LM2831XMFSLASHNOPB datasheet(10/38 Pages) TI

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VIN

TON

Inductor

Current

D = TON/TSW

VSW

TOFF

TSW

IPK

Voltage

LM2831

SNVS422D – AUGUST 2006 – REVISED SEPTEMBER 2015

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Feature Description (continued)

is proportional to the difference between the feedback voltage and VREF. When the PWM comparator output goes

high, the output switch turns off until the next switching cycle begins. During the switch off-time, inductor current

discharges through the Schottky catch diode, which forces the SW pin to swing below ground by the forward

voltage (VD) of the Schottky catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant

output voltage.

Figure 18. Typical Waveforms

7.3.2 Soft Start

This function forces VOUT to increase at a controlled rate during start up. During soft start, the error amplifier’s

reference voltage ramps from 0 V to its nominal value of 0.6 V in approximately 600 µs. This forces the regulator

output to ramp up in a controlled fashion, which helps reduce inrush current.

7.3.3 Output Overvoltage Protection

The overvoltage comparator compares the FB pin voltage to a voltage that is 15% higher than the internal

reference VREF. Once the FB pin voltage goes 15% above the internal reference, the internal PMOS control

switch is turned off, which allows the output voltage to decrease toward regulation.

7.3.4 Undervoltage Lockout

Undervoltage lockout (UVLO) prevents the LM2831 from operating until the input voltage exceeds 2.73 V

(typical). The UVLO threshold has approximately 430 mV of hysteresis, so the part will operate until VIN drops

below 2.3 V (typical). Hysteresis prevents the part from turning off during power up if VIN is non-monotonic.

7.3.5 Current Limit

The LM2831 uses cycle-by-cycle current limiting to protect the output switch. During each switching cycle, a

current limit comparator detects if the output switch current exceeds 2.5 A (typical), and turns off the switch until

the next switching cycle begins.

7.3.6 Thermal Shutdown

Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature

exceeds 165°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature

drops to approximately 150°C.

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