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LM2594
x t on
[ Load
in
O
in
peak
2L 1
V
O
x 1.0 , and f osc + 52 kHz.
fosc
where t on +
in
O
across the regulator is the absolute sum of the input and
output voltages and this must be limited to a maximum of
40 V.
This circuit configuration is able to deliver approximately
0.25 A to the output when the input voltage is 12 V or higher.
At lighter loads the minimum input voltage required drops
to approximately 4.7 V, because the buck ? boost regulator
topology can produce an output voltage that, in its absolute
value, is either greater or less than the input voltage.
Since the switch currents in this buck ? boost configuration
are higher than in the standard buck converter topology, the
available output current is lower.
This type of buck ? boost inverting regulator can also
require a larger amount of startup input current, even for
light loads. This may overload an input power source with
a current limit less than 1.0 A.
Such an amount of input startup current is needed for at
least 2.0 ms or more. The actual time depends on the output
voltage and size of the output capacitor.
Because of the relatively high startup currents required by
this inverting regulator topology, the use of a delayed startup
or an undervoltage lockout circuit is recommended.
Using a delayed startup arrangement, the input capacitor
can charge up to a higher voltage before the switch ? mode
regulator begins to operate.
The high input current needed for startup is now partially
supplied by the input capacitor C in .
It has been already mentioned above, that in some
situations, the delayed startup or the undervoltage lockout
features could be very useful. A delayed startup circuit
applied to a buck ? boost converter is shown in Figure 28.
Figure 30 in the “Undervoltage Lockout” section describes
an undervoltage lockout feature for the same converter
topology.
Design Recommendations:
The inverting regulator operates in a different manner
than the buck converter and so a different design procedure
has to be used to select the inductor L1 or the output
capacitor C out .
The output capacitor values must be larger than what is
normally required for buck converter designs. Low input
voltages or high output currents require a large value output
capacitor (in the range of thousands of m F).
The recommended range of inductor values for the
inverting converter design is between 68 m H and 220 m H. To
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.
The following formula is used to obtain the peak inductor
current:
I (V ) |V |) V
I )
in
|V |
V ) |V |
Under normal continuous inductor current operating
conditions, the worst case occurs when V in is minimal.
12 to 40 V
Feedback
R4
Unregulated
DC Input
+V in
LM2594
L1
100 m H
C in
100 m F/50 V
C1
0.1 m F
ON/OFF
R2
47k
GND
D1
1N5819
C out
220 m F
R3
C FF
? 12 V @ 0.25 A
Regulated
Output
Figure 24. Inverting Buck ? Boost Develops with Delayed Startup
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