LM2577N-12 National Semiconductor, LM2577N-12 Datasheet - Page 20

• Manufacturer Part Number: LM2577N-12
• Description: SIMPLE SWITCHER Step-Up Voltage Regulator
• Manufacturer: National Semiconductor

www.national.com
Application Hints
C. Calculate the minimum value of C
D. Calculate the maximum ESR of the +V
output capacitors in parallel.
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 Regu-
lator Design Procedurefor more information regarding the
selection of C
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
OUT
Duty Cycle
Primary Current Variation
Peak Primary Current
Switch Voltage when Off
Diode Reverse Voltage
Average Diode Current
Peak Diode Current
Short Circuit Diode Current
Power Dissipation of
LM1577/LM2577
V
.
OUT
= 1.23V (1 + R1/R2)
(Continued)
C
FIGURE 19. Flyback Regulator Formulas
OUT
and −V
V
I
SW(OFF)
I
D(AVE)
I
P(PK)
D(PK)
V
P
D
I
OUT
R
D
P
20
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
R1 and R2 so that
4. Diode Selection
The switching diode in a flyback converter must withstand
the reverse voltage specified by the following equation.
A suitable diode must have a reverse voltage rating greater
than this. In addition it must be rated for more than the aver-
age and peak diode currents listed in Figure 19 .
5. Input Capacitor Selection
The primary of a flyback transformer draws discontinuous
pulses of current from the input supply. As a result, a flyback
regulator generates more noise at the input supply than a
step-up regulator, and this requires a larger bypass capacitor
to decouple the LM1577/LM2577 V
most applications, a low ESR, 1.0 µF cap will be sufficient, if
it is connected very close to the V
V
DS011468-78
OUT
+
N (V
I
LOAD
IN
−
V
SAT
)
IN
IN
and Ground pins.
pin from this noise. For
OUT
, select