AMS2576 Datasheet, PDF (6/9 Pages) Advanced Monolithic Systems Ltd – 3.3V, 5V, 12Vand adjustable versions

inductance decreases rapidly and the inductor
begins to look mainly resistive (the DC resistance
of the winding). This will cause the switch current
to rise very rapidly. Different inductor types have
different saturation characteristics, and this
should be well considered when selecting as
inductor.
Feedback Connection
For fixed output voltage version, the FB (feedback)
pin must be connected to VOUT. For the
adjustable version, it is important to place the
output voltage ratio resistors near AMS2576
as possible in order to minimize the noise
introduction.
Enable Input
It is required that the ENABLE must not be left
open. For normal operation, connect this pin to
a ”LOW” voltage (typically, below 1.6V). On the
other hand, for standby mode, connect this pin
with a “HIGH” voltage.
This pin can be safely pulled up to +VIN without a
resistor in series with it.
Grounding
AMS2576
lead length and PCB traces is always the first
thought. Further more, an additional small LC
filter (30µH & 100µF) (as shown in Figure 3) will
possibly provide a 10X reduction in output ripple
voltage and transients.
Heatsink and Thermal Consideration
Although the AMS2576 requires only a small
heatsink for most cases, the following thermal
consideration is important for all operation. With
the package thermal resistances Ó¨ja and Ó¨jc ,
total power dissipation can be estimated as
follows:
PD= (Vin x Iq) + (Vout / Vin) (Iout x Vsat);
When no heatsink is used, the junction
temperature rise can be determined by the
following:
∆TJ = PD x Өja
With the ambient temperature, the actual junction
temperature will be:
Tj = ∆Tj + Ta
To maintain output voltage stability, the power
ground connections must be low-impedance. For
the 5-lead TO-220 and TO-263 style package,
both the tab and pin 3 are ground and rather
connection may be used.
Thermal Characteristics
The output ripple voltage is due mainly to the
inductor sawtooth ripple current multiplied by the
ESR of the output capacitor. The output ripple
voltage of a switching power supply will contain a
sawtooth ripple voltage at the switcher frequency,
typically about 1% of the output voltages, and
may also contain short voltage spokes of the
sawtooth waveform.
If the actual operating junction temperature is out
of the safe operating junction temperature
(typically 125 ℃ ), then a heatsink is required.
When using a heatsink, the junction temperature
rise will be reduced by the following:
∆Tj= PD x (Өjc + Ө interface + Ө heatsink );
Also one can see from the above, it is important
to choose an heatsink with adequate size and
thermal resistance, such that to maintain the
regulator’s junction temperature below the
maximum operating temperature.
Due to the fast switching action, and the parasitic
inductance of the output filter capacitor, there is
voltage spikes presenting at the peaks of the
sawtooth waveform. Cautions must be taken for
stray capacitance. Wiring inductance, and even
the scope probes used for transients evaluation.
To minimize these voltage spikes, shortening the
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