Datasheet

LM2833

www.ti.com

SNVS505E –MAY 2008–REVISED APRIL 2013

reduce from the maximum value accordingly, to maintain regulation. It takes a finite amount of time for this

reduction of duty cycle and this can result in a transient in output voltage for a short duration, as shown in

Figure 27. In applications where this output voltage overshoot is undesirable, one simple solution is to add a

feed-forward capacitor (C

) across the top feedback resistor R1 to speed Gm Amplifier recovery. In practice, a

27nF to 100nF ceramic capacitor is usually a good choice to remove the overshoot completely or limit the

overshoot to an insignificant level during startup, as shown in Figure 28. Another more effective solution is to

control EN pin voltage by a separate logic signal, and pull the signal high only after V

is fully established. In this

way, the chip can execute a normal, complete soft start process, minimizing any output voltage overshoot. Under

some circumstances at cold temperature, this approach may also be required to minimize any unwanted output

voltage transients that may occur when the input voltage rises slowly. For a fast rising input voltage (100µs for

example), there is no need to control EN separately or add a feed-forward capacitor since the soft-start can bring

up output voltage smoothly as shown in Figure 29.

During startup, the LM2833 gradually increases the switching frequency from 400kHz (LM2833X) or 800kHz

(LM2833Z) to the nominal fixed value, as the feedback voltage increases (see FREQUENCY FOLDBACK section

for more information). Since the internal corrective ramp signal adjusts its slope dynamically, and is proportional

to the switching frequency during startup, a larger output capacitance may be required to insure a smooth output

voltage rise, at low programmed output voltage and high output load current.

Figure 27. Startup Response to V

Figure 28. Startup Response to V

with C

Product Folder Links: LM2833