SYSTEM

CONTROLLER

LM3205

0.8V to 3.6V

L1

3.3 PH

C1*

10 PF

C2

4.7 PF

SW

FB

EN

PGND

SGND

DAC

ON/OFF

2.7V to 5.5V

LM3205

SNVS388E – JULY 2005 – REVISED APRIL 2013

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OPERATION DESCRIPTION

The LM3205 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 current-mode buck architecture, with synchronous rectification for high efficiency. It is designed for a

maximum load capability of 650mA in PWM mode.

Maximum load range may vary from this depending on input voltage, output voltage and the inductor chosen.

Efficiency is typically around 96% for a 400mA load with 3.4V output, 4.2V input. The output voltage is

dynamically programmable from 0.8V (typ.) to 3.6V (typ.) by adjusting the voltage on the control pin 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 LM3205 is constructed using a chip-scale 8-pin DSBGA or 10-pin WSON package. These packages 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 (2MHz) reduces the size of external components. As

shown in Figure 1, 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 in

the Applications Information 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 in the Device Information section.)

Figure 31. Typical Operating System Circuit

CIRCUIT OPERATION

Referring to Figure 1 and Figure 30, the LM3205 operates as follows. During the first part of each switching

cycle, the control block in the LM3205 turns on the internal PFET (P-channel MOSFET) 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 NFET (N-channel MOSFET) 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

high, and releases it 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.

12

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