Texas Instruments LM2833Z LLP - 3MHz 3.0A Step-Down DC-DC Switching Regulator Evaluation Module LM2833ZSDEVAL LM2833ZSDEVAL Datenbogen
Produktcode
LM2833ZSDEVAL
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
reduction of duty cycle and this can result in a transient in output voltage for a short duration, as shown in
. In applications where this output voltage overshoot is undesirable, one simple solution is to add a
feed-forward capacitor (C
FF
) 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
overshoot to an insignificant level during startup, as shown in
. Another more effective solution is to
control EN pin voltage by a separate logic signal, and pull the signal high only after V
IN
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
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
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
(LM2833Z) to the nominal fixed value, as the feedback voltage increases (see
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.
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
IN
Figure 28. Startup Response to V
IN
with C
FF
Copyright © 2008–2013, Texas Instruments Incorporated
11
Product Folder Links: