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LTC4267 Datasheet(PDF) 23 Page - Linear Technology |
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LTC4267 Datasheet(HTML) 23 Page - Linear Technology |
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23 / 32 page  LTC4267 23 4267f The shunt regulator can sink up to 5mA through the PVCC pin to PGND. The values of RSTART and CPVCC must be selected for the application to withstand the worst-case load conditions and drop on PVCC, ensuring that the PVCC turn-off threshold is not reached. CPVCC should be sized sufficiently to handle the switching current needed to drive NGATE while maintaining minimum switching voltage. actual current needed to power the LTC4267 switching regulator goes through Q1 and PVCC sources current on an “as-needed” basis. The static current is then limited only to the current through RB and D1. APPLICATIO S I FOR ATIO Figure 12. Powering the LTC4267 Switching Regulator via the Shunt Regulator Figure 13. Powering the LTC4267 Switching Regulator with an External Preregulator VPORTP PVCC PGND POUT VPORTN LTC4267 –48 FROM PSE RSTART CPVCC + – PGND 4267 F14 External Preregulator The circuit in Figure 13 shows a third way to power the LTC4267 switching regulator circuit. An external series preregulator consists of a series pass transistor Q1, zener diode D1, and a bias resistor RB. The preregulator holds PVCC at 7.6V nominal, well above the maximum rated PVCC turn-off threshold of 6.8V. Resistor RSTART momentarily charges the PVCC node up to the PVCC turn-on threshold, enabling the switching regulator. The voltage on CPVCC begins to decline as the switching regulator draws its normal supply current, which exceeds the delivery of RSTART. After some time, the output voltage approaches the desired value. By this time, the pass transistor Q1 catches the declining voltage on the PVCC pin, and provides virtually all the supply current required by the LTC4267 switching regulator. CPVCC should be sized sufficiently to handle the switching current needed to drive NGATE while maintaining minimum switching voltage. The external preregulator has improved efficiency over the simple resistor-shunt regulator method mentioned previously. RB can be selected so that it provides a small current necessary to maintain the zener diode voltage and the maximum possible base current Q1 will encounter. The VPORTP PVCC PGND POUT VPORTN LTC4267 –48 FROM PSE RSTART CPVCC + – PGND PGND PGND Q1 D1 8.2V RB 4267 F15 Compensating the Main Loop In an isolated topology, the compensation point is typically chosen by the components configured around the external error amplifier. Shown in Figure 14, a series RC network is connected from the compare voltage of the error am- plifier to the error amplifier output. In PD designs where transient load response is not critical, replace RZ with a short. The product of R2 and CC should be sufficiently large to ensure stability. When fast settling transient response is critical, introduce a zero set by RZCC. The PD designer must ensure that the faster settling response of the output voltage does not compromise loop stability. In a nonisolated design, the LTC4267 incorporates an internal error amplifier where the ITH/RUN pin serves as a compensation point. In a similar manner, a series RC network can be connected from ITH/RUN to PGND as shown in Figure 15. CC and RZ are chosen for optimum load and line transient response. Figure 14. Main Loop Compensation for an Isolated Design R1 R2 CC RZ TO OPTO- ISOLATOR 4267 F14 VOUT |
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Similar Description - LTC4267 |
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