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LTC1430I Datasheet(PDF) 10 Page - Linear Technology |
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LTC1430I Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LTC1430 INTERNAL CIRCUITRY VCC/PVCC2 LTC1430 (8-LEAD) PVCC1 M1 L1 M2 G1 G2 PVCC COUT VOUT LTC1430 • F07 + Figure 7. 8-Lead Power Supplies OS-CON part number 10SA220M (220 µF/10V) capacitors feature 2.3A allowable ripple current at 85 °C and 0.035Ω ESR; three in parallel at the input and six at the output will meet the above requirements. Input Supply Considerations/Charge Pump The 16-lead LTC1430 requires four supply voltages to operate: PVCC for the main power input, PVCC1 and PVCC2 for MOSFET gate drive and a clean, low ripple VCC for the LTC1430 internal circuitry (Figure 6). In many applica- tions, PVCC and PVCC2 can be tied together and fed from a common high power supply, provided that the supply voltage is high enough to fully enhance the gate of external MOSFET M2. This can be the 5V system supply if a logic level MOSFET is used for M2. VCC can usually be filtered with an RC from this same high power supply; the low quiescent current (typically 350 µA) allows the use of relatively large filter resistors and correspondingly small filter capacitors. 100 Ω and 4.7µF usually provide ad- equate filtering for VCC. For both versions of the LTC1430, PVCC1 must be higher than PVCC by at least one external MOSFET VGS(ON) to fully enhance the gate of M1. This higher voltage can be provided with a separate supply (typically 12V) which should power up after PVCC, or it can be generated with a simple charge pump (Figure 4). The charge pump consists of a 1N4148 diode from PVCC to PVCC1 and a 0.1µF capacitor from PVCC1 to the switching node at the drain of M2. This circuit provides 2PVCC – VF to PVCC1 while M1 is ON and PVCC – VF while M1 is OFF where VF is the ON voltage of the 1N4148 diode. Ringing at the drain of M2 can cause transients above 2PVCC at PVCC1; if PVCC is higher than 7V, a 12V zener diode should be included from PVCC1 to PGND to prevent transients from damaging the circuitry at PVCC2 or the gate of M1. More complex charge pumps can be constructed with the 16-lead versions of the LTC1430 to provide additional voltages for use with standard threshold MOSFETs or very low PVCC voltages. A tripling charge pump (Figure 5) can provide 2PVCC and 3PVCC voltages. These can be con- nected to PVCC2 and PVCC1 respectively, allowing stan- dard threshold MOSFETs to be used with 5V at PVCC or 5V logic level threshold MOSFETs to be used with 3.3V at PVCC. VCC can be driven from the same potential as PVCC2, allowing the entire system to run from a single 3.3V supply. Tripling charge pumps require the use of Schottky diodes to minimize forward drop across the diodes at start-up. The tripling charge pump circuit will tend to rectify any ringing at the drain of M2 and can provide well more than 3PVCC at PVCC1; all tripling (or higher multiply- ing factor) circuits should include a 12V zener clamp diode DZ to prevent overvoltage at PVCC1. INTERNAL CIRCUITRY VCC LTC1430 (16-LEAD) PVCC2 PVCC1 M1 L1 M2 G1 G2 PVCC COUT VOUT LTC1430 • F06 + Figure 6. 16-Lead Power Supplies The 8-lead versions of the LTC1430 have the PVCC2 and VCC pins tied together inside the package (Figure 7). This pin, brought out as VCC/PVCC2, has the same low ripple requirements as the 16-lead part, but must also be able to supply the gate drive current to M2. This can be obtained by using a larger RC filter from the PVCC pin; 22Ω and 10µF work well here. The 10 µF capacitor must be VERY close to the part (preferably right underneath the unit) or output regulation may suffer. APPLICATIO S I FOR ATIO |
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Similar Description - LTC1430I |
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