LM3281

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SNVSA38 – NOVEMBER 2014

Device Functional Modes (continued)

7.4.3 Analog Bypass Mode

The LM3281 contains an internal BPFET (Bypass FET) transistor connected from the battery directly to the

output for bypassing the PWM DC-DC converter when VIN approaches VOUT. In Analog Bypass mode, this

BPFET is turned on just enough for the PWM DC-DC to maintain regulation by providing a parallel path from the

battery directly to the load for maximum usable battery range and extended operating time while maintaining

regulation. When the part is in dropout and is operating in full bypass mode, the output voltage will be the input

voltage less the voltage drop across the resistance of the BPFET in parallel with the PFET + Switch Inductor.

Analog Bypass mode is more efficient than operating in PWM mode at 100% duty cycle because the combined

resistance of the circuit is significantly less than the series resistance of just the PWM PFET and inductor. This

translates into higher voltage available at the output in Analog Bypass mode for a given battery voltage. The

bypass operation is very system resource friendly in that the bypass PFET is gradually turned on automatically

when the input voltage gets close to the output voltage (while always maintaining regulation), a typical scenario

of a discharging battery. Likewise, it is also automatically gradually turned off when the input voltage rises, a

typical scenario when connecting a charger.

7.4.4 ECO (Economy) Mode

At light load current, the converter enters ECO mode operation with reduced quiescent supply current to maintain

high efficiency. During ECO mode operation, a switching burst brings the output just above target voltage. This

period of switching is followed by no switching in which the output coasts to just below target voltage, and then

this cycle is repeated. The frequency of how often the switching burst occurs is dependent on the load current.

PWM operation resumes once the load current reaches a specific threshold.

7.4.5 Shutdown Mode

Setting the EN digital input pin low (< 0.4 V) places the LM3281 in Shutdown mode where it consumes less than

0.1

μA current typically. In shutdown, the PFET switch, the NFET synchronous rectifier, the BPFET, reference

voltage source, control, and bias circuitry of the LM3281 are turned off. Setting EN high (> 1.2 V) enables normal

operation.

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