LM3243

SNVS782C – OCTOBER 2010 – REVISED AUGUST 2015

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Device Functional Modes (continued)

During the second portion of each cycle, the control block turns the PFET switch off, blocking current flow from

the input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the

The output filter capacitor stores charge when the inductor current is greater than the load current and releases it

when the inductor current is less than the load current, smoothing the voltage across the load.

At the next rising edge of the clock, the cycle repeats. An increase of load pulls the output voltage down,

increasing the error signal. As the error signal increases, the peak inductor current becomes higher, thus

increasing the average inductor current. The output voltage is therefore regulated by modulating the PFET switch

on-time to control the average current sent to the load. The circuit generates a duty-cycle modulated rectangular

signal that is averaged using a low pass filter formed by the inductor and output capacitor. The output voltage is

equal to the average of the duty-cycle modulated rectangular signal.

7.4.2 PFM Mode

With MODE = HIGH, the LM3243 automatically transitions to from PWM into PFM operation if the average

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

hysteresis window exists between PFM and PWM transitions.

After a transient event, the part temporarily operates in 2.7 MHz (typical) 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 (typical), then the part will

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

7.4.3 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 (typical)

2.7 MHz (typical)

Variable

Frequency at loads = 95 mA (typical)

2.7 MHz (typical)

2.7 MHz (typical)

2.5 × VCON

2.5 × VCON

75 mA (minimum in PFM) or 2.5 A (minimum in

Maximum load steady state

2.5 A (min.)

PWM)

7.4.4 Shutdown Mode

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

(typical) 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 undervoltage lock-out (UVLO) 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.

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