mode, an 1200mA (max.) cycle-by-cycle current limit is nor-

mally used. If an excessive load pulls the output voltage

down to approximately 0.375V, then the device switches to a

timed current limit mode. In timed current limit mode the

internal PFET switch is turned off after the current compara-

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

3.5us to 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

output can be set from 0.8V(typ.) to 3.6V(typ.) by changing

the voltage on the analog V

PA 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 Out-

put Voltage in the Application Information section for further

details.

Thermal Overload Protection

The LM3205 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 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 device and is considered bad practice.

Application Information

SETTING THE OUTPUT VOLTAGE

The LM3205 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 (typ.) to

3.6V (typ.) by setting the voltage on the V

following formula:

V

When V

will follow proportionally by 2.5 times of V

If V

tion may occur because of insufficient slope compensation. If

V

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

is used. Refer to datasheet curve (V

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

INDUCTOR SELECTION

A 3.3µH inductor with saturation current rating over 1200mA

and low inductance drop at the full DC bias condition is

recommended for almost all applications. The inductor’s DC

resistance should be less than 0.2

Ω for good efficiency. For

low dropout voltage, lower DCR inductors are advanta-

geous. 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 an inductor with the lowest acceptable limit (as of

Nov./05). Table 1 suggests some inductors and suppliers.

TABLE 1. Suggested inductors and their suppliers

Model

Size (WxLxH) [mm]

Vendor

NR3015T3R3M

3.0 x 3.0 x 1.5

Taiyo-Yuden

DO3314-332MXC

3.3 x 3.3 x 1.4

Coilcraft

If a smaller inductance inductor is used in the application, the

LM3205 may become unstable during line and load tran-

sients and V

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. 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 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 character-

istics of the capacitors must be considered when selecting

the voltage rating and case size of the capacitor. A few

manufactures 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

LM3205 should be carefully 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.

TABLE 2. Suggested capacitors and their suppliers

Model

Vendor

0805ZD475KA 4.7µF, 10V

AVX

C1608X5R0J475M, 4.7µF, 6.3V

TDK

C2012X5R0J106M,10µF, 6.3V

TDK

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