1
FIGURE 2. Voltage Doubling Principle
is charging and discharging at a current approxi-
and S
and C
is the sum of the ON resistance of the internal
1
1
is approximately twice as the output current,
and S
2
4
. Since the switching current charging and
are closed, C
3
are closed, the sum of the input
2
1
and S
charges to the supply
1
gives the 2V+ output
SW
4
is typically 8Ω for
are open; at the
1
and S
1
out
will be
3
10128114
is a
are
6
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
High capacitance, low ESR capacitors can reduce both the
output resistance and the voltage ripple.
The Schottky diode D
from turning-on its own parasitic diode and potentially
latching-up. During start-up, D
the output capacitor to V
decreasing the start-up time. Therefore, the Schottky diode
D
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.
SHUTDOWN MODE
A shutdown (SD) pin is available to disable the device and
reduce the quiescent current to 0.1 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
resistance, and the power efficiency is
Where I
and I
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
(Table 1) are recommended to maximize efficiency, reduce
the output voltage drop and voltage ripple.
1
should have enough current carrying capability to charge
L
2
R
Q
out
(V+) is the quiescent power loss of the IC device,
is the conversion loss associated with the switch
1
2
:
is only needed to protect the device
IN
minus the diode drop thereby
1
will also quickly charge up
out
R
out
), the output volt-