LM3208 datasheet(14/17 Pages) NSC

14 / 17 page

DSON(P) Management

(Continued)

device uses only a small part of the PFET to minimize drive

loss of the PFET. When V

CON is greater than 0.42V (typ.),

the entire PFET is used to minimize R

DSON(P) loss. This

threshold has about 20mV (typ.) of hysteresis.

CON,ON

The output is disabled when V

CON is below 125mV (typ.). It

is enabled when V

CON is above 150mV (typ.). The threshold

has about 25mV (typ.) of hysteresis.

Current Limiting

A current limit feature allows the LM3208 to protect itself and

external components during overload conditions. In PWM

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

mally used when V

CON is above 0.42V (typ.), and an 800mA

(typ.) is used when V

CON is below 0.40V (typ.). If an exces-

sive load pulls the output voltage down to approximately

0.375V, then the device switches to a timed current limit

mode when V

CON is above 0.42V (typ.). In timed current limit

mode the 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 LM3208 features dynamically adjustable output voltage

to eliminate the need for external feedback resistors. The

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

on the analog V

CON pin. This feature is useful in PA applica-

tions where peak power is needed only when the handset is

far away from the base station or when data is being trans-

mitted. In other instances, the transmitting power can be

reduced. Hence the supply voltage to the PA can be re-

duced, promoting longer battery life. See Setting the Output

Voltage in the Application Information section for further

details. The LM3208 moves into Pulse Skipping mode when

duty cycle is over 92% and the output voltage ripple in-

creases slightly.

Thermal Overload Protection

The LM3208 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 LM3208 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:

OUT =2.5xVCON

When V

CON is between 0.32V and 1.44V, the output voltage

will follow proportionally by 2.5 times of V

CON.

If V

CON is over 1.44V (VOUT = 3.6V), sub-harmonic oscilla-

tion may occur because of insufficient slope compensation. If

CON voltage is less than 0.32V (VOUT = 0.8V), the output

voltage may not be regulated due to the required on-time

being less than the minimum on-time (55ns). The output

voltage can go lower than 0.8V providing a limited V

IN range

is used. Refer to datasheet curve (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. When the control pin voltage

is more 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 25mV (typ.) hysteresis. The quiescent

current when (V

CON = 0V and VEN = Hi) is around 600µA.

ESTIMATION OF MAXIMUM OUTPUT CURRENT

CAPABILITY

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

rent can be estimated by:

IND_PP =(VIN -VOUT )xVOUT /(L1xFSW xVIN)

Where, Fsw is switching frequency.

Therefore, maximum output current can be calculated by:

OUT_MAX =ILIM -0.5xIIND_PP

For the worst case calculation, the following parameters

should be used:

SW (Lowest switching frequency): 1.8MHz

LIM (Lowest current limit value): 985mA

L1 (Lowest inductor value): refer to inductor data-sheet.

Note that inductance will drop with DC bias current and

temperature. The worst case is typically at 85˚C.

For example, V

IN = 4.2V, VOUT = 3.2V, L1 = 2.0µH (Induc-

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

SW =

1.8MHz , I

LIM = 985mA.

IND_PP = 212mA

OUT_MAX = 985 – 106 = 876mA

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

account when calculating maximum output current. Special

attention 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, tran-

sient response requirements may not be met for output

current greater than 650mA.

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 recom-

mended. The lower limit of acceptable inductance is 1.7µH

at 1200mA over the operating temperature range. Full atten-

tion should be paid to this limit, because some small induc-

tors show large inductance drops at high DC bias. These

cannot be used with the LM3208. FDK MIPW3226D3R0M is

an example of an inductor with the lowest acceptable limit

(as of Oct./05).Table 1 suggests some inductors and suppli-

ers.

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