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1-A SIMPLE STEP-DOWN SWITCHING VOLTAGE REGULATOR
SLVS569E – JANUARY 2005 – REVISED JANUARY 2006
APPLICATION INFORMATION (continued)
Output Voltage Ripple and Transients
As with any switching power supply, the output of the LM2575 has a sawtooth-ripple voltage at the switching
frequency. Typically about 1% of the output voltage, this ripple is due mainly to the inductor sawtooth-ripple
current and the ESR of the output capacitor (see note on C
OUT
). Furthermore, the output also may contain small
voltage spikes at the peaks of the sawtooth waveform. This is due to the fast switching of the output switch and
the parasitic inductance of C
OUT
. These voltage spikes can be minimized through the use of low-inductance
capacitors.
There are several ways to reduce the output ripple voltage: a larger inductor, a larger C
OUT
, or both. Another
method is to use a small LC filter (20
µH
and 100
µF)
at the output. This filter can reduce the output ripple
voltage by a factor of 10 (see
Feedback Connection
FEEDBACK must be connected between the two programming resistors. Again, both of these resistors should be
in close proximity to the regulator, and each should be less than 100 kΩ to minimize noise pickup.
ON/OFF Input
ON/OFF should be grounded or be a low-level TTL voltage (typically <1.6 V) for normal operation. To shut down
the LM2575 and put it in standby mode, a high-level TTL or CMOS voltage should be supplied to this pin.
ON/OFF should not be left open and safely can be pulled up to V
IN
with or without a pullup resistor.
Grounding
The power and ground connections of the LM2575 must be low impedance to help maintain output stability. With
the 16-pin package, all the ground pins (including signal and power grounds) should be soldered directly to wide
PCB copper traces to ensure low-inductance connections and good thermal dissipation.
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