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Detailed Device Description
The LM2685 CMOS charge pump voltage converter oper-
ates as an input voltage doubler, +5V regulator and inverter
for an input voltage in the range of +2.85V to +6.5V. It
delivers maximum load currents of 50mA and 15mA for the
regulated +5V and the inverted output voltages respectively,
with an operating current of only 800A. It also has a typical
shutdown current of 6A. All these performance qualities
make the LM2685 an ideal device for battery powered sys-
tems.
The LM2685 has three main functional blocks: a voltage
doubler, a low dropout (LDO) regulator, and a voltage in-
verter. Figure 1 shows the LM2685 functional block diagram.
VOLTAGE DOUBLER
The voltage doubler stage doubles the input voltage V
IN,
within the range of +2.85V to +5.4V. For V
IN above 5.4V, the
doubler shuts off and the input voltage is passed directly to
V
DBL via an internal power switch.
The doubler contains four large CMOS switches which are
switched in a sequence to double the input supply voltage.
Figure 2 illustrates the voltage conversion scheme. When S2
and S4 are closed, C1 charges to the supply voltage V
IN.
During this time interval, switches S1 and S3 are open. In the
next time interval, S2 and S4 are opened at the same time,
S1 and S3 are closed, the sum of the input voltage V
IN and
the voltage across C1 gives the 2V
In and the voltage across
C2 gives the 2V
IN at VDBL output. VDBL supplies the LDO
regulator. It is recommended not to load V
DBL when V05 has
a load of 50mA. For proper operation, the sum of V
DBL and
V
05 loads must not be more than 50mA.
The Schottky diode D1 is only needed for start-up. The
internal oscillator circuit uses the V
DBL and GND pins. The
voltage across them must be larger than 1.8V to ensure the
operation of the oscillator. During start-up, D1 is used to
charge up the voltage at V
DBL pin to start the oscillator; it
also protects the device from turning on its own parasitic
diode and potentially latching up. The diode should have
enough current carrying capability to change capacitor C3 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 ramp
is less than 10V/ms, a smaller schottky diode like
MBR0520LT1 can be used to reduce the circuit size.
+5 LDO REGULATOR
V
DBL is the input to an LDO regulator that regulates it to a +5
output voltage at V
05.VPSW is tied to V05 through a series
switch PSW. The LDO output capacitor (4.7F Tantalum)
may be tied to either V
05 or VPSW.
INVERTER
From the V
05 output, a 5V output is created at VNEG by
means of an inverting charge pump. This negative output is
unregulated, meaning that it’s output will droop as the load
current at V
NEG increases. The inverter contains four large
10110003
FIGURE 1. Functional Block Diagram
10110004
FIGURE 2. Voltage Doubler Principle
LM2685
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相关代理商/技术参数
LM2686MTC 制造商:Texas Instruments 功能描述:VOLT CONVERTER SMD 2686 TSSOP14
LM2686MTC 制造商:Texas Instruments 功能描述:VOLT CONVERTER SMD 2686 TSSOP14
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LM2687LD 制造商:Texas Instruments 功能描述:
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