LM2577N-12 National Semiconductor, LM2577N-12 Datasheet - Page 13

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

Application Hints
STEP-UP (BOOST) REGULATOR
Figure 4 shows the LM1577-ADJ/LM2577-ADJ used as a
Step-Up Regulator. This is a switching regulator used for
producing an output voltage greater than the input supply
voltage. The LM1577-12/LM2577-12 and LM1577-15/
LM2577-15 can also be used for step-up regulators with 12V
or 15V outputs (respectively), by tying the feedback pin di-
rectly to the regulator output.
A basic explanation of how it works is as follows. The
LM1577/LM2577 turns its output switch on and off at a fre-
quency of 52 kHz, and this creates energy in the inductor (L).
When the NPN switch turns on, the inductor current charges
up at a rate of V
switch turns off, the lower end of the inductor flies above V
discharging its current through diode (D) into the output ca-
pacitor (C
stored in the inductor during the switch on time is transferred
to the output during the switch off time. The output voltage is
controlled by the amount of energy transferred which, in turn,
is controlled by modulating the peak inductor current. This is
done by feeding back a portion of the output voltage to the
error amp, which amplifies the difference between the feed-
back voltage and a 1.230V reference. The error amp output
voltage is compared to a voltage proportional to the switch
current (i.e., inductor current during the switch on time).
The comparator terminates the switch on time when the two
voltages are equal, thereby controlling the peak switch cur-
rent to maintain a constant output voltage.
Voltage and current waveforms for this circuit are shown in
Figure 5 , and formulas for calculating them are given in Fig-
ure 6 .
FIGURE 5. Step-Up Regulator Waveforms
OUT
) at a rate of (V
IN
/L, storing current in the inductor. When the
(Continued)
OUT
− V
IN
)/L. Thus, energy
DS011468-11
IN
,
13
V
I
STEP-UP REGULATOR DESIGN PROCEDURE
The following design procedure can be used to select the ap-
propriate external components for the circuit in Figure 4 ,
based on these system requirements.
Given:
Before proceeding any further, determine if the LM1577/
LM2577 can provide these values of V
when operating with the minimum value of V
limits for V
equations.
These limits must be greater than or equal to the values
specified in this application.
1. Inductor Selection (L)
LOAD
Duty Cycle
Average
Inductor
Current
Inductor
Current
Ripple
Peak
Inductor
Current
Peak Switch
Current
Switch
Voltage
When Off
Diode
Reverse
Voltage
Average
Diode
Current
Peak Diode
Current
Power
Dissipation
of
LM1577/2577
F
V
V
I
and
= Forward Biased Diode Voltage
LOAD(max)
IN (min)
OUT
A. Voltage Options:
1. For 12V or 15V output
= Output Load Current
= Regulated output voltage
FIGURE 6. Step-Up Regulator Formulas
V
V
OUT
OUT
= Minimum input supply voltage
OUT
= Maximum output load current
and I
60V
10 x V
V
I
I
IND(AVE)
I
I
SW(OFF)
IND(PK)
SW(PK)
I
D(AVE)
D(PK)
V
P
I
D
IND
LOAD(max)
R
D
IN(min)
are given by the following
V
V
OUT
OUT
OUT
I
LOAD
− V
+ V
and I
IN
SAT
. The upper
F
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LOAD(max)