LM3281

SNVSA38 – NOVEMBER 2014

www.ti.com

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 VIN 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 VIL (below 0.2 V) until VIN is at least at 2.6 V.

7.4 Device Functional Modes

The LM3281 includes five steady-state modes of operation depending on MODE, VIN, and ILOAD conditions: PWM

(Pulse Width Modulation), Forced PWM, ECO (ECOnomy), Analog Bypass, and Shutdown. Two protection

mechanisms include current limiting and thermal overload protection. Finally, soft-start operation is active to

prevent excessive input current only when the part is first enabled.

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 (VIN – VOUT)/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 –VOUT/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 IQ 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.

12

Submit Documentation Feedback

Copyright © 2014, Texas Instruments Incorporated

Product Folder Links: LM3281