Datasheet

'V
OUT
= 'I
L
x
1
8 x F
S
x C
OUT
§
¨
©
ESR +
§
¨
©
LM5576, LM5576-Q1
www.ti.com SNVS447I –JANUARY 2007–REVISED APRIL 2013
This procedure provides a guide to select the value of L1. The nearest standard value (33µH) will be used. L1
must be rated for the peak current (I
PK+
) to prevent saturation. During normal loading conditions, the peak current
occurs at maximum load current plus maximum ripple. During an overload condition the peak current is limited to
4.2A nominal (5.1A maximum). The selected inductor (see Table 1) has a conservative 6.2 Amp saturation
current rating. For this manufacturer, the saturation rating is defined as the current necessary for the inductance
to reduce by 30%, at 20°C.
C3 (C
RAMP
)
With the inductor value selected, the value of C3 (C
RAMP
) necessary for the emulation ramp circuit is:
C
RAMP
= L x 10
-5
where
• L is in Henrys (10)
With L1 selected for 33µH the recommended value for C3 is 330pF.
C9, C10
The output capacitors, C9 and C10, smooth the inductor ripple current and provide a source of charge for
transient loading conditions. For this design a 22µF ceramic capacitor and a 150µF SP organic capacitor were
selected. The ceramic capacitor provides ultra low ESR to reduce the output ripple voltage and noise spikes,
while the SP capacitor provides a large bulk capacitance in a small volume for transient loading conditions. An
approximation for the output ripple voltage is:
(11)
D1
A Schottky type re-circulating diode is required for all LM5576 applications. Ultra-fast diodes are not
recommended and may result in damage to the IC due to reverse recovery current transients. The near ideal
reverse recovery characteristics and low forward voltage drop are particularly important diode characteristics for
high input voltage and low output voltage applications common to the LM5576. The reverse recovery
characteristic determines how long the current surge lasts each cycle when the buck switch is turned on. The
reverse recovery characteristics of Schottky diodes minimize the peak instantaneous power in the buck switch
occurring during turn-on each cycle. The resulting switching losses of the buck switch are significantly reduced
when using a Schottky diode. The reverse breakdown rating should be selected for the maximum V
IN
, plus some
safety margin.
The forward voltage drop has a significant impact on the conversion efficiency, especially for applications with a
low output voltage. “Rated” current for diodes vary widely from various manufacturers. The worst case is to
assume a short circuit load condition. In this case the diode will carry the output current almost continuously. For
the LM5576 this current can be as high as 4.2A. Assuming a worst case 1V drop across the diode, the maximum
diode power dissipation can be as high as 4.2W. For the reference design a 100V Schottky in a DPAK package
was selected.
C1, C2
The regulator supply voltage has a large source impedance at the switching frequency. Good quality input
capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current
during the on-time. When the buck switch turns on, the current into the VIN pin steps to the lower peak of the
inductor current waveform, ramps up to the peak value, then drops to zero at turn-off. The average current into
VIN during the on-time is the load current. The input capacitance should be selected for RMS current rating and
minimum ripple voltage. A good approximation for the required ripple current rating necessary is I
RMS
> I
OUT
/ 2.
Quality ceramic capacitors with a low ESR should be selected for the input filter. To allow for capacitor
tolerances and voltage effects, two 2.2µF, 100V ceramic capacitors will be used. If step input voltage transients
are expected near the maximum rating of the LM5576, a careful evaluation of ringing and possible spikes at the
device VIN pin should be completed. An additional damping network or input voltage clamp may be required in
these cases.
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