LM3205

SNVS388D – JULY 2005 – REVISED MAY 2009

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INTERNAL SYNCHRONOUS RECTIFICATION

While in PWM mode, the LM3205 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.

With medium and heavy loads, 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,

eliminating the need for an external diode.

CURRENT LIMITING

A current limit feature allows the LM3205 to protect itself and external components during overload conditions. In

PWM mode, an 1200mA (max.) cycle-by-cycle current limit is normally 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 comparator 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

seen in some products when the output voltage is pulled low in serious overload conditions.

DYNAMICALLY ADJUSTABLE OUTPUT VOLTAGE

The LM3205 features dynamically adjustable output voltage to eliminate the need for external feedback resistors.

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 Output Voltage in the Application

Information section for further details.

THERMAL OVERLOAD PROTECTION

The LM3205 has a thermal overload protection function that operates to protect itself 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. When the temperature drops below 125°C, normal

operation resumes. Prolonged operation in thermal overload conditions may damage the device and is

considered bad practice.

APPLICATION INFORMATION

SETTING THE OUTPUT VOLTAGE

The LM3205 features a pin-controlled variable output voltage 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

(1)

the required on-time being less than the minimum on-time (50ns). The output voltage can go lower than 0.8V

curve is for a typical part and there could be part-to-part variation for output voltages less than 0.8V over the

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