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LTM4651 Scheda tecnica(PDF) 16 Page - Linear Technology |
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LTM4651 Scheda tecnica(HTML) 16 Page - Linear Technology |
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16 / 34 page  LTM4651 16 4651f For more information www.linear.com/LTM4651 VIN may be rapidly discharged and CINOUT is used, the resulting capacitor-divider network formed by CINOUT and CINL||CINHmaytransientlydragVOUT–excessivelynegative. It is recommended to straddle VOUT–andPGNDwithaTVS diode, if output voltage excursions during VIN-discharge are anticipated. Frequency Adjustment The default switching frequency (fSW) of the LTM4651 is 400kHz. This is suitable for mainly low-VIN or low-VOUT– applications (VIN < 5V or |VOUT–| < 5V). For a practical design, the LTM4651’s inductor ripple current (∆PK-PK) is suggested to be less than ~2APK-PK. From EQ. 2, it follows that fSW should be chosen such that: 1 L • ∆IPK-PK • 1 VIN – 1 VOUT– (12) In some cases, the value of fSW yielded by EQ. 12 violates the supported minimum on time of the LTM4651 (see EQ. 3). If this occurs, choose fSW instead according to: fSW < D TON(MIN) (13) The primary consequence of using a lower switching frequency than that dictated by EQ. 12 is that the output current capability of the LTM4651 is reduced, according to EQ. 5. ToconfiguretheLTM4651forahigherswitchingfrequency than 400kHz default, apply a resistor, RfSET, between the fSET pin and SVOUT–. RfSET is given (in MΩ) by: RfSET(MΩ)= 1 10pF •[fSW(MHz)–0.4(MHz)] (14) The relationship of RfSET to programmed fSW is shown in Figure 2. See Table 1 and Table 8 for Recommended fSW and as- sociated RfSET values for various combinations of VIN and VOUT–. APPLICATIONS INFORMATION Figure 2. Relationship Between RfSET and Target fSW VOUT– (V) –0.5 –3.3 –5 –8 –12 –15 –20 –24 3.6 400kHz, No RfSET 400kHz, No RfSET 400kHz, No RfSET 400kHz, No RfSET 400kHz, No RfSET 400kHz, No RfSET 425kHz, 4.3MΩ 450kHz, 2.2MΩ 5 450kHz, 2.2MΩ 475kHz, 1.3MΩ 500kHz, 1MΩ 525kHz, 806kΩ 550kHz, 665kΩ 12 550kHz, 665kΩ 700kHz, 332kΩ 825kHz, 237kΩ 875kHz, 210kΩ 900kHz, 200kΩ 1MHz, 165kΩ 24 Drive CLKIN with a 200kHz Clock, No RfSET 450kHz, 2.2MΩ 600kHz, 499kΩ 800kHz, 249kΩ 1.1MHz, 143kΩ 1.2MHz, 124kΩ 1.4MHz, 100kΩ 1.5MHz, 90.9kΩ 36 Not Recommended Due to On- Time Criteria Violation 500kHz, 1MΩ 850kHz, 221kΩ 1.2MHz, 124kΩ 1.4MHz, 100kΩ 1.6MHz, 82.5kΩ N/A 48 900kHz, 200kΩ N/A Due to SOA Criteria Violation Table 1. Recommended Switching Frequency (fSW) and RfSET Values for Common Combinations of VIN and VOUT– RfSET NOT USED RfSET (kΩ) 10 100 1k 10k 0.1 1 10 4651 F02 |
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