7.3.1 Small Solution Size

Solution size less than 7.5 mm² is possible using the LM3281 in combination with only three small passive components.

7.3.2 Automatic Analog Bypass with Low Dropout

An internal bypass transistor under analog control automatically engages as V_IN falls below the V_OUT target. Output stays regulated in analog bypass mode until full dropout. The parallel impedance of this additional bypass transistor with normal DC-DC output path reduces V_OUT voltage drop-out, maximizing V_OUT supply voltage to the load at low V_IN conditions. The analog implementation provides a smooth transition among regulation and bypass modes, avoiding V_OUT distortion.

7.3.3 Low I_Q

An ECOnomy (ECO) mode of operation draws 16 µA (typ.) quiescent current, permitting the LM3281 to be used in “always-on” applications. This low I_Q is achieved over the entire input supply range of 5.5 V to 2.6 V, irrespective of whether LM3281 is operating in regulation (ECO Mode or Analog Bypass mode) or in full dropout (full bypass).

7.3.4 Forced PWM Operation

ECO mode provides low I_Q while PWM mode optimizes output voltage ripple and transient performance. When high, the MODE pin permits automatic mode selection based on load current. When MODE is pulled low the LM3281 enters “forced PWM” operation with very low ripple and optimized transient response. Alternately, the MODE pin can be tied high in an application to allow the device to always select a mode of operation automatically.

7.3.5 High Maximum Current

Load current of 1.2 A is supported with short bursts (of < 100 µS with < 10% duty cycle) up to 1.4 A.

A wide variety of load current use cases are accommodated by the LM3281. Examples are described in Table 1 and Table 2: one for high ambient temperature all the time, and one for a more typical ambient temperature use case. Many alternate use case scenarios are available; please contact TI to discuss the load current relevant for a given application.

For the high ambient temperature of 85°C for the entire device operational lifetime, see Table 1:

For a more typical ambient temperature distribution of T_A ≤ 70°C for ≥ 80% of the operational lifetime and 70°C < T_A ≤ 85% for ≤ 20% of the operational lifetime, see Table 2:

7.3.6 High-Capacitance Load and Line Transient Performance

The LM3281 is internally compensated to drive loads with large bypass capacitance, including transceiver modules, without sacrificing transient performance. Please reference Total Effective Output Capacitance (COUT + CLOAD1 + CLOAD2) regarding output capacitance requirements.

7.3.7 Soft Start

During start-up a soft-start feature prevents high input current which could cause supply voltage bus drops and interfere with other subsystems sharing the supply bus. Soft start is especially valuable in applications where large load capacitance must be charged on start-up. Loading of the output during start-up condition will extend the soft-start time. Excessive loading may even prevent the output from reaching the target voltage, and the device may therefore stay in the soft-start condition indefinitely.

7.3.8 Thermal Overload Protection

The LM3281 device has a thermal overload protection that protects the device from short-term misuse and overload conditions. If the junction temperature exceeds 150°C, the LM3281 shuts itself down. Normal operation resumes after the temperature drops below 125°C. Prolonged operation in thermal overload condition may damage the device and is therefore not recommended.

7.3.9 Current Limit

The current limit feature allows the LM3281 to protect itself and external components during overload conditions. In PWM mode, the cycle-by-cycle current limit of the SW pin is 1.9-A peak, and the bypass current limit is 1.3 A.  Thus, the total current limit is 2.2 A (typ.).  During the start-up condition or when the output voltage is less than 0.34 V, the SW pin current limit is reduced to 0.85 A peak, and the bypass current is disabled.  If excessive load prevents the output from rising above 0.34 V for more than 40 μs, the LM3281 enters the short-circuit-protection state.

7.3.10 Power-On Reset

Some applications may require tying the EN pin directly to the VIN pin. For this reason, the LM3281 features a Power on Reset (POR) that ensures that the part will enter a deterministic state when power is first applied. When the EN pin is tied directly to the VIN pin, the input power supply needs to rise fast enough for the POR circuit to work properly. The V_IN voltage should not stay between 1.2 V and 2.6 V for longer than 20 µs. This is not required if the EN pin voltage remains below V_IL (below 0.2 V) until V_IN is at least at 2.6 V.

7.4.1 PWM Mode

When the LM3281 operates in PWM mode, the switching frequency is constant, and the switcher regulates the output voltage by changing the energy per cycle to support the load required. During the first portion of each switching cycle, the control block in the LM3281 turns on the internal PFET switch. This allows current to flow from the input through the inductor and to the output filter capacitor and load. The inductor limits the current to a ramp with a slope of (V_IN – V_OUT)/L, by storing energy in its magnetic field. 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 NFET and to the output filter capacitor and load, which ramps the inductor current down with a slope of –V_OUT/L. 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 Forced PWM (FPWM) Mode

To maintain high efficiency at lighter loads, LM3281 automatically goes into what is called ECO mode which has low I_Q but higher ripple compared to PWM mode. If an application requires very low ripple and/or fast transient response, LM3281 can be forced to operate in PWM mode even at lighter loads. When high, the MODE pin permits automatic PWM or ECO mode operation based on load current. When MODE is pulled low the LM3281 enters “forced PWM” operation with very low ripple and optimized transient response. If automatic PWM/ECO mode operation is desired, the MODE pin can be permanently tied high in an application to allow the device to always select a mode of operation automatically based on the load current conditions.

It should be noted that LM3281 transient performance is quite good in ECO mode, and it may not be necessary to operate in FPWM mode for transient performance reasons alone. Normally, FPWM operation is selected for lower output voltage ripple.

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.