LM3242

SNOSB48D – OCTOBER 2011 – REVISED MARCH 2013

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OPERATION DESCRIPTION

Device Information

The LM3242 is a simple, step-down DC-DC converter optimized for powering RF power amplifiers (PAs) in

mobile phones, portable communicators, and similar battery powered RF devices. It is designed to allow the RF

PA to operate at maximum efficiency over a wide range of power levels from a single Li-Ion battery cell. It is

based on a voltage-mode buck architecture, with synchronous rectification for high efficiency. It is designed for a

maximum load capability of 750 mA in PWM mode. Maximum load range may vary from this depending on input

voltage, output voltage and the inductor chosen.

There are five modes of operation depending on the current required: PWM (Pulse Width Modulation), ECO

(ECOnomy), BP (Bypass), Sleep, and Shutdown. (See Table 1.) The LM3242 operates in PWM mode at higher

load current conditions. Lighter loads cause the device to automatically switch into ECO mode. Shutdown mode

turns the device off and reduces battery consumption to 0.1 µA (typ.).

mA load with 3.3V output, 3.9V input. The output voltage is dynamically programmable from 0.4V to 3.6V by

adjusting the voltage on the control pin (VCON) without the need for external feedback resistors. This ensures

longer battery life by being able to change the PA supply voltage dynamically depending on its transmitting

power.

Additional features include current overload protection and thermal overload shutdown.

The LM3242 is constructed using a chip-scale 9-bump DSBGA package. This package offers the smallest

possible size, for space-critical applications such as cell phones, where board area is an important design

consideration. Use of a high switching frequency (6MHz, typ.) reduces the size of external components. As

shown in the Typical Application Circuit, only three external power components are required for implementation.

Use of a DSBGA package requires special design considerations for implementation. (See DSBGA Package

Assembly and Use section.) Its fine bump-pitch requires careful board design and precision assembly equipment.

Use of this package is best suited for opaque-case applications, where its edges are not subject to high-intensity

ambient red or infrared light. Also, the system controller should set EN low during power-up and other low supply

voltage conditions. (See Shutdown Mode below.)

Circuit Operation

Referring to the Typical Application Circuit and Figure 2, the LM3242 operates as follows. During the first part of

each switching cycle, the control block in the LM3242 turns on the internal top-side PFET switch. This allows

current to flow from the input through the inductor to the output filter capacitor and load. The inductor limits the

part of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and then turns

the bottom-side NFET synchronous rectifier on. In response, the inductor’s magnetic field collapses, generating a

voltage that forces current from ground through the synchronous rectifier to the output filter capacitor and load.

As the stored energy is transferred back into the circuit and depleted, the inductor current ramps down with a

when low, smoothing the voltage across the load.

The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the

load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and

synchronous rectifier at SW to a low-pass filter formed by the inductor and output filter capacitor. The output

voltage is equal to the average voltage at the SW pin.

PWM Mode Operation

While in PWM mode operation, the converter operates as a voltage-mode controller with input voltage feed

forward. This allows the converter to achieve excellent load and line regulation. The DC gain of the power stage

is proportional to the input voltage. To eliminate this dependence, feed forward inversely proportional to the input

voltage is introduced. While in PWM mode, the output voltage is regulated by switching at a constant frequency

and then modulating the energy per cycle to control power to the load. At the beginning of each clock cycle the

PFET switch is turned on and the inductor current ramps up until the comparator trips and the control logic turns

off the switch. The current limit comparator can also turn off the switch in case the current limit of the PFET is

exceeded. Then the NFET switch is turned on and the inductor current ramps down. The next cycle is initiated by

the clock turning off the NFET and turning on the PFET.

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