LT1070 Design Manual (Linear Technology) - 9
| Autoren | Carl Nelson |
| Hersteller | Linear Technology |
| Beschreibung | Application Note 19. This design manual is an extensive discussion of all standard switching configurations for the LT1070; including buck, boost, flyback, forward, inverting and "Cuk." The manual includes comprehensive information on the LT1070, the external components used with it, and complete formulas for calculating component values. |
| Seiten / Seite | 80 / 9 — Figure 3. Feedback Pin. Figure 4 |
| Dateiformat / Größe | PDF / 656 Kb |
| Dokumentensprache | Englisch |
Figure 3. Feedback Pin. Figure 4
Textversion des Dokuments
Application Note 19 SEPARATE GROUND PATH GND VSW GND VSW VC FB VIN VC FB VIN SWITCH CURRENT PATH TO TO SWITCH CURRENT OUTPUT OUTPUT OUTPUT PATH—KEEP RESISTANCE DIVIDER LOW AN19 F03a AN19 F03b
Figure 3 Feedback Pin
The feedback pin has a second function; it is used to program the LT1070 for normal or flyback-regulated The feedback pin is the inverting input to a single stage operation (see description of block diagram). In Figure 4, error amplifier. The noninverting input to this amplifier is Q53 is biased with a base voltage approximately 1V. This internally tied to a 1.244V reference as shown in Figure 4. clamps the feedback pin to about 0.4V when current is Input bias current of the amplifier is typically 350nA with drawn out of the pin. A current of ≈10μA or higher through the output of the amplifier in its linear region. The amplifier Q53 forces the regulator to switch from normal operation is a gm type, meaning that it has high output impedance to flyback mode, but this threshold current can vary from with controlled voltage-to-current gain (gm ≈ 4400μmhos). 3μA to 30μA. The LT1070 is in flyback mode during normal DC voltage gain with no load is ≈ 800. start-up until the feedback pin rises above 0.45V. The resistor divider used to set output voltage will draw 2.3V current out of the feedback pin until the output voltage is TO LOGIC up to about 33% of its regulated value. STEERING If it is desired to run the LT1070 in the fully isolated flyback Q53 ≈ 1V mode, a single resistor is tied from the feedback pin to – 30Ω ground. The feedback pin then sits at a voltage of FEEDBACK ≈0.4V for PIN R = 8.2k. The actual voltage depends on resistor value ERROR VC PIN AMP + since the feedback pin has about 200Ω output impedance 5.6V in this mode. 500μA in the resistor will drop the feedback pin voltage from 0.4V to 0.3V. Minimum current through 1.244V the resistor to guarantee flyback operation is 50μA. Actual GND PIN resistor value is chosen to fine-trim flyback regulated AN19 F04 voltage. (See discussion of isolated flyback mode opera- tion and graphs of feedback pin characteristics.)
Figure 4
an19fc AN19-9
Figure 3 Feedback Pin
The feedback pin has a second function; it is used to program the LT1070 for normal or flyback-regulated The feedback pin is the inverting input to a single stage operation (see description of block diagram). In Figure 4, error amplifier. The noninverting input to this amplifier is Q53 is biased with a base voltage approximately 1V. This internally tied to a 1.244V reference as shown in Figure 4. clamps the feedback pin to about 0.4V when current is Input bias current of the amplifier is typically 350nA with drawn out of the pin. A current of ≈10μA or higher through the output of the amplifier in its linear region. The amplifier Q53 forces the regulator to switch from normal operation is a gm type, meaning that it has high output impedance to flyback mode, but this threshold current can vary from with controlled voltage-to-current gain (gm ≈ 4400μmhos). 3μA to 30μA. The LT1070 is in flyback mode during normal DC voltage gain with no load is ≈ 800. start-up until the feedback pin rises above 0.45V. The resistor divider used to set output voltage will draw 2.3V current out of the feedback pin until the output voltage is TO LOGIC up to about 33% of its regulated value. STEERING If it is desired to run the LT1070 in the fully isolated flyback Q53 ≈ 1V mode, a single resistor is tied from the feedback pin to – 30Ω ground. The feedback pin then sits at a voltage of FEEDBACK ≈0.4V for PIN R = 8.2k. The actual voltage depends on resistor value ERROR VC PIN AMP + since the feedback pin has about 200Ω output impedance 5.6V in this mode. 500μA in the resistor will drop the feedback pin voltage from 0.4V to 0.3V. Minimum current through 1.244V the resistor to guarantee flyback operation is 50μA. Actual GND PIN resistor value is chosen to fine-trim flyback regulated AN19 F04 voltage. (See discussion of isolated flyback mode opera- tion and graphs of feedback pin characteristics.)
Figure 4
an19fc AN19-9