LM2832 datasheet(11/27 Pages) NSC

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Applications Information

THEORY OF OPERATION

The LM2832 is a constant frequency PWM buck regulator IC

that delivers a 2.0A load current. The regulator has a preset

switching frequency of 1.6MHz or 3.0MHz. This high fre-

quency allows the LM2832 to operate with small surface

mount capacitors and inductors, resulting in a DC/DC con-

verter that requires a minimum amount of board space. The

LM2832 is internally compensated, so it is simple to use and

requires few external components. The LM2832 uses

current-mode control to regulate the output voltage. The

following operating description of the LM2832 will refer to the

Simplified Block Diagram (Figure 1) and to the waveforms in

Figure 2. The LM2832 supplies a regulated output voltage by

switching the internal PMOS control switch at constant fre-

quency and variable duty cycle. A switching cycle begins at

the falling edge of the reset pulse generated by the internal

oscillator. When this pulse goes low, the output control logic

turns on the internal PMOS control switch. During this on-

time, the SW pin voltage (V

SW) swings up to approximately

IN, and the inductor current (IL) increases with a linear

slope. I

L is measured by the current sense amplifier, which

generates an output proportional to the switch current. The

sense signal is summed with the regulator’s corrective ramp

and compared to the error amplifier’s output, which is pro-

portional to the difference between the feedback voltage and

REF. 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 (V

D)ofthe

Schottky catch diode. The regulator loop adjusts the duty

cycle (D) to maintain a constant output voltage.

SOFT-START

This function forces V

OUT to increase at a controlled rate

during start up. During soft-start, the error amplifier’s refer-

ence voltage ramps from 0V to its nominal value of 0.6V in

approximately 600 µs. This forces the regulator output to

ramp up in a controlled fashion, which helps reduce inrush

current.

OUTPUT OVERVOLTAGE PROTECTION

The over-voltage comparator compares the FB pin voltage

to a voltage that is 15% higher than the internal reference

REF. 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 regula-

tion.

UNDERVOLTAGE LOCKOUT

Under-voltage lockout (UVLO) prevents the LM2832 from

operating until the input voltage exceeds 2.73V (typ). The

UVLO threshold has approximately 430 mV of hysteresis, so

the part will operate until V

IN drops below 2.3V (typ). Hys-

teresis prevents the part from turning off during power up if

IN is non-monotonic.

CURRENT LIMIT

The LM2832 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

3.25A (typ), and turns off the switch until the next switching

cycle begins.

THERMAL SHUTDOWN

Thermal shutdown limits total power dissipation by turning

off the output switch when the IC junction temperature ex-

ceeds 165˚C. After thermal shutdown occurs, the output

switch doesn’t turn on until the junction temperature drops to

approximately 150˚C.

20197566

FIGURE 2. Typical Waveforms

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