Operating Mode Selection Control

The BYP digital input pin is used to select between PWM/

Auto-bypass and Bypass operating mode. Setting BYP pin

device in PWM/Auto-bypass mode.

Bypass and PWM operation overlap during the transition

between the two modes. This transition time is approxi-

mately 31µs when changing from PWM to Bypass mode,

and 15µs when changing from Bypass to PWM mode. This

helps prevent under or overshoots during the transition pe-

riod between PWM and Bypass modes.

Shutdown Mode

a 0.1µA (typ.) Shutdown mode. During shutdown, the PFET

switch, NFET synchronous rectifier, reference voltage

source, control and bias circuitry of the LM3204 are turned

EN should be set low to turn off the LM3204 during power-up

and under voltage conditions when the power supply is less

than the 2.7V minimum operating voltage. The LM3204 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 benefit of including UVLO (Under

Voltage Lock-Out) circuitry.

Dynamically Adjustable Output

Voltage

The LM3204 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

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.

Over Voltage Protection

The LM3204 has an over voltage comparator that prevents

the output voltage from rising too high, when the device is

left in PWM mode under light-load conditions, during output

voltage steps, or during startup. When the output voltage

rises to 330mV over its target, the OVP comparator inhibits

PWM operation to skip pulses until the output voltage returns

to the target. During the over voltage protection mode, both

the PWM PFET and the NFET synchronous rectifier are off.

When the part comes out of the over voltage protection

mode, the NFET synchronous rectifier remains off for ap-

proximately 3.5µs to avoid inductor current going negative.

Internal Synchronous Rectification

While in PWM mode, the LM3204 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.

With medium and heavy loads, the internal NFET synchro-

nous 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. There is no zero

cross detect, which means that the NFET can conduct cur-

rent in both directions and inductor current is always con-

tinuous. The advantage of this method is that the part re-

mains in PWM mode at light loads or no load conditions. The

NFET has a current limit. The NFET is designed to conduct

through its intrinsic body diode during transient intervals

before it turns on, eliminating the need for an external diode.

Current Limiting

A current limit feature allows the LM3204 to protect itself and

external components during overload conditions. In PWM

mode, a 940mA (max.) cycle-by-cycle current limit is nor-

mally used. If an excessive load pulls the output voltage

down to below approximately 0.375V, indicating a possible

short to ground, 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 comparator trips, and the

beginning of the next cycle is inhibited for 3.5µs to force the

instantaneous inductor current to ramp down to a safe value.

After the 3.5µs interval, the internal PFET is turned on again.

This cycle is repeated until the load is reduced and the

output voltage exceeds approximately 0.375V. Therefore,

the device may not startup if an excessive load is connected

to the output when the device is enabled. The synchronous

rectifier is off in the timed current limit mode. Timed current

limit prevents the loss of current control seen in some prod-

ucts when the output voltage is pulled low in serious over-

load conditions.

A current limit is also provided for the NFET. This is approxi-

mately −500mA. Both the NFET and the PFET are turned off

in negative current limit until the PFET is turned on again at

the beginning of the next cycle. The negative current limit

inhibits buildup of excessive negative inductor current.

In the Bypass mode, the bypass current limit is 1000mA

(typ.). The output voltage drops when the bypass current

limit kicks in.

Thermal Overload Protection

The LM3204 has a thermal overload protection function to

protect the device from short-term misuse and overload

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, and the Bypass PFET is

turned off in Bypass mode. When the temperature drops

below 130˚C, normal operation resumes. Prolonged opera-

tion in thermal overload conditions may damage the device.

Application Information

SETTING THE OUTPUT VOLTAGE

The LM3204 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

formula:

V

When V

age will follow proportionally by 3 times of V

If V

tion may occur because of insufficient slope compensation.

www.national.com

13