LM3205

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SNVS388E – JULY 2005 – REVISED APRIL 2013

While in operation, 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 current. This is done by comparing the signal from the current-sense

amplifier with a slope compensated 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 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 50ns (typ.)

SHUTDOWN MODE

Setting the EN digital pin low (<0.5V) places the LM3205 in a 0.01µA (typ.) Shutdown mode. During shutdown,

the PFET switch, NFET synchronous rectifier, reference voltage source, control and bias circuitry of the LM3205

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

EN should be set low to turn off the LM3205 during power-up and under voltage conditions when the power

supply is less than the 2.7V minimum operating voltage. The LM3205 is designed for compact portable

applications, such as mobile phones. In such applications, the system controller determines power supply

sequencing and requirements for small package size outweigh the additional size required for inclusion of UVLO

(Under Voltage Lock-Out) circuitry.

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.

Copyright © 2005–2013, Texas Instruments Incorporated

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