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LM2594
PROCEDURE (ADJUSTABLE OUTPUT VERSION: LM2594) (CONTINUED)
Procedure
Example
V ) V
OUT D 1000
IN
OUT
SAT
IN
SAT
D
5 ) 0.5
E
T + 6
6.7 V
m s
4. Inductor Selection (L1)
A. Use the following formula to calculate the inductor Volt x
microsecond [V x m s] constant:
E T + V * V * V V
V * V ) V 150 kHz
B. Match the calculated E x T value with the corresponding
m s
4. Inductor Selection (L1)
A. Calculate E x T [V x m s] constant:
E T + 12 * 5 * 1.0
12 * 1 ) 0.5
5.5
11.5
1000
150 kHz
V
m s
number on the vertical axis of the Inductor Value Selection
Guide shown in Figure 19. This E x T constant is a
measure of the energy handling capability of an inductor and
is dependent upon the type of core, the core area, the
number of turns, and the duty cycle.
C. Next step is to identify the inductance region intersected by
the E x T value and the maximum load current value on the
horizontal axis shown in Figure 19.
D. Select an appropriate inductor from Table 3.
The inductor chosen must be rated for a switching
frequency of 150 kHz and for a current rating of 1.15 x I Load .
The inductor current rating can also be determined by
calculating the inductor peak current :
B. E x T = 19.2 [V x m s]
C. I Load(max) = 0.5 A
Inductance Region = L20
D. Proper inductor value = 100 m H
Choose the inductor from Table 3.
I
p(max)
+ I
Load(max)
)
V
in
* V out ton
2L
V out
t on + x 1.0
f osc
in
where t on is the “on” time of the power switch and
V
5. Output Capacitor Selection (C out )
A. Since the LM2594 is a forward ? mode switching regulator
with voltage mode control, its open loop has 2 ? pole ? 1 ? zero
frequency characteristic. The loop stability is determined by
the output capacitor (capacitance, ESR) and inductance
values.
For stable operation use recommended values of the output
capacitors in Table 1.
Low ESR electrolytic capacitors between 180 m F and
1000 m F provide best results.
B. The capacitors voltage rating should be at least 1.5 times
greater than the output voltage, and often much higher
voltage rating is needed to satisfy low ESR requirement
6. Feedforward Capacitor (C FF )
It provides additional stability mainly for higher input voltages. For
Cff selection use Table 1. The compensation capacitor between
5. Output Capacitor Selection (C out )
A. In this example is recommended Nichicon PM
capacitors: 220 m F/25 V
6. Feedforward Capacitor (C FF )
In this example is recommended feedforward capacitor
1.5 nF.
0.6 nF and 15 nF is wired in parallel with the output voltage setting
resistor R2, The capacitor type can be ceramic, plastic, etc..
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