LM1577, LM2577
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SNOS658D –JUNE 1999–REVISED APRIL 2013
STEP-UP (BOOST) REGULATOR
Figure 32 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 VIN/L, storing current in the inductor. When the switch turns off, the lower end of
the inductor flies above VIN, discharging its current through diode (D) into the output capacitor (COUT) at a rate of
(VOUT − VIN)/L. Thus, 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 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 feedback 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 current to maintain a constant output voltage.
Voltage and current waveforms for this circuit are shown in Figure 33, and formulas for calculating them are
given in Table 1.
Figure 33. Step-Up Regulator Waveforms
Table 1. Step-Up Regulator Formulas(1)
Duty Cycle
D
Average Inductor Current
Inductor Current Ripple
Peak Inductor Current
IIND(AVE)
ΔIIND
IIND(PK)
ISW(PK)
Peak Switch Current
Switch Voltage When Off
Diode Reverse Voltage
Average Diode Current
Peak Diode Current
VSW(OFF)
VR
VOUT + VF
VOUT − VSAT
ID(AVE)
ILOAD
ID(PK)
PD
Power Dissipation of LM1577/2577
(1) VF = Forward Biased Diode Voltage
ILOAD = Output Load Current
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