LM3243

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SNVS782B – OCTOBER 2010 – REVISED FEBRUARY 2013

Dynamic Adjustment of Output Voltage

The output voltage of the LM3243 can be dynamically adjusted by changing the voltage on the VCON pin. In RF

PA applications, peak power is required when the handset is far away from the base station. To maximize the

power savings, the LM3243 output should be set just high enough to achieve the desired PA linearity. Hence,

during low-power requirements, reduction of supply voltage to the PA can reduce power consumption from the

PA, making the operation more efficient and promote longer battery life. Please refer to the Setting the Output

Voltage section for further details.

Mode Selection

Table 1 shows the LM3243 parameters for the given modes (PWM or PFM/PWM).

Table 1. Parameters under Different Modes

Parameter/Mode

PWM

PFM/PWM

MODE pin

LOW

HIGH

BP pin

LOW

LOW

Frequency at loads = 75 mA (typ.)

2.7 MHz (typ.)

Variable

Frequency at loads = 95 mA (typ.)

2.7 MHz (typ.)

2.7 MHz (typ.)

2.5 x VCON

2.5 x VCON

Max Load Steady State

2.5A (min.)

75 mA (min in PFM) or 2.5A (min. in PWM)

Internal Synchronous Rectification

The LM3243 uses an internal NFET as a synchronous rectifier to reduce rectifier forward voltage drop, thus

increasing efficiency. The reduced forward voltage drop in the internal NFET synchronous rectifier significantly

improves efficiency for low output voltage operation. The NFET is designed to conduct through its intrinsic body

diode during the transient intervals, eliminating the need of an external diode.

Current Limit

The LM3243 current limit feature protects the converter during current overload conditions. Both SW and ACB

pins have positive and negative current limits. The positive and negative current limits bound the SW and ACB

2.1 max.) limits the maximum peak inductor current possible. The output voltage will fall out of regulation only

Timed Current Limit

If the load or output short circuit pulls the output voltage to 0.3V or lower and the peak inductor current sustains

internal PFET switch is turned off. After approximately 30 µs, the device will return to the normal operation.

Thermal Overload Protection

The LM3243 IC has a thermal overload protection that protects itself from short-term misuse and overload

conditions. If the junction temperature exceeds 150°C, the LM3243 shuts down. Normal operation resumes after

the temperature drops below 130°C. Prolonged operation in thermal overload condition may damage the device

and is therefore not recommended.

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