LM3243

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SNVS782C – OCTOBER 2010 – REVISED AUGUST 2015

7 Detailed Description

7.1 Overview

The LM3243 is a high-efficiency step-down DC-DC converter optimized to power the RF power amplifier (PA) in

cell phones, portable communication devices, or battery-powered RF devices with a single Li-Ion battery. It

operates in fixed-frequency pulse width modulation (PWM) mode for 2G transmissions (with MODE = LOW),

automatic mode transition between PFM and PWM mode for 3G/4G RF PA operation (with MODE = HIGH),

forced bypass mode (with BP = HIGH), or in shutdown mode (with EN = LOW).

The fixed-frequency PWM mode provides high efficiency and very low output voltage ripple. In PFM mode, the

converter operates with reduced switching frequencies and lower supply current to maintain high efficiencies.

The forced bypass mode allows the user to drive the output directly from the input supply through a bypass FET.

The shutdown mode turns the LM3243 off and reduces current consumption to 0.02 µA (typical).

In PWM and PFM modes of operation, the output voltage of the LM3243 can be dynamically programmed from

0.4 V to 3.6 V (typical) by adjusting the voltage on VCON. Current overload protection and thermal overload

protection are also provided.

The LM3243 was engineered with Active Current assist and analog Bypass (ACB). This unique feature allows

the converter to support maximum load currents of 2.5 A (minimum) while keeping a small footprint inductor and

meeting all of the transient behaviors required for operation of a multi-mode RF Power Amplifier. The ACB circuit

provides an additional current path when the load current exceeds 1.4 A (typical) or as the switcher approaches

dropout. Similarly, the ACB circuit allows the converter to respond with faster VCON output voltage transition

times by providing extra output current on rising and falling output edges. The ACB circuit also performs the

function of analog bypass. Depending upon the input voltage, output voltage and load current, the ACB circuit

automatically and seamlessly transitions the converter into analog bypass while maintaining output voltage

regulation and low output voltage ripple. Full bypass (100% duty cycle operation) will occur if the total dropout

The LM3243 device’s 16-pin DSBGA package is the best solution for space-constrained applications such as cell

phones and other hand-held devices. The high switching frequency, 2.7 MHz (typical) in PWM mode, reduces

the size of input capacitors, output capacitors and of the inductor. Use of a DSBGA package is best suited for

opaque case applications and requires special design considerations for implementation. (Refer to DSBGA

Package Assembly and Use.) Because the LM3243 does not implement UVLO, the system controller should set

EN = LOW during power-up and UVLO conditions. (Refer to Shutdown Mode).

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