Datasheet

LM2592HV
www.ti.com SNVS075C –MAY 2001–REVISED APRIL 2013
Example 2: (V
IN
> 40V) LM2592HV-5.0, V
IN
= 48V, Output 5V @ 1.5A
1. A first pass inductor selection is based upon Inductance and the switch currrent limit. We choose an inductor
with the Inductance value indicated by the nomograph (see Figure 26) and a current rating equal to I
CLIM
. We
therefore quick-select a 68μH/4A inductor (designed for 150 kHz operation) for this application.
2. We should confirm that it is rated to handle e
CLIM
by the procedure shown in AN-1197 and that the losses are
acceptable. Here e
CLIM
is:
(3)
Example 3: (V
IN
≤ 40V) LM2592HV-ADJ, V
IN
= 20V, Output 10V @ 2A
1. Since input voltage is less than 40V, a first pass inductor selection is based upon Inductance and rated max
load current. We choose an inductor with the Inductance value indicated by the nomographFigure 27 and a
current rating equal to the maximum load. But we first need to calculate Et for the given application. The Duty
cycle is
where
• V
D
is the drop across the Catch Diode (≊ 0.5V for a Schottky)
• and V
SAT
the drop across the switch (≊1.5V) (4)
So
(5)
And the switch ON time is:
where
• f is the switching frequency in Hz (6)
So
(7)
Therefore, looking at Figure 25 we quick-select a 47μH/2A inductor (designed for 150 kHz operation) for this
application.
2. We should confirm that it is rated to handle 200 μJ (see Figure 27) by the procedure shown in AN-1197 and
that the losses are acceptable. (If the DC Input voltage had been greater than 40V we would need to consider
e
CLIM
as in Example 2).
This completes the simplified inductor selection procedure. For more general applications and better
optimization, the designer should refer to AN-1197. Table 1 provides helpful contact information on suggested
Inductor manufacturers who may be able to recommend suitable parts, if the requirements are known.
FEEDFORWARD CAPACITOR
(Adjustable Output Voltage Version)
C
FF
- A Feedforward Capacitor C
FF
, shown across R2 in Test Circuit and Layout Guidelines is used when the
output voltage is greater than 10V or when C
OUT
has a very low ESR. This capacitor adds lead compensation to
the feedback loop and increases the phase margin for better loop stability.
If the output voltage ripple is large (> 5% of the nominal output voltage), this ripple can be coupled to the
feedback pin through the feedforward capacitor and cause the error comparator to trigger the error flag. In this
situation, adding a resistor, R
FF
, in series with the feedforward capacitor, approximately 3 times R1, will attenuate
the ripple voltage at the feedback pin.
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