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FIGURE 3. Typical Operating System Circuit

Circuit Operation

Referring to Figure 1 and Figure 2, the LM3218 operates as

follows: During the first part of each switching cycle, the con-

trol block in the LM3218 turns on the internal PFET (P-

channel MOSFET) switch. This allows current to flow from the

input through the inductor to the output filter capacitor and

load. The inductor limits the current to a ramp with a slope of

around (V

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 MOSFET) synchronous rectifier

on. In response, the inductor’s magnetic field collapses, gen-

erating 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

when the inductor current 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 rect-

angular wave formed by the power MOSFET switch and

synchronous rectifier to a low-pass filter formed by the induc-

tor and output filter capacitor. The output voltage is equal to

the average voltage at the terminal of the power MOSFET

inverter.

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 control

the peak inductor current. This is done by comparing the sig-

nal from the current-sense amplifier with a slope compensat-

ed 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 in-

creases the average current sent to the output and adjusts for

the increase in the load. Before appearing at the PWM com-

parator, a slope compensation ramp from the oscillator is

subtracted from the error signal for stability of the current

feedback loop. The minimum on time of PFET is 55 ns (typ.)

Shutdown Mode

Setting the EN digital pin low (<0.5V) places the LM3218 in

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

switch, NFET synchronous rectifier, reference voltage

source, control and bias circuitry of the LM3218 are turned

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

EN should be set low to turn off the LM3218 during power-up

and under-voltage conditions when the power supply is less

than the 2.7V minimum operating voltage. The LM3218 is de-

signed 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 LM3218 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, elim-

inating the need for an external diode.

R

The LM3218 has a unique R

improve efficiency in the low output current region up to 100

mA. When the V

vice uses only a small part of the PFET to minimize drive loss

of the PFET. When V

tire PFET is used to minimize R

has about 20 mV (typ.) of hysteresis.

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