LM3208 datasheet(13/17 Pages) NSC

13 / 17 page

Operation Description (Continued)

Circuit Operation

Referring to Figure 1 and Figure 2, the LM3208 operates as

follows. During the first part of each switching cycle, the

control block in the LM3208 turns on the internal PFET

(P-channel MOSFET) switch. This allows current to flow

from the input through the inductor to the output filter capaci-

tor and load. The inductor limits the current to a ramp with a

slope of around (V

IN -VOUT) / L, by storing energy in a

magnetic field. During the second part of each cycle, the

controller turns the PFET switch off, blocking current flow

from the input, and then turns the NFET (N-channel MOS-

FET) synchronous rectifier on. In response, the inductor’s

magnetic field collapses, generating a voltage that forces

current from ground through the synchronous rectifier to the

output filter capacitor and load. As the stored energy is

transferred back into the circuit and depleted, the inductor

current ramps down with a slope around V

OUT / L. The

output filter capacitor stores charge when the inductor cur-

rent is high, and releases it when low, smoothing the voltage

across the load.

The output voltage is regulated by modulating the PFET

switch on time to control the average current sent to the load.

The effect is identical to sending a duty-cycle modulated

rectangular wave formed by the switch and synchronous

rectifier at SW to a low-pass filter formed by the inductor and

output filter capacitor. The output voltage is equal to the

average voltage at the SW pin.

While in operation, the output voltage is regulated by switch-

ing at a constant frequency and then modulating the energy

per cycle to control power to the load. Energy per cycle is set

by modulating the PFET switch on-time pulse width to con-

trol the peak inductor current. This is done by comparing the

signal from the current-sense amplifier with a slope compen-

sated error signal from the voltage-feedback error amplifier.

At the beginning of 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 synchronous 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 increase in the load.

Before appearing at the PWM comparator, a slope compen-

sation ramp from the oscillator is subtracted from the error

signal for stability of the current feedback loop. The minimum

on time of PFET is 55ns (typ.)

Shutdown Mode

Setting the EN digital pin low (<0.5V) places the LM3208 in

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

switch, NFET synchronous rectifier, reference voltage

source, control and bias circuitry of the LM3208 are turned

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

EN should be set low to turn off the LM3208 during power-up

and under voltage conditions when the power supply is less

than the 2.7V minimum operating voltage. The LM3208 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 LM3208 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 an 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.

DSON(P) Management

The LM3208 has a unique R

DSON(P) management function to

improve efficiency in the low output current region up to

100mA. When the V

CON voltage is less than 0.40V (typ.), the

20166336

FIGURE 3. Typical Operating System Circuit

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