Detailed Device Description

The LM2686 CMOS charge pump voltage converter oper-

ates as an input voltage doubler, +5V regulator for an input

voltage in the range of +2.85V to +6.5V. It delivers maximum

load currents of 50mA for the regulated +5V, with an operat-

ing current of only 450µA. It also has a typical shutdown cur-

rent of 6µA. All these performance qualities make the

LM2686 an ideal device for battery powered systems.

The LM2686 has two main functional blocks: a voltage dou-

bler and a low dropout (LDO) regulator.

Figure 1 shows the

LM2686 functional block diagram.

Voltage Doubler

The voltage doubler ties directly to V

voltage in the range from +2.85V to +5.4V up to 5.7V to

10.8V at the V

off and the input voltage is passed directly to VDBL via an in-

ternal 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

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

the voltage across C1 gives the 2V

C2 gives the 2V

regulator. It is recommended not to load V

a load of 50mA. For proper operation, the sum of V

V

+5 LDO Regulator

V

output voltage at V

switch PSW. The LDO output capacitor (4.7µF Tantalum)

may be connected to either V

Shutdown and Load Disconnect

In addition to the nominal charge pump and regulator func-

tions, the LM2686 features shutdown and load disconnect

circuitry. CE (chip enable) and SD (shutdown positive) per-

form the same task with opposite input polarities. When CE

is low or SD is high, all circuit blocks are disabled and V

05

falls to ground potential. This is the same result as when the

die temperature exceeds 150˚C, and the device’s internal

thermal shutdown is triggered.

The LM2686 incorporates a low impedance switch tied to the

V

disconnect and this is achievable via the switch. Switch PSW

connects V

closed, allowing 5V loads to be tied to either V

Forcing CE low or SD high opens the PSW.

Application Information

Capacitor Selection

The output resistance and ripple voltage are dependent on

the capacitance and ESR values of the external capacitors.

Voltage Doubler External Capacitors

The selection of capacitors are based on the specifications

of the dropout voltage (which equals I

voltage ripple, and the converter efficiency.

where R

MOSFET switches as shown in

Figure 2.

The peak-to-peak output voltage ripple is determined by the

oscillator frequency, the capacitance and ESR of the capaci-

tor C3.

DS101141-3

FIGURE 1. Functional Block Diagram

DS101141-4

FIGURE 2. Voltage Doubler Principle

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