Circuit Operation

Referring to Figure 1, the LM3204 operates as follows. Dur-

ing the first part of each switching cycle, the control block in

the LM3204 turns on the internal PFET (P-channel MOS-

FET) 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 col-

lapses, 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

charge when the inductor current is going high, and releases

it when inductor current is going 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.

PWM Mode

While in PWM (Pulse Width Modulation) mode, the output

voltage is regulated by switching 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 cur-

rent. This is done by comparing the PFET drain current to a

slope-compensated reference current generated by the 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. The minimum on-

time of PFET in PWM mode is 50ns (typ.).

Bypass Mode

The LM3204 contains an internal PFET switch for bypassing

the PWM DC-DC converter during Bypass mode. In Bypass

mode, this PFET is turned on to power the PA directly from

the battery for maximum RF output power. Bypass mode is

more efficient than operating in PWM mode at 100% duty

cycle because the resistance of the bypass PFET is less

than the series resistance of the PWM PFET and inductor.

This translates into higher voltage available on the output in

Bypass mode, for a given battery voltage. The part can be

placed in bypass mode by sending BYP pin high. This is

called Forced Bypass Mode and it remains in bypass mode

until BYP pin goes low.

Alternatively the part can go into Bypass mode automatically.

This is called Auto-bypass mode or Automatic Bypass mode.

The bypass switch turns on when the difference between the

input voltage and programmed output voltage is less than

250mV (typ.) for more than the bypass delay time of 15µs

(typ.). The bypass switch turns off when the input voltage is

higher than the programmed output voltage by 450mV (typ.)

for longer than the bypass delay time. The bypass delay time

is provided to prevent false triggering into Automatic Bypass

mode by either spikes or dips in V

system resource friendly in that the Bypass PFET is turned

on automatically when the input voltage gets close to the

output voltage, typical scenario of a discharging battery. It is

also turned off automatically when the input voltage rises,

typical scenario of a charger connected. Another scenario

could be changes made to V

PFET to turn on and off automatically. It is recommended to

connect BYPOUT pin directly to the output capacitor with a

separate trace and not to the FB pin.

If the V

FET is turned off.

20138936

FIGURE 1. Typical Operating System Circuit

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