0

0

t

t

Inductor

Current

SW

Voltage

LM2833

SNVS505D – MAY 2008 – REVISED JANUARY 2009

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APPLICATION INFORMATION

THEORY OF OPERATION

The LM2833 is a constant frequency PWM buck regulator IC that delivers a 3.0A load current. The regulator is

available in preset switching frequencies of 1.5MHz or 3.0MHz. This high frequency allows the LM2833 to

operate with small surface mount capacitors and inductors, resulting in a DC/DC converter that requires a

minimum amount of board space. The LM2833 is internally compensated, therefore it is simple to use and

requires few external components. The LM2833 uses peak current-mode control to regulate the output voltage.

The following description of operation of the LM2833 will refer to the Typical Application Circuit, to the waveforms

in Figure 26 and simplified block diagram in Figure 25. The LM2833 supplies a regulated output voltage by

switching the internal PMOS power switch at a constant frequency 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

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 proportional

internal power switch turns off until the next switching cycle begins. During the switch off-time, the inductor

current discharges through the catch diode D1, which forces the SW pin to swing below ground by the forward

voltage.

Figure 26. SW Pin Voltage and Inductor Current Waveforms

SOFT-START/SHUTDOWN

The LM2833 has both enable and shutdown modes that are controlled by the EN pin. Connecting a voltage

source greater than 1.8V to the EN pin enables the operation of the LM2833, while reducing this voltage below

0.4V places the part in a low quiescent current (300nA typical) shutdown mode. There is no internal pull-up on

EN pin, therefore an external signal is required to initiate switching. Do not allow this pin to float or rise to 0.3V

than UVLO, there is 15µs delay before switching starts. During this delay the LM2833 will go through a power on

reset state after which the internal soft-start process commences. During soft-start, the error amplifier’s reference

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

ramp up in a controlled fashion, which helps reduce inrush current seen at the input and minimizes output

voltage overshoot.

of the LM2833 whenever the input voltage is applied. However, when an input voltage of slow rise time is used to

power the application and if both the input voltage and the output voltage are not fully established before the soft-

start time elapses, the control circuit will command maximum duty cycle operation of the internal power switch to

bring up the output voltage rapidly. When the feedback pin voltage exceeds 0.6V, the duty cycle will have to

10

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