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LM3647 Scheda tecnica(PDF) 8 Page - National Semiconductor (TI) |
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LM3647 Scheda tecnica(HTML) 8 Page - National Semiconductor (TI) |
8 / 17 page 8 www.national.com nection between RESET and SYSOK is optional but highly recommended for safe operation of the LM3647. 4.3.5 Dimensioning the RCIN circuitry The RC-circuitry is designed to time the charger so it charges and regulates at the correct frequency. The values of the R and C are important, because a change in the RC-value gives a higher or lower operating frequency which affects charge quality. The capacitance should be a ceramic type and preferably an NP0 type, which gives the least frequency deviation with temperature change. 4.3.6 Dimensioning the DISCHARGE circuitry The discharge-circuitry controls the discharge rate during the discharge phase (if used). The discharge output turns on the transistor Q2, and a current flows from the battery through the discharge resistor R8. The current through R8 depends on battery voltage and the value of R8. This value depends on the maximum discharge rate for the battery-pack. The ap- proximate value can be calculated using the formula below: The resistor R7 keeps the transistor Q2 off until the LM3647 has been powered up and is in control of the circuitry. 4.3.7 BUZZER output circuitry The buzzer-circuitry turns the transistor Q3 on when the buzzer should sound. If the current consumption for the buzz- er is lower than 0.3mA then the buzzer may be directly con- nected to the BUZZER-pin. Please note that the BUZZER- pin does not generate a PWM-signal, such buzzers must have their own drive-circuitry. If an electromagnetic buzzer is used, then the transistor may need a reverse-biased diode to protect it from harmful voltage spikes. 4.3.8 PWM filter circuitry The PWM-pin can either output a fast PWM-signal, or a slow on/off output (for controlling external constant current source, Ni-Cd/Ni-MH mode only). Fast PWM-mode: The RC-network R6, C9 and R5, C1||C2 lowpass-filters the PWM-signal from the LM3647 to a DC-level that is fed into the operational amplifier. The resistor R22 is required to pre- vent DC-output before the LM3647 has control of the RC-net- works. R8 MaximumBatteryVoltage MaximumDisch eRate arg ------------------------------------------------------------------- ≈ Q3 |
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