LM3243

SNVS782B – OCTOBER 2010 – REVISED FEBRUARY 2013

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PFM Mode

With MODE = HIGH, the LM3243 automatically transitions to from PWM into PFM (Pulse Frequency Modulation)

the fixed output voltage by transferring a fixed amount of energy during each cycle and modulating the frequency

to control the total power delivered to the output. The converter switches only as needed to support the demand

of the load current, therefore maximizing efficiency. If the load current should increase during PFM mode to more

than 95 mA (typ.), the part will automatically transition into constant frequency PWM mode. A 20 mA (typ.)

hysteresis window exists between PFM and PWM transitions.

After a transient event, the part temporarily operates in 2.7 MHz (typ.) fixed-frequency PWM mode to quickly

charge or discharge the output. This is true for start-up conditions or if MODE pin is toggled LOW-to-HIGH. Once

the output reaches its target output voltage, and the load is less than 75 mA (typ.), then the part will seamlessly

transition into PFM mode (assuming it is not in forced bypass or auto bypass condition).

Active Current Assist and Analog Bypass (ACB)

The 3GPP time mask requirement for 2G requires high current to be sourced by the LM3243. These high

currents are required for a small time during transients or under a heavy load. Over-rating the switching inductor

for these higher currents would increase the solution size and will not be an optimum solution. So to allow an

optimal inductor size for such a load, an alternate current path is provided from the input supply through the ACB

current required to support the load. The ACB circuit also minimizes the dropout voltage by having the analog

4.0A (max.) peak current limit.

Bypass Operation

When BP = LOW the part will be in automatic bypass mode which will automatically determine the amount of

bypass needed to maintain voltage regulation. When the input supply voltage to the LM3243 is lowered to a level

where the commanded duty cycle is higher than what the converter is capable of providing, the part will go into

pulse-skipping mode. The switching frequency will be reduced to maintain a low and well-behaved output voltage

ripple. The analog bypass circuit will allow the converter to stay in regulation until full bypass is reached (100%

duty cycle operation). The converter comes out of full bypass and back into analog bypass regulation mode with

a similar reverse process.

Shutdown Mode

To shut down the LM3243 pull the EN pin LOW (<0.5V). In shutdown mode, the current consumption is 0.02 µA

(typ.) and the PFET switch, NFET synchronous rectifier, reference voltage source, control and bias circuit are

turned OFF. To enable LM3243 pull EN HIGH (>1.2V), and the mode of operation will be dependent on the

voltage applied to the MODE pin.

Since the LM3243 does not feature a UVLO (Under Voltage Lock-Out) circuit, the EN pin should be set LOW to

turn off the LM3243 during power-up and during UVLO conditions. For cell-phone applications, the system

controller determines the power supply sequence; thus, it is up to the system controller to ensure proper

sequencing by using all of the available pins and functions properly.

Mode Pin

The MODE pin changes the state of the converter to one of the two allowed modes of operation. Setting the

MODE pin HIGH (>1.2V) sets the device for automatic transition between PFM/PWM mode operation. In this

mode, the converter operates in PFM mode to maintain the output voltage regulation at very light loads and

transitions into PWM mode at loads exceeding 95 mA (typ.). The PWM switching frequency is 2.7 MHz (typ.).

Setting the MODE pin LOW (<0.5V) sets the device for PWM mode operation. The switching operation is in

PWM mode only, and the switching frequency is also 2.7 MHz (typ.).

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