Datasheet

= (20 - 1.5 - 10) x
0.55
Et =
(V
IN
- V
SAT
- V
OUT
) x t
ON
150000
x 10
6
VPsecs
= 31.3 VPsecs
t
ON
=
D
f
x 10
6
Ps
D =
10 + 0.5
20 - 1.5 + 0.5
= 0.55
D =
V
OUT
+ V
D
V
IN
- V
SAT
+ V
D
e
CLIM
=
1
2
x 100 x 3
2
= 450 PJ
LM2591HV
www.ti.com SNVS074D –MAY 2001–REVISED APRIL 2013
Example 2: (V
IN
> 40V) LM2591HV-5.0, V
IN
= 48V, Output 5V @ 1A
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 (Figure 26) and a current rating equal to I
CLIM
. We
therefore quick-select a 100μH/3A 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 SNVA038 and that the
losses are acceptable. Here e
CLIM
is:
(3)
Example 3: (V
IN
≤ 40V) LM2591HV-ADJ, V
IN
= 20V, Output 10V @ 1A
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 nomograph Figure 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)
• 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 100μH/1A inductor (designed for 150 kHz operation) for this
application.
2. We should confirm that it is rated to handle 100 μJ (see Figure 27) by the procedure shown in AN-1197
SNVA038 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 above).
Note that we have taken V
SAT
as 1.5V which includes an estimated resistive drop across the inductor.
This completes the simplified inductor selection procedure. For more general applications and better
optimization, the designer should refer to AN-1197 SNVA038. 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.
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