LM3262

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SNVS875O – AUGUST 2012 – REVISED DECEMBER 2015

Feature Description (continued)

7.3.2 Internal Synchronous Rectification

While in PWM mode, the LM3262 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 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.

7.3.3 Current Limiting

The current limit feature allows the LM3262 to protect itself and external components during overload conditions.

In PWM mode, the cycle-by-cycle current limit is a 1450 mA (typical). If an excessive load pulls the output

voltage down to less than 0.3 V (typical), the NFET synchronous rectifier is disabled, and the current limit is

reduced to 530 mA (typical). Moreover, when the output voltage becomes less than 0.15 V (typical), the

switching frequency will decrease to 3 MHz, thereby preventing excess current and thermal stress.

7.3.4 Dynamically Adjustable Output Voltage

The LM3262 features dynamically adjustable output voltage to eliminate the need for external feedback resistors

by controlling this voltage using the analog VCON pin. The input impedance of this pin can be approximated by a

series 50-K

Ω resistor and 10-pF capacitor. The output can be set from 0.4 V to 3.6 V by changing the voltage on

the analog VCON pin. This feature 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 for further details. The LM3262 moves into pulse-skipping mode when duty cycle is

over approximately 92% or less than approximately 15%, and the output voltage ripple increases slightly.

7.3.5 Thermal Overload Protection

The LM3262 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. 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.

7.3.6 Soft Start

The LM3262 has a soft-start circuit that limits in-rush current during start-up. During start-up, the switch current

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