lm2706 National Semiconductor Corporation, lm2706 Datasheet - Page 12

• Manufacturer Part Number: lm2706
• Description: Miniature, Variable, Step-down Dc-dc Converter With Bypass For Rf Power Amplifiers
• Manufacturer: National Semiconductor Corporation

www.national.com
Dynamically Adjustable Output
Voltage
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
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
DO1608C-103
P1174.103T
ELL6RH100M
CDRH5D18-100
Part Number
(Continued)
CON
output voltage control pin, as directed in
TABLE 3. Suggested Inductors and Their Suppliers
Coilcraft
Pulse
Panasonic
Sumida
Vendor
12
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.
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 0.3Ω for good efficiency. Table 3 lists suggested
inductors and suppliers.
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.
0.65
847-639-6400
858-674-8100
714-373-7366
847-956-0666
V
V
CON
V
<
CON
Phone
CON
V
TABLE 2. Output Voltage Selection
CON
≤ 0.55
≥ 1.7
(V)
<
FIGURE 4. V
1.5
V
OUT
OUT
= 1.75 V
vs V
V
847-639-1469
858-674-8262
714-373-7323
847-956-0702
OUT
3.25
CON
1.5
FAX
CON
(V)
OUT
200409A9
+ 0.45V
and V
CON
.