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LM3200 Datasheet(Fiches technique) 13 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Numéro de pièce LM3200
Description  Miniature, Adjustable, Step-Down DC-DC Converter with Bypass Mode for RF Power Amplifiers
Télécharger  15 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
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Current Limiting (Continued)
fore, the device may not startup if an excessive load is
connected to the output when the device is enabled. The
synchronous rectifier is off in the timed current limit 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.
A current limit is also provided for the NFET. This is approxi-
mately −500 mA. Both the NFET and the PFET are turned off
in negative current limit until the PFET is turned on again at
the beginning of the next cycle. The negative current limit
inhibits buildup of excessive inductor current. In the Bypass
mode, the bypass current limit is 1000 mA(typ). The output
voltage drops when the bypass current limit kicks in.
Thermal Overload Protection
The LM3200 has a thermal overload protection function that
operates to protect the device from short-term misuse and
overload conditions. When the junction temperature exceeds
around 150˚C, the device inhibits operation. Both the PFET
and the NFET are turned off in PWM mode, and the Bypass
PFET is turned off in Bypass mode. When the temperature
drops below 130˚C, normal operation resumes. Prolonged
operation in thermal overload conditions may damage the
device and is considered bad practice.
Application Information
SETTING THE OUTPUT VOLTAGE
The LM3200 features a pin-controlled variable output volt-
age to eliminate the need for external feedback resistors. It
can be programmed for an output voltage from 0.8V to 3.6V
by setting the voltage on the V
CON pin, as in the following
formula:
V
OUT =3xVCON
When V
CON is between 0.267V and 1.20V, the output volt-
age will follow proportionally by 3 times of V
CON.
If V
CON is over 1.20V (VOUT = 3.6V), sub-harmonic oscilla-
tion may occur because of insufficient slope compensation.
If V
CON voltage is less than 0.267V (VOUT = 0.8V), the output
voltage may not be regulated due to the required on-time
being less than the minimum on-time (50ns). The output
voltage can go lower than 0.8V providing a limited V
IN range
is used. Refer to datasheet curve (Low V
CON Voltage vs
Output Voltage) for details. This curve is for a typical part and
there could be part to part variation for output voltages less
than 0.8V over the limited V
IN range. In addition, if VCON is
less than approx. 0.15V, the LM3200 output is turned off, but
the internal bias circuits are still active.
INDUCTOR SELECTION
A 2.2 µH inductor with saturation current rating over 940 mA
is recommended for almost all applications. The inductor
resistance should be less than 0.3
Ω for better efficiency.
Table 1 lists suggested inductors and suppliers.
TABLE 1. Suggested Inductors and Their Suppliers
Model
Size (WxLxH) [mm]
Vendor
DO3314-222MX
3.3 x 3.3 x 1.4
Coilcraft
VLF3010AT-2R2M1R0 2.6 x 2.8 x 1.0
TDK
MIPW3226D2R2M
3.2 x 2.6 x 1.0
FDK
Model
Size (WxLxH) [mm]
Vendor
LPO3310-222MX
3.3 x 3.3 x 1.0
Coilcraft
If a higher value inductor is used the LM3200 may become
unstable and exhibit large under or over shoot during line,
load and V
CON transients. If smaller inductance value is
used, slope compensation maybe insufficient causing sub-
harmonic oscillations. The device has been tested with in-
ductor values in the range 1.55µH to 3.1µH to account for
inductor tolerances.
For low-cost applications, an unshielded bobbin inductor can
be used. For noise-critical applications, an unshielded or
shielded-bobbin inductor should be used. A good practice is
to layout the board with footprints accommodating both
types for design flexibility. This allows substitution of an
unshielded inductor, in the event that noise from low-cost
bobbin models is unacceptable. 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 a corresponding magnetic field. This can cause
poor efficiency, regulation errors or stress to a DC-DC con-
verter like the LM3200.
CAPACITOR SELECTION
The LM3200 is designed to be used with ceramic capacitors.
Use a 10 µF ceramic capacitor for the input and a 4.7 µF
ceramic capacitor for the output. Ceramic capacitors such as
X5R, X7R and B are recommended for both filters. These
provide an optimal balance between small size, cost, reliabil-
ity and performance for cell phones and similar applications.
Table 2 lists suggested capacitors and suppliers.
TABLE 2. Suggested Capacitors and Their Suppliers
Model
Vendor
LMK212BJ475MG
Taiyo-Yuden
C2012X5R1A475K
TDK
GRM188R61A475K
Murata
C3216X5R1A106K
TDK
The DC bias characteristics of the capacitor must be consid-
ered when making the selection. If smaller case size such as
0603 is selected, the dc bias could reduce the cap value by
as much as 40%, in addition to the 20% tolerances and 15%
temperature coefficients. Request dc bias curves from
manufacturer when making selection.The device has been
designed to be stable with output capacitors as low as 3 µF
to account for capacitor tolerances.This value includes dc
bias reduction, manufacturing tolerences and temp coeffi-
cients.
The input filter capacitor supplies AC current drawn by the
PFET switch of the LM3200 in the first part of each cycle and
reduces the voltage ripple imposed on the input power
source. The output filter capacitor absorbs the AC inductor
current, helps maintain a steady output voltage during tran-
sient load changes and reduces output voltage ripple. These
capacitors must be selected with sufficient capacitance and
sufficiently low ESR (Equivalent Series Resistance) to per-
form these functions. The ESR of the filter capacitors is
generally a major factor in voltage ripple.
MICRO SMD PACKAGE ASSEMBLY AND USE
Use of the Micro SMD package requires specialized board
layout, precision mounting and careful re-flow techniques, as
detailed in National Semiconductor Application Note 1112.
www.national.com
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