Datasheet

'V
OUT
= 'I
L
x
1
8 x F
S
x C
OUT
§
¨
©
ESR +
§
¨
©
L1 =
5V x (75V ± 5V)
0.4A x 300 kHz x 75V
= 39 PH
L1 =
V
OUT
x (V
IN(max)
± V
OUT
)
I
RIPPLE
x F
S
x V
IN(max)
I
PK+
L1 Current
0 mA
I
PK-
I
O
I
RIPPLE
1/Fs
LM5575
SNVS471G –JANUARY 2007–REVISED APRIL 2013 www.ti.com
Figure 16. Inductor Current Waveform
To keep the circuit in continuous conduction mode (CCM), the maximum ripple current I
RIPPLE
should be less
than twice the minimum load current, or 0.4Ap-p. Using this value of ripple current, the value of inductor (L1) is
calculated using the following:
(6)
(7)
This procedure provides a guide to select the value of L1. The nearest standard value (47µH) 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
2.1A nominal (2.5A maximum). The selected inductor (see Table 1) has a conservative 3.25 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
(8)
Where L is in Henrys
With L1 selected for 47µH the recommended value for C3 is 470pF.
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 10µF ceramic capacitor and a 120µF AL organic capacitor were
selected. The ceramic capacitor provides ultra low ESR to reduce the output ripple voltage and noise spikes,
while the AL capacitor provides a large bulk capacitance in a small volume for transient loading conditions. An
approximation for the output ripple voltage is:
(9)
D1
A Schottky type re-circulating diode is required for all LM5575 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 LM5575. 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.
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