Application Information

SETTING THE OUTPUT VOLTAGE

The LM2614 can be used with external feedback resistors

and an analog signal to vary the output voltage. Select an

output voltage from 1.0V to 3.6V by setting the voltage on

the VCON as directed in Table 1.

TABLE 1. Output Voltage Selection

VCON (V)

VOUT (V)

VCON = 0V

V

V

Refer to Figure 12 for the relation between VOUT and

VCON.

When the control voltage is between 1.85V and 0V, the

output voltage will vary in a monotonic fashion with respect

to the voltage on the control pin as per the equation in Table

1. Select the value of R2 to allow at least 100 times the

feedback pin bias current to flow through it.

EXTERNAL COMPENSATION

The LM2614 uses external components connected to the

EANEG and EAOUT pins to compensate the regulator (Fig-

ure 4). Typically, all that is required is a series connection of

one capacitor (C4) and one resistor (R3). A capacitor (C5)

can be connected across the EANEG and EAOUT pins to

improve the noise immunity of the loop. C5 reacts with R3 to

give a high frequency pole. C4 reacts with the high open loop

gain of the error amplifier and the resistance at the EANEG

pin to create the dominant pole for the system, while R3 and

C4 react to create a zero in the frequency response. The

pole rolls off the loop gain, to give a bandwidth somewhere

between 10kHz and 50kHz, this avoids a 100kHz parasitic

pole contributed by the current mode controller. Typical val-

ues in the 220pF to 1nF (C4) range are recommended to

create a pole on the order of 10Hz or less.

The next dominant pole in the system is formed by the output

capacitance (C2) and the parallel combination of the load

resistance and the effective output resistance of the regula-

tor. This combined resistance (Ro) is dominated by the small

signal output resistance, which is typically in the range of 3

Ω

to 15

Ω. The exact value of this resistance, and therefore this

load pole depends on the steady state duty cycle and the

internal ramp value. Ideally we want the zero formed by R3

and C4 to cancel this load pole, such that R3=RoC2/C4. Due

to the large variation in Ro, this ideal case can only be

achieved at one operating condition. Therefore a compro-

mise of about 5

Ω for Ro should be used to determine a

starting value for R3. This value can then be optimized on

the bench to give the best transient response to load

changes and changes in VCON, under all conditions. Typical

values are 10pF for C5 and 220pF to 470pF for C4, to

ensure good response from dropout conditions to V

OUT-

(min).

INDUCTOR SELECTION

Use a 10µH inductor with saturation current rating higher

than the peak current rating of the device. The inductor’s

resistance should be less than 0.3

Ω for good efficiency.

Table 2 lists suggested inductors and suppliers.

TABLE 2. 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

P0770.103T

Pulse

858-674-8100

858-674-8262

For low-cost applications, an unshielded inductor is sug-

gested. For noise critical applications, a toroidal or shielded

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 shielded

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

models is unacceptable.

The saturation current rating is the current level beyond

which an inductor loses its inductance. Different manufactur-

ers specify the saturation current rating differently. Some

specify saturation current point to be when inductor value

falls 30% from its original value, others specify 10%. It is

always better to look at the inductance versus current curve

and make sure the inductor value doesn’t fall below 30% at

the peak current rating of the LM2614. Beyond this rating,

the inductor loses 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 LM2614. Satu-

ration occurs when the magnetic flux density from current

through the windings of the inductor exceeds what the in-

ductor’s core material can support with a corresponding

magnetic field.

VOUT vs VCON

(Circuit in Figure 4)

20036721

FIGURE 12.

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