Current Limiting

A current limit feature allows the LM2618 to protect itself and

external components during overload conditions. Current

limiting is implemented using an independent internal com-

parator. In PWM mode, cycle-by-cycle current limiting is

normally used. If an excessive load pulls the output voltage

down to approximately 0.7V, then the device switches to a

timed current limit mode. In timed current limit mode the

internal P-FET switch is turned off after the current compara-

tor trips and the beginning of the next cycle is inhibited for

2.5µs to force the instantaneous inductor current to ramp

down to a safe value. PFM mode also uses timed current

limit operation. The synchronous rectifier is off in this mode.

Timed current limit prevents the loss of current control seen

in some products when the output voltage is pulled low in

serious overload conditions.

Current Limiting and PWM Mode

Transient Response

Considerations

The LM2618 was designed for fast response to moderate

load steps. Harsh transient conditions during loads above

300mA can cause the inductor current to swing up to the

maximum current limit, resulting in PWM mode jitter or insta-

bility from activation of the current limit comparator. To avoid

this jitter or instability, do not power-up or start the LM2618

into a full load (loads near or above 400mA). Do not change

operating modes or output voltages when operating at a full

load. Avoid extremely sharp and wide-ranging load steps to

Pin Selectable Output Voltage

The LM2618 features pin-selectable output voltage to elimi-

nate the need for external feedback resistors. The output

can be set to 1.80V, 1.83V, 1.87V or 1.92V by configuring the

VID0 and VID1 pins. See Setting the Output Voltage in the

Application Information section for further details.

Soft-Start

The LM2618 has soft start to reduce current inrush during

power-up and startup. This reduces stress on the LM2618

and external components. It also reduces startup transients

on the power source.Soft start is implemented by ramping up

the internal reference in the LM2618 to gradually increase

the output voltage.

Thermal Shutdown Protection

The LM2618 has a thermal shutdown protection to protect

from short-term misuse and overload conditions. When the

junction temperature exceeds 150˚C, the device shuts down,

restarting in soft start after the temperature drops below

130˚C. Prolonged operation in thermal overload conditions

may damage the device and is considered bad practice.

Application Information

Setting The Output Voltage

The LM2618 features pin-selectable output voltage to elimi-

nate the need for external feedback resistors. Select an

output voltage of 1.80V, 1.83V, 1.87V or 1.92V by configur-

ing the VID0 and VID1 pins, as directed in Table 1.

TABLE 1. VID0 and VID1 Output Voltage Selection

Settings

V

Logic Level

VID0

VID1

1.92

0

1

1.87

0

0

1.87

N.C.

N.C.

1.83

1

0

1.80

1

1

VID0 and VID1 are digital inputs. They may be set high by

connecting to VDD or low by connecting to SGND. Option-

ally, VID0 and VID1 may be driven by digital gates that

provide over 1.3V for a high state and less than 0.4V for a

low state to ensure valid logic levels. The VID0 and VID1

inputs each have an internal 1.8 µA pull-down that pulls them

low for a default 1.87V output, when left unconnected. Leav-

ing these pins open is acceptable, but setting the pins high or

low is recommended.

Inductor Selection

A 10µH inductor with a saturation current rating over the

maximum current limit is recommended for most applica-

tions. 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

Model

Vendor

Phone

FAX

DO1608C-103

Coilcraft

847-639-6400

847-639-1469

P1174.103T

Pulse Engineering

858-674-8100

858-674-8262

P0770.103T

Pulse Engineering

858-674-8100

858-674-8262

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 overlapping footprints of 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 loses its inductance. Beyond this rating,

the inductor loses its ability to limit current through the PFET

switch to a ramp and allows the switch current to increase

rapidly. This can cause poor efficiency, regulation errors or

stress to DC-DC converters like the LM2618. 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 energy storage in a corre-

sponding magnetic field.

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