Dynamically Adjustable Output

be reduced helping maintain longer battery life. See Setting

the Output Voltage in the Application Information section for

further details.

Soft-Start

The LM2706 has soft start to reduce inrush during power-up

and startup. This reduces stress on the LM2706 and external

components. It also reduces startup transients on the power

source. Soft start is implemented by ramping up the refer-

ence input to the error amplifier of the LM2706 to gradually

increase the output voltage. The reference ramps up in

around 600 µs.

Thermal Overload Protection

The LM2706 has a thermal overload protection function that

operates to protect itself from short-term misuse and over-

load conditions. When the junction temperature exceeds

around 150˚C, the device initiates a soft-start cycle which is

completed after the temperature drops below 130˚C. Pro-

longed operation in thermal overload conditions may dam-

age the device and is considered bad practice.

Application Information

SETTING THE OUTPUT VOLTAGE

The LM2706 features a pin-controlled variable output volt-

age to eliminate the need for external feedback resistors.

Select an output voltage from 1.5V to 3.25V by setting the

voltage on the V

Table 2.

When the control pin voltage is between 0.65V and 1.5V, the

output voltage will vary in a monotonic fashion with respect

to the voltage on the control pin as per the above equation.

Internally the control pin is buffered and then clamped before

it is fed to the error amplifier inputs. If voltage on the control

pin is less than 0.55V, the output voltage is regulated at 1.5V

and if the voltage is greater than 1.7V, the output is regulated

at 3.25V.

TABLE 2. Output Voltage Selection

V

V

V

CON

≤ 0.55

1.5

V

V

CON

≥ 1.7

3.25

Refer to Figure 4 for the relation between V

INDUCTOR SELECTION

Use a 10 µH inductor with a saturation current rating of

atleast 750 mA. The inductor’s resistance should be less

than around 0.3

Ω for good efficiency. Table 3 lists suggested

inductors and suppliers.

TABLE 3. Suggested Inductors and Their Suppliers

Part Number

Vendor

Phone

FAX

DO1608C-103

Coilcraft

847-639-6400

847-639-1469

P1174.103T

Pulse

858-674-8100

858-674-8262

ELL6RH100M

Panasonic

714-373-7366

714-373-7323

CDRH5D18-100

Sumida

847-956-0666

847-956-0702

For low-cost applications, an unshielded bobbin inductor is

suggested. For noise critical applications, a toroidal or

shielded-bobbin inductor should be used. A good practice is

to lay out the board with footprints accommodating both

types for design flexibility. This allows substitution of a low-

noise toroidal inductor, in the event that noise from low-cost

bobbin models is unacceptable. The saturation current rating

is the current level beyond which an inductor looses its

inductance. Beyond this rating, the inductor looses its ability

to limit current through the PWM switch to a ramp. This can

cause poor efficiency, regulation errors or stress to DC-DC

converters like the LM2706. Saturation occurs when the

magnetic flux density from current through the windings of

the inductor exceeds what the inductor’s core material can

support with a corresponding magnetic field.

CAPACITOR SELECTION

Use a 4.7 µF ceramic input capacitor and a 4.7 µF ceramic

output capacitor. These provide an optimal balance between

small size, cost, reliability and performance for cell phones

and similar applications. Table 4 lists suggested capacitors

and suppliers.

200409A9

FIGURE 4. V

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