LM3205TLEV National Semiconductor, LM3205TLEV Datasheet - Page 13

• Manufacturer Part Number: LM3205TLEV
• Description: BOARD EVALUATION LM3205TL
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3205TLEV

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 0.8 ~ 3.6V
• Current - Output: 650mA
• Voltage - Input: 2.7 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 2MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3205
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

force the instantaneous inductor current to ramp down to a
safe value. The synchronous rectifier is off in 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.
Dynamically Adjustable Output
Voltage
The LM3205 features dynamically adjustable output voltage
to eliminate the need for external feedback resistors. The out-
put can be set from 0.8V(typ.) to 3.6V(typ.) by changing the
voltage on the analog V
applications where peak power is needed only when the
handset is far away from the base station or when data is
being transmitted. In other instances the transmitting power
can be reduced. Hence the supply voltage to the PA can be
reduced, promoting longer battery life. See Setting the Output
Voltage in the Application Information section for further de-
tails.
Thermal Overload Protection
The LM3205 has a thermal overload protection function that
operates to protect itself 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. When the temperature
drops below 125°C, normal operation resumes. Prolonged
operation in thermal overload conditions may damage the de-
vice and is considered bad practice.
Application Information
SETTING THE OUTPUT VOLTAGE
The LM3205 features a pin-controlled variable output voltage
to eliminate the need for external feedback resistors. It can
be programmed for an output voltage from 0.8V (typ.) to 3.6V
(typ.) by setting the voltage on the V
formula:
When V
will follow proportionally by 2.5 times of V
If V
may occur because of insufficient slope compensation. If
V
voltage may not be regulated due to the required on-time be-
ing less than the minimum on-time (50ns). The output voltage
can go lower than 0.8V providing a limited V
Refer to datasheet curve (V
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
INDUCTOR SELECTION
A 3.3µH inductor with saturation current rating over 1200mA
and low inductance drop at the full DC bias condition is rec-
ommended for almost all applications. The inductor’s DC
resistance should be less than 0.2Ω for good efficiency. For
low dropout voltage, lower DCR inductors are advantageous.
The lower limit of acceptable inductance is 2.3µH at 1200mA
over the operating temperature range. Full attention should
be paid to this limit, because some small inductors show large
inductance drops at high DC bias. These can not be used with
the LM3205. Taiyo-Yuden NR3015T3R3M is an example of
CON
CON
voltage is less than 0.32V (V
is over 1.44V (V
CON
is between 0.32V and 1.44V, the output voltage
IN
range.
V
OUT
OUT
CON
= 2.5 x V
= 3.6V), sub-harmonic oscillation
CON
pin. This feature is useful in PA
Voltage vs Output Voltage)
CON
CON
OUT
pin, as in the following
= 0.8V), the output
CON
IN
.
range is used.
13
an inductor with the lowest acceptable limit (as of Nov./05).
Table 1 suggests some inductors and suppliers.
If a smaller inductance inductor is used in the application, the
LM3205 may become unstable during line and load transients
and V
For low-cost applications, an unshielded bobbin inductor is
suggested. For noise-critical applications, a toroidal or shield-
ed-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. Saturation occurs when the mag-
netic flux density from current through the windings of the
inductor exceeds what the inductor’s core material can sup-
port with a corresponding magnetic field. This can cause poor
efficiency, regulation errors or stress to a DC-DC converter
like the LM3205.
CAPACITOR SELECTION
The LM3205 is designed for use with ceramic capacitors for
its input and output filters. Use a 10µF ceramic capacitor for
input and a 4.7µF ceramic capacitor for output. They should
maintain at least 50% capacitance at DC bias and tempera-
ture conditions. Ceramic capacitors types such as X5R, X7R
are recommended for both filters. These provide an optimal
balance between small size, cost, reliability and performance
for cell phones and similar applications. Table 2 lists some
suggested part numbers and suppliers. DC bias characteris-
tics of the capacitors must be considered when selecting the
voltage rating and case size of the capacitor. A few manufac-
tures can supply 4.7µF capacitors in the 0805 case size which
maintain at least 50% of their value, but TDK is currently the
only manufacturer which can provide such capacitors in the
0603 case size. As of November, 2005, no manufacture can
supply 10µF capacitors in the 0603 case size which maintain
50% of their value. If it is necessary to choose a 0603-size
capacitor for V
fully evaluated on the system board. Output capacitors with
smaller case sizes mitigate piezo electric vibrations when the
output voltage is stepped up and down at fast rates. However,
they have a larger percentage drop in value with dc bias. Use
of multiple 2.2µF or 1µF capacitors in parallel may also be
considered.
The input filter capacitor supplies AC current drawn by the
PFET switch of the LM3205 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-
NR3015T3R3M
DO3314-332MXC
TABLE 2. Suggested capacitors and their suppliers
TABLE 1. Suggested inductors and their suppliers
CON
C1608X5R0J475M, 4.7µF, 6.3V
C2012X5R0J106M,10µF, 6.3V
Model
transient response times may get affected.
0805ZD475KA 4.7µF, 10V
IN
, the operation of the LM3205 should be care-
Model
3.0 x 3.0 x 1.5
3.3 x 3.3 x 1.4
Size (WxLxH) [mm]
Taiyo-
Yuden
Coilcraft
www.national.com
Vendor
Vendor
AVX
TDK
TDK