LM2660M National Semiconductor, LM2660M Datasheet - Page 7

• Manufacturer Part Number: LM2660M
• Description: DC/DC Charge Pump Converter IC
• Manufacturer: National Semiconductor

Specifications of LM2660M

• No. Of Pins: 8
• Mounting Type: Surface Mount
• Supply Voltage Min: 1.5V
• Peak Reflow Compatible (260 C): No
• Current Rating: 100A
• Supply Voltage Max: 5.5V
• Leaded Process Compatible: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Information
The peak-to-peak output voltage ripple is determined by the
oscillator frequency, and the capacitance and ESR of the
output capacitor C
Again, using a low ESR capacitor will result in lower ripple.
POSITIVE VOLTAGE DOUBLER
The LM2660/LM2661 can operate as a positive voltage dou-
bler (as shown in the Basic Application Circuits). The dou-
bling function is achieved by reversing some of the connec-
tions to the device. The input voltage is applied to the GND
pin with an allowable voltage from 2.5V to 5.5V. The V+ pin
is used as the output. The LV pin and OUT pin must be
connected to ground. The OSC pin can not be driven by an
external clock in this operation mode. The unloaded output
voltage is twice of the input voltage and is not reduced by the
diode D
The Schottky diode D
internal oscillator circuit uses the V+ pin and the LV pin
(connected to ground in the voltage doubler circuit) as its
power rails. Voltage across V+ and LV must be larger than
1.5V to insure the operation of the oscillator. During start-up,
D
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 LM2660/LM2661 as a precision
voltage divider. Since the off-voltage across each switch
equals V
CHANGING OSCILLATOR FREQUENCY
For the LM2660, the internal oscillator frequency can be
selected using the Frequency Control (FC) pin. When FC is
open, the oscillator frequency is 10 kHz; when FC is con-
nected to V+, the frequency increases to 80 kHz. A higher
oscillator frequency allows smaller capacitors to be used for
equivalent output resistance and ripple, but increases the
typical supply current from 0.12 mA to 1 mA.
The oscillator frequency can be lowered by adding an exter-
nal capacitor between OSC and GND. (See Typical Perfor-
mance Characteristics.) Also, in the inverter mode, an exter-
nal clock that swings within 100 mV of V+ and GND can be
used to drive OSC. Any CMOS logic gate is suitable for
driving OSC. LV must be grounded when driving OSC. The
maximum external clock frequency is limited to 150 kHz.
The switching frequency of the converter (also called the
charge pump frequency) is half of the oscillator frequency.
Note: OSC cannot be driven by an external clock in the voltage-doubling
1
is used to charge up the voltage at V+ pin to start the
mode.
1
’s forward drop.
IN
/2, the input voltage can be raised to +11V.
1
2
:
should have enough current carrying
1
is only needed for start-up. The
(Continued)
7
SHUTDOWN MODE
For the LM2661, a shutdown (SD) pin is available to disable
the device and reduce the quiescent current to 0.5 µA.
Applying a voltage greater than 2V to the SD pin will bring
the device into shutdown mode. While in normal operating
mode, the SD pin is connected to ground.
CAPACITOR SELECTION
As discussed in the Simple Negative Voltage Converter
section, the output resistance and ripple voltage are depen-
dent 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
switch on-resistance, the two external capacitors and their
ESRs.
Since the switching current charging and discharging C
approximately twice as the output current, the effect of the
ESR of the pumping capacitor C
output resistance. The output capacitor C
discharging at a current approximately equal to the output
current, therefore, its ESR only counts once in the output
resistance. However, the ESR of C
output voltage ripple. Therefore, low ESR capacitors (Table
3) are recommended for both capacitors to maximize effi-
ciency, reduce the output voltage drop and voltage ripple.
For convenience, C
same.
The output resistance varies with the oscillator frequency
and the capacitors. In Figure 3, the output resistance vs.
oscillator frequency curves are drawn for three different tan-
talum capacitors. At very low frequency range, capacitance
plays the most important role in determining the output re-
sistance. Once the frequency is increased to some point
(such as 20 kHz for the 150 µF capacitors), the output
resistance is dominated by the ON resistance of the internal
switches and the ESRs of the external capacitors. A low
Open
V+
Open or V+
N/A
Open
External Capacitor
External Clock
(inverter mode only)
TABLE 1. LM2660 Oscillator Frequency Selection
TABLE 2. LM2661 Oscillator Frequency Selection
L
FC
2
R
Q
OUT
(V+) is the quiescent power loss of the IC device,
OSC
is the conversion loss associated with the
Open
Open
External Capacitor
External Clock
(inverter mode only)
1
and C
80 kHz
See Typical Performance
Characteristics
External Clock Frequency
OSC
2
are usually chosen to be the
1
is multiplied by four in the
Oscillator
2
directly affects the
10 kHz
80 kHz
See Typical
Performance
Characteristics
External Clock
Frequency
2
is charging and
Oscillator
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1
is