lm2577sx-adj National Semiconductor Corporation, lm2577sx-adj Datasheet - Page 15

• Manufacturer Part Number: lm2577sx-adj
• Description: Simple Switcher Step-up Voltage Regulator
• Manufacturer: National Semiconductor Corporation

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
directly 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
frequency 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
When the switch turns off, the lower end of the inductor flies
above V
output capacitor (C
energy 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 trans-
ferred 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 feedback voltage and a 1.230V ref-
erence. The error amp output voltage is compared to a
voltage proportional to the switch current (i.e., inductor cur-
rent 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
Figure 6.
FIGURE 5. Step-Up Regulator Waveforms
IN
, discharging its current through diode (D) into the
OUT
) at a rate of (V
IN
/L, storing current in the inductor.
(Continued)
OUT
− V
01146811
IN
)/L. Thus,
15
V
I
STEP-UP REGULATOR DESIGN PROCEDURE
The following design procedure can be used to select the
appropriate 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)
A. Voltage Options:
1. For 12V or 15V output
From Figure 7 (for 12V output) or Figure 8 (for 15V
output), identify inductor code for region indicated by
V
IN (min)
OUT
IN (min)
= Output Load Current
= Regulated output voltage
FIGURE 6. Step-Up Regulator Formulas
V
V
OUT
OUT
and I
= Minimum input supply voltage
OUT
= Maximum output load current
≤ 60V
≤ 10 x V
LOAD (max)
and I
V
I
I
IND(AVE)
I
I
IND(PK)
SW(OFF)
SW(PK)
I
∆I
D(AVE)
D(PK)
V
P
D
IND
R
D
LOAD(max)
IN(min)
. The shaded region indicates con-
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)