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LTC1435 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1435 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LTC1435 additional circuitry is required to derive INTVCC power from the output. The following list summarizes the four possible connec- tions for EXTVCC: 1. EXTVCC left open (or grounded). This will cause INTVCC to be powered from the internal 5V regulator resulting in an efficiency penalty of up to 10% at high input voltages. 2. EXTVCC connected directly to VOUT. This is the normal connection for a 5V regulator and provides the highest efficiency. 3. EXTVCC connected to an output-derived boost network. For 3.3V and other low voltage regulators, efficiency gains can still be realized by connecting EXTVCC to an output-derived voltage which has been boosted to greater than 4.8V. This can be done with either the inductive boost winding as shown in Figure 4a or the capacitive charge pump shown in Figure 4b. The charge pump has the advantage of simple magnetics. 4. EXTVCC connected to an external supply. If an external supply is available in the 5V to 10V range (EXTVCC ≤ VIN), it may be used to power EXTVCC providing it is compatible with the MOSFET gate drive requirements. When driving standard threshold MOSFETs, the exter- nal supply must always be present during operation to prevent MOSFET failure due to insufficient gate drive. APPLICATIONS INFORMATION INTVCC Regulator An internal P-channel low dropout regulator produces the 5V supply which powers the drivers and internal circuitry within the LTC1435. The INTVCC pin can supply up to 15mA and must be bypassed to ground with a minimum of 2.2 µF tantalum or low ESR electrolytic. Good bypassing is necessary to supply the high transient currents required by the MOSFET gate drivers. High input voltage applications, in which large MOSFETs are being driven at high frequencies, may cause the maximum junction temperature rating for the LTC1435 to be exceeded. The IC supply current is dominated by the gate charge supply current when not using an output derived EXTVCC source. The gate charge is dependent on operating frequency as discussed in the Efficiency Consid- erations section. The junction temperature can be esti- mated by using the equations given in Note 1 of the Electrical Characteristics. For example, the LTC1435 is limited to less than 17mA from a 30V supply: TJ = 70°C + (17mA)(30V)(100°C/W) = 126°C To prevent maximum junction temperature from being exceeded, the input supply current must be checked when operating in continuous mode at maximum VIN. EXTVCC Connection The LTC1435 contains an internal P-channel MOSFET switch connected between the EXTVCC and INTVCC pins. The switch closes and supplies the INTVCC power when- ever the EXTVCC pin is above 4.8V, and remains closed until EXTVCC drops below 4.5V. This allows the MOSFET driver and control power to be derived from the output during normal operation (4.8V < VOUT < 9V) and from the internal regulator when the output is out of regulation (start-up, short circuit). Do not apply greater than 10V to the EXTVCC pin and ensure that EXTVCC < VIN. Significant efficiency gains can be realized by powering INTVCC from the output, since the VIN current resulting from the driver and control currents will be scaled by a factor of Duty Cycle/Efficiency. For 5V regulators this supply means connecting the EXTVCC pin directly to VOUT. However, for 3.3V and other lower voltage regulators, Figure 4a. Secondary Output Loop and EXTVCC Connection R6 R5 EXTVCC SFB SGND VIN TG BG PGND LTC1435 N-CH N-CH + CIN VIN 1N4148 + 1 µF + COUT VSEC L1 1:N RSENSE VOUT OPTIONAL EXT VCC CONNECTION 5V ≤ VSEC ≤ 9V LTC1435 • F04a SW |
Similar Part No. - LTC1435 |
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Similar Description - LTC1435 |
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