V

CON,ON

The output is disabled when V

is enabled when V

has about 25 mV (typ.) of hysteresis.

Current Limiting

A current limit feature allows the LM3218 to protect itself and

external components during overload conditions. In PWM

mode, an 1100 mA (typ.) cycle-by-cycle current limit is nor-

mally used when V

(typ.) is used when V

load pulls the output voltage down to approximately 0.375V,

then the device switches to a timed current limit mode when

V

internal PFET switch is turned off after the current comparator

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 LM3218 features dynamically adjustable output voltage

to eliminate the need for external feedback resistors. The out-

put can be set from 0.8V to 3.6V by changing the voltage on

the analog V

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, promot-

ing longer battery life. See Setting the Output Voltage in the

Application Information section for further details. The

LM3218 moves into Pulse Skipping mode when duty cycle is

over 92% and the output voltage ripple increases slightly.

Thermal Overload Protection

The LM3218 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 LM3218 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 to 3.6V by

setting the voltage on the V

V

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 (55 ns). 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

than 0.15V (typ.), the switches are turned on. When it is less

than 0.125V (typ.), the switches are turned off. This on/off

function has 25 mV (typ.) hysteresis. The quiescent current

when (V

ESTIMATION OF MAXIMUM OUTPUT CURRENT

CAPABILITY

Referring to Figure 3, the Inductor peak-to-peak ripple current

can be estimated by:

I

Where, Fsw is switching frequency.

Therefore, maximum output current can be calculated by:

I

For the worst case calculation, the following parameters

should be used:

F

I

L1 (Lowest inductor value): refer to inductor datasheet. Note

that inductance will drop with DC bias current and tempera-

ture. The worst case is typically at 85°C.

For example, V

tance value at 985 mA DC-bias current and 85°C), F

MHz , I

I

I

The effects of switch, inductor resistance and dead time are

ignored. In real application, the ripple current would be 10%

to 15% higher than ideal case. This should be taken into ac-

count when calculating maximum output current. Special at-

tention needs to be paid that a delta between maximum output

current capability and the current limit is necessary to satisfy

transient response requirements. In practice, transient re-

sponse requirements may not be met for output current

greater than 650 mA.

INDUCTOR SELECTION

The inductor is an integrated LTCC 2.6 µH substrate within

the LM3218 module and has a saturation current rating over

1200 mA. The integrated inductor’s low 1.2 mm maximum

height provides ease of use into small design constraints. In-

tegrating the inductor can eliminate layout issues associated

with DC/DC converters and reduce potential EMI problems.

CAPACITOR SELECTION

The LM3218 is designed for use with ceramic capacitors for

its input and output filters. Use a 10 µF ceramic capacitor for

input and a 4.7 µF ceramic capacitor for output. They should

maintain at least 50% capacitance at DC bias and tempera-

ture conditions. Ceramic capacitor types such as X5R, X7R

and B are recommended for both filters. Table 1 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. If it is necessary

to choose a 0603-size capacitor for C

ation of the LM3218 should be carefully evaluated on the

system board. Use of multiple 2.2 µF or 1 µF capacitors in

parallel may also be considered.

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