TL3577 Datasheet, PDF (12/21 Pages) Texas Instruments – 100-kHz CURRENT-MODE SIMPLE STEP-UP/FLYBACK SWITCHING REGULATOR

TL3577
100-kHz CURRENT-MODE SIMPLE STEP-UP/FLYBACK SWITCHING REGULATOR
SLVS633 – OCTOBER 2006
www.ti.com
APPLICATION INFORMATION (continued)
Step-Up (Boost) Regulator
Figure 2 shows a step-up switching regulator utilizing the TL3577. The regulator produces an output voltage
higher than the input voltage. The TL3577 turns its switch on and off at a fixed frequency of 100 kHz, thus
storing energy in the inductor (L). When the NPN switch is on, the inductor current is charged at a rate of V_IN/L.
When the switch is off, the voltage at the SWITCH terminal of the inductor rises above V_IN, discharging the
stored current through the output diode (D) into the output capacitor (C_OUT) at a rate of (V_OUT – V_IN)/L. The
energy stored in the inductor is thus transferred to the output. The output voltage is controlled by the amount of
energy transferred, which is controlled by modulating the peak inductor current. This modulation is accomplished
by feeding a portion of the output voltage to an error amplifier that amplifies the difference between the feedback
voltage and an internal 1.23-V precision reference voltage. The output of the error amplifier is compared to a
voltage that is proportional to the switch current or the inductor current during the switch-on time. A comparator
terminates the switch-on time when the two voltages are equal and, thus, controls the peak switch current to
maintain a constant output voltage. Figure 5 shows voltage and current waveforms for the circuit. Formulas for
calculation are shown in Table 1.
Step-Up Regulator Design Procedure
Given:
V_IN(min) = Minimum input supply voltage
V_OUT = Regulated output voltage
V_SW(OFF)
Switch
Voltage
V_SAT
0V
Diode
Voltage
Inductor
Current
V_F
0V
V_R
I_IND(AVG)
∆I_IND
Switch
Current
0
I_SW(PK)
0
Diode
Current
I_D(PK)
I_D(AVG)
0
Figure 5. Step-Up Regulator Waveforms
12
Submit Documentation Feedback