Application Information

SETTING THE OUTPUT VOLTAGE

The LM2619 can be used with external feedback resistors to

set the output voltage.Select the value of R2 to allow atleast

100 times the feedback pin bias current to flow through it.

V

EXTERNAL COMPENSATION

The LM2619 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, under all conditions. Typical values are 10pF for

C5, 220pF to 1nF for C4 and 22K to 100K for R3.

A

R

M

D = VOUT/VIN , D’ = 1-D , duty cycle

M

during PFET on time

R

Ω , effective resistance at inverting input

of error amp

R

1

where R

power stage

f

f

C2

f

control

f

C5

f

α = R2/(R1+ R2)

f

where f

equation for crossover frequency assumes that f

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 1 lists suggested inductors and suppliers.

TABLE 1. Suggested Inductors and Their Suppliers

Part Number

Vendor

Phone

FAX

DO1608C-103

Coilcraft

847-639-6400

847-639-1469

ELL6SH100M

Panasonic

714-373-7366

714-373-7323

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 LM2619. 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 LM2619. 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.

CAPACITOR SELECTION

Use a 10µF ceramic input capacitor. Use X7R or X5R types,

do not use Y5V.

Use of tantalum capacitors is not recommended.

Ceramic capacitors provide an optimal balance between

small size, cost, reliability and performance for cell phones

and similar applications. A 22µF ceramic output capacitor is

recomended for applications that require increased toler-

ance to heavy load transients. A 10µF ceramic output ca-

pacitor can be used in applications where the worst case

load transient step is less than 200mA. Use of a 10µF output

capacitor trades off smaller size for an increase in output

voltage ripple, and undershoot during load transients. Table

2 lists suggested capacitors and suppliers.

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