lm2618 National Semiconductor Corporation, lm2618 Datasheet - Page 12

• Manufacturer Part Number: lm2618
• Description: 400ma Sub-miniature, High Efficiency, Synchronous Pwm & Pfm Programmable Dc-dc Converter
• Manufacturer: National Semiconductor Corporation

www.national.com
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
full load, such as from
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.
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,
DO1608C-103
P1174.103T
P0770.103T
CDRH5D18-100
Model
<
30mA to
>
350mA.
TABLE 2. Suggested Inductors and Their Suppliers
Coilcraft
Pulse Engineering
Pulse Engineering
Sumida
Vendor
12
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.
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 0.3Ω for
good efficiency. Table 2 lists suggested inductors and
suppliers.
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.
TABLE 1. VID0 and VID1 Output Voltage Selection
847-639-6400
858-674-8100
858-674-8100
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Phone
V
OUT
1.92
1.87
1.87
1.83
1.80
(V)
Settings
VID0
N.C.
0
0
1
1
Logic Level
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858-674-8262
858-674-8262
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VID1
N.C.
1
0
0
1
FAX