LM1575, LM2575-N, LM2575HV
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SNVS106E –MAY 1999–REVISED APRIL 2013
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
The output voltage of a switching power supply will contain a sawtooth ripple voltage at the switcher frequency,
typically about 1% of the output voltage, and may also contain short voltage spikes at the peaks of the sawtooth
waveform.
The output ripple voltage is due mainly to the inductor sawtooth ripple current multiplied by the ESR of the output
capacitor. (See INDUCTOR SELECTION)
The voltage spikes are present because of the fast switching action of the output switch, and the parasitic
inductance of the output filter capacitor. To minimize these voltage spikes, special low inductance capacitors can
be used, and their lead lengths must be kept short. Wiring inductance, stray capacitance, as well as the scope
probe used to evaluate these transients, all contribute to the amplitude of these spikes.
An additional small LC filter (20 μH & 100 μF) can be added to the output (as shown in Figure 37) to further
reduce the amount of output ripple and transients. A 10 × reduction in output ripple voltage and transients is
possible with this filter.
FEEDBACK CONNECTION
The LM2575 (fixed voltage versions) feedback pin must be wired to the output voltage point of the switching
power supply. When using the adjustable version, physically locate both output voltage programming resistors
near the LM2575 to avoid picking up unwanted noise. Avoid using resistors greater than 100 kΩ because of the
increased chance of noise pickup.
ON /OFF INPUT
For normal operation, the ON /OFF pin should be grounded or driven with a low-level TTL voltage (typically
below 1.6V). To put the regulator into standby mode, drive this pin with a high-level TTL or CMOS signal. The
ON /OFF pin can be safely pulled up to +VIN without a resistor in series with it. The ON /OFF pin should not be
left open.
GROUNDING
To maintain output voltage stability, the power ground connections must be low-impedance (see Figure 26). For
the TO-3 style package, the case is ground. For the 5-lead TO-220 style package, both the tab and pin 3 are
ground and either connection may be used, as they are both part of the same copper lead frame.
With the CDIP or SOIC packages, all the pins labeled ground, power ground, or signal ground should be
soldered directly to wide printed circuit board copper traces. This assures both low inductance connections and
good thermal properties.
HEAT SINK/THERMAL CONSIDERATIONS
In many cases, no heat sink is required to keep the LM2575 junction temperature within the allowed operating
range. For each application, to determine whether or not a heat sink will be required, the following must be
identified:
1. Maximum ambient temperature (in the application).
2. Maximum regulator power dissipation (in application).
3. Maximum allowed junction temperature (150°C for the LM1575 or 125°C for the LM2575). For a safe,
conservative design, a temperature approximately 15°C cooler than the maximum temperature should be
selected.
4. LM2575 package thermal resistances θJA and θJC.
Total power dissipated by the LM2575 can be estimated as follows:
PD = (VIN) (IQ) + (VO/VIN) (ILOAD) (VSAT
)
where
•
•
•
•
IQ (quiescent current) and VSAT can be found in the Characteristic Curves shown previously,
VIN is the applied minimum input voltage,
VO is the regulated output voltage
and ILOAD is the load current.
(10)
19
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