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LTC1267-ADJ5 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1267-ADJ5 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LTC1267 LTC1267-ADJ/LTC1267-ADJ5 APPLICATIO S I FOR ATIO the voltage comparator. This causes Burst Mode opera- tion to be activated when the LTC1267 would normally be in continuous operation. The effect is most pronounced with low values of RSENSE and can be improved by oper- ating at higher frequencies with lower values of L. The output remains in regulation at all times. EXT VCC Pin Connection The LTC1267 contains an internal PNP switch connected between the EXT VCC and VCC pins. The switch closes and supplies the VCC power whenever the EXT VCC pin is higher in voltage than the 4.5V internal regulator. This allows the MOSFET driver and control power to be derived from the output during normal operation and from the internal regulator when the output is out of regulation (start-up, short circuit). Significant efficiency gain can be realized by powering VCC 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 LTC1267, LTC1267-ADJ or LTC1267-ADJ5 this simply means connecting the EXT VCC pin directly to VOUT of the 5V regulator. The following list summarizes the four possible connec- tions for EXT VCC: 1. EXT VCC left open. This will cause VCC to be powered only from the internal 4.5V regulator, resulting in re- duced MOSFET gate drive levels and an efficiency penalty of up to 10% at high input voltages. 2. EXT VCC connected directly to highest VOUT of the two regulators. This is the normal connection for LTC1267/ LTC1267-ADJ/LTC1267-ADJ5 and provides the high- est efficiency. 3. EXT VCC connected to an output-derived boost net- work. For 3.3V and other low voltage regulators, effi- ciency gains can still be realized by connecting EXT VCC to an output-derived voltage which has been boosted to greater than 4.5V. This can be done either with the inductive boost winding shown in Figure 5a or the capacitive charge pump shown in Figure 5b. The charge pump has the advantage of simple magnetics and generally provides the highest efficiency at the expense of a slightly higher parts count. + VIN VIN CIN L 1:1 RSENSE 1N4148 + COUT VOUT 3.3V + 1 µF P-CH LTC1267 • F05A N-CH EXT VCC PGATE 3 PDRIVE 3 LTC1267 NGATE 3 PGND3 • • Figure 5a. Inductive Boost Circuit for EXT VCC + VIN VIN CIN RSENSE L + COUT VOUT 3.3V BAT 85 VN2222LL BAT 85 BAT 85 0.22 µF 1 µF P-CH LTC1267 • F05B N-CH EXT VCC PGATE 3 PDRIVE 3 LTC1267 NGATE 3 PGND3 + Figure 5b. Capacitive Charge Pump for EXT VCC 4. EXT VCC connected to an external supply. If an external supply is available in the 5V to 10V range it may be used to power EXT VCC providing it is compatible with the MOSFET gate drive requirements. When driving stan- dard threshold MOSFETs, the external supply must always be present during operation to prevent MOSFET failure due to insufficient gate drive. Under the condition that EXT VCC is connected to VOUT1 which is greater than 5.5V, to power down the whole regulator, both the pins MSHDN and SHDN1 have to be pulled high. If SHDN1 is left floating or grounded the EXT VCC may self-power from VOUT1, preventing com- plete shutdown. LTC1267 Adjustable Applications When an output voltage other than 3.3V or 5V is required, the LTC1267-ADJ and LTC1267-ADJ5 adjustable ver- sions are used with an external resistive divider from VOUT to the VFB1, 2 pins. This is shown in Figure 6. The regulated voltage is determined by: VOUT = 1.25V ) )1+ R2 R1 |
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