LM3207 datasheet(15/19 Pages) NSC

15 / 19 page

PWM Operation (Continued)

each cycle, the clock turns on the PFET switch, causing the

inductor current to ramp up. When the current sense signal

ramps past the error amplifier signal, the PWM comparator

turns off the PFET switch and turns on the NFET synchro-

nous rectifier, ending the first part of the cycle. If an increase

in load pulls the output down, the error amplifier output

increases, which allows the inductor current to ramp higher

before the comparator turns off the PFET. This increases the

average current sent to the output and adjusts for the in-

crease in the load. Before appearing at the PWM compara-

tor, a slope compensation ramp from the oscillator is sub-

tracted from the error signal for stability of the current

feedback loop. The minimum on time of PFET in PWM mode

is 50ns (typ.)

Shutdown Mode

Setting the EN digital pin low (<0.5V) places the LM3207 in

a 0.01µA (typ.) Shutdown mode. During shutdown, the PFET

switch, NFET synchronous rectifier, reference voltage

source, control and bias circuitry of theLM3207 are turned

off. Setting EN high (>1.2V) enables normal operation.

EN should be set low to turn off the LM3207 during power-up

and under voltage conditions when the power supply is less

than the 2.7V minimum operating voltage. The LM3207 is

designed for compact portable applications, such as mobile

phones. In such applications, the system controller deter-

mines power supply sequencing and requirements for small

package size outweigh the additional size required for inclu-

sion of UVLO (Under Voltage Lock-Out) circuitry.

Internal Synchronous Rectification

While in PWM mode, the LM3207 uses an internal NFET as

a synchronous rectifier to reduce rectifier forward voltage

drop and associated power loss. Synchronous rectification

provides a significant improvement in efficiency whenever

the output voltage is relatively low compared to the voltage

drop across and ordinary rectifier diode.

The internal NFET synchronous rectifier is turned on during

the inductor current down slope in the second part of each

cycle. The synchronous rectifier is turned off prior to the next

cycle. The NFET is designed to conduct through its intrinsic

body diode during transient intervals before it turns on, elimi-

nating the need for an external diode.

Current Limiting

A current limit feature allows the LM3207 to protect itself and

external components during overload conditions. In PWM

mode, a 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 evident in some products when the

output voltage is pulled low in serious overload conditions.

Dynamically Adjustable Output

Voltage

The LM3207 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

CON pin. This feature is useful in

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 LM3207 has a thermal overload protection function that

operates to protect itself from short-term misuse and over-

load conditions. When the junction temperature exceeds

150˚C, the device inhibits operation. The PFET and NFET

are turned off in PWM mode. The LDO is turned off as well.

When the temperature drops below 130˚C, normal operation

resumes. Prolonged operation in thermal overload condi-

tions may damage the device and is considered bad prac-

tice.

LDO Operation

An LDO is used to provide a regulated 2.875V Vref supply to

a RF PA. The LDO can be enabled only after the PWM is

running. The LDO will automatically be disabled whenever

the EN or EN

LDO is disabled. Included in the LDO are active

charge and discharge circuits to quickly move a 100nF ca-

pacitor to meet the 3us timing requirements, or an 220nF

capacitor to meet the 5us timing requirements. The charging

and discharging currents are controlled to minimize supply

disturbances. The LM3207 was designed specifically to work

with a 100nF or a 220nF ceramic capacitor and no bypass

capacitor.

Application Information

SETTING THE DC-DC CONVERTER OUTPUT VOLTAGE

The LM3207 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

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 (50ns). 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 have part-to-part variation for output voltages less than

0.8V over the limited V

IN range.

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