LM3281 Datasheet, PDF (13/26 Pages) Texas Instruments – 3.3-V, 1.2-A, 6-MHz Miniature Step-Down DC-DC Converter

www.ti.com
LM3281
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 V_IN approaches V_OUT. 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.
Copyright © 2014, Texas Instruments Incorporated
Product Folder Links: LM3281
Submit Documentation Feedback
13