LM3208

SNVS404B – APRIL 2006 – REVISED MARCH 2013

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Internal Synchronous Rectification

While in PWM mode, the LM3208 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, eliminating the need for an external

diode.

threshold has about 25mV (typ.) of hysteresis.

Current Limiting

A current limit feature allows the LM3208 to protect itself and external components during overload conditions. In

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. 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 LM3208 features dynamically adjustable output voltage to eliminate the need for external feedback resistors.

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. The LM3208 moves into Pulse Skipping mode when duty cycle is over

92% and the output voltage ripple increases slightly.

Thermal Overload Protection

The LM3208 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 LM3208 features a pin-controlled variable output voltage to eliminate the need for external feedback

as in the following formula:

(1)

14

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