Datasheet

LM1577, LM2577
www.ti.com SNOS658D –JUNE 1999–REVISED APRIL 2013
Table 5. Flyback Regulator Formulas (continued)
Peak Primary Current
I
P(PK)
(19)
Switch Voltage when Off
V
SW(OFF)
(20)
Diode Reverse Voltage V
R
V
OUT
+
N (V
IN
−
V
SAT
)
Average Diode Current I
D(AVE)
I
LOAD
Peak Diode Current
I
D(PK)
(21)
Short Circuit Diode Current
(22)
Power Dissipation of LM1577/LM2577
P
D
(23)
C. Calculate the minimum value of C
C
(24)
D. Calculate the maximum ESR of the +V
OUT
and −V
OUT
output capacitors in parallel.
(25)
This formula can also be used to calculate the maximum ESR of a single output regulator.
At this point, refer to this same section in the STEP-UP REGULATOR DESIGN PROCEDURE section for more
information regarding the selection of C
OUT
.
3. Output Voltage Selection
This section is for applications using the LM1577-ADJ/LM2577-ADJ. Skip this section if the LM1577-12/LM2577-
12 or LM1577-15/LM2577-15 is being used.
With the LM1577-ADJ/LM2577-ADJ, the output voltage is given by
V
OUT
= 1.23V (1 + R1/R2) (26)
Resistors R1 and R2 divide the output voltage down so it can be compared with the LM1577-ADJ/LM2577-ADJ
internal 1.23V reference. For a desired output voltage V
OUT
, select R1 and R2 so that
(27)
4. Diode Selection
The switching diode in a flyback converter must withstand the reverse voltage specified by the following
equation.
(28)
A suitable diode must have a reverse voltage rating greater than this. In addition it must be rated for more than
the average and peak diode currents listed in Table 5.
5. Input Capacitor Selection
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