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LTC1471 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1471 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC1470/LTC1471 APPLICATIONS INFORMATION winding is rectified by diode D2, added to the main 5V output and applied to the input of the LT1313. (Note that the auxiliary winding must be phased properly as shown in Figure 3.) When the 12V output is activated by a TTL high on either VPP enable lines, the 5V section of the LTC1142HV is forced into continuous mode operation. A resistor divider composed of R2, R3 and switch Q3 forces an offset which is subtracted from the internal offset at the Sense– input (pin 14) of the LTC1142HV. When this external offset cancels the built-in 25mV offset, Burst Mode TM operation is inhibited and the LTC1142HV is forced into continuous mode operation. (See LTC1142HV data sheet for further detail.) In this mode, the 15V auxiliary supply can be loaded without regard to the loading on the 5V output of the LTC1142HV. ContinuousmodeoperationisonlyinvokedwhentheLT1313 is programmed to 12V. If the LT1313 is programmed to 0V, 3.3V or 5V, power is obtained directly from the main power source (battery pack) through diode D1. Again, the LT1313 output can be loaded without regard to the loading of the main 5V output. R4 and C4 absorb transient voltage spikes associated with the leakage inductance inherent in T1’s secondary winding and ensure that the auxiliary supply does not exceed 20V. Auxiliary Power from the LTC1142 3.3V Output For low-battery count applications (< 6.5V) it is necessary to modify the circuit of Figure 3. As the input voltage falls, the 5V duty cycle increases to the point where there is simply not enough time to transfer energy from the 5V primary winding to the auxiliary winding. For applications where 12V load currents exist in conjunction with these low input voltages, use the circuit shown in Figure 4. In this circuit, the auxiliary 15V supply is generated from an overwinding on the 3.3V inductor of the LTC1142 regulator output. In Figure 3, power is drawn directly from the batteries through D1 when the regulator is in Burst Mode operation and the VPP pins require 3.3V or 5V. In this circuit, however, Q3 and Q4 force the LTC1142 3.3V regulator into continuous mode operation whenever 3.3V, 5V or 12V is programmed at the VPPOUT pins of the LT1313. (See the LT1312 and LT1313 data sheets for further detail.) Figure 4. Deriving 15V from the 3.3V Output of the LTC1142 for VPP Power Burst Mode is a trademark of Linear Technology Corporation. 24 23 6 1 28 R3 12k AENVPP0 AENVPP1 C2 1000pF C1 68 µF C5 68 µF C3 220 µF D3 MBRS130T3 D2 MBRS1100 3.3V OUTPUT 15V AUX SUPPLY T1* 15 µH *CTX02-12753 COILTRONICS (407) 241-7876 R4 0.033 Ω D4 18V Q1 Q2 Q3 2N7002 Q4 2N7002 R1 100 Ω 3.37:1 VIN 5.4V TO 11V R2 100 Ω + + 1470/71 F04 + VIN PDRIVE NDRIVE SENSE+ SENSE– 1/2 LTC1142 (3.3V REG) BENVPP0 BENVPP1 HC86 HC86 |
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