LM3242

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SNOSB48E – OCTOBER 2011 – REVISED AUGUST 2015

Feature Description (continued)

7.3.2 Internal Synchronous Rectification

While in PWM mode, the LM3242 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 LM3242 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 0.3V (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 0.15V (typical), the switching

frequency decreases to 3 MHz, thereby preventing excess current and thermal stress.

7.3.4 Dynamically Adjustable Output Voltage

The LM3242 features dynamically adjustable output voltage to eliminate the need for external feedback resistors.

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 in Application

and Implementation for further details. The LM3242 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 LM3242 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 LM3242 has a soft-start circuit that limits in-rush current during start-up. During start-up the switch current

7.4 Device Functional Modes

Table 1. Description Of Modes

MODE

EN

BPEN

VCON

Shutdown

0

X

X

X

Sleep

1

0

< 80 mV

X

Pulse Width Modulation (PWM)

1

0

> 100 mA

Economy (ECO)

1

0

< 50 mA

Bypass (BP)

1

0

X

1

1

X

X

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