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LTC4425EDD Datasheet(PDF) 9 Page - Linear Technology |
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LTC4425EDD Datasheet(HTML) 9 Page - Linear Technology |
9 / 20 page LTC4425 9 4425p OPERATION The LTC4425 is a linear charger designed to charge a two-cell supercap stack by employing a constant-current, constant-voltage, and constant-temperature architecture. It has two modes of operation: charge current profile mode (also referred to as normal mode) and LDO mode. In LDO mode, the LTC4425 charges the top of the stack to an externally programmed output voltage with a fixed charge current that is also externally programmable. In charge current profile mode, the LTC4425 charges the top of the stack to the input voltage VIN with a charge current that varies based on the input-to-output differential voltage. LDO Mode In LDO mode, the output voltage VOUT is programmed by an external resistor divider network consisting of RFB1 and RFB2 via the FB pin and the charge current is programmed by an external resistor RPROG via the PROG pin. Please refer to the Block Diagram shown in Figure 1. The charger control circuitry consists of a constant- current amplifier and a constant-voltage amplifier. When the part is enabled to charge a discharged supercap stack, initially the constant-current amplifier is in control and servos the PROG pin voltage to 1V. The current through the PROG resistor gets multiplied by approximately 1000, the ratio of the sense MOSFET (MPSNS) and the power MOSFET (MPSW), to charge the supercap stack. As the output voltage VOUT gets close to the programmed value, the constant-voltage amplifier takes over and backs off the charge current as necessary to maintain the FB pin voltage equal to an internal reference voltage of 1.2V. Since the PROG pin current is always about 1/1000 of the charge current, the PROG pin voltage continues to give an indication of the actual charge current even when the constant-voltage amplifier is in control. Charge Current Profile or Normal Mode The LTC4425 is in charge current profile mode when the FB pin is shorted to the input voltage VIN. In this mode of operation, the constant-voltage amplifier is internally disabled but the charge current is still programmed by the external RPROG resistor. The charger provides 1/10 of the programmed charge current if the input-to-output voltage differential (VIN–VOUT) is more than 750mV to limit the power dissipation within the chip. As this differential voltage decreases from 750mV, the charge current increases linearly to its full programmed value when VOUT is within 250mV or closer to VIN. As VOUT rises further, the voltage across the charger FET gets too small to support the full charge current. So the charge current gradually falls off and the charger FET enters into its triode (ohmic) region of operation (see Figure 2). Since the charger FET RDS(ON) is approximately 50mΩ, with a programmed charge current of 2A, the FET will enter the ohmic (triode) region and the charge current will start to fall off when VOUT is within about 100mV of VIN. Figure 2. Different Regions of Charge Current Profile IDEAL DIODE CONTROL REGION OHMIC REGION FULL CHARGE CURRENT REGION LINEAR CHARGE CURRENT REGION 1/10 CHARGE CURRENT REGION 0.3A 2A 0.2A 15 100 250 750 VIN – VOUT (mV) 4425 F02 The Ideal Diode Controller When the input-to-output differential approaches 15mV, the ideal diode controller takes over the control from the constant-current amplifier and backs off the charge cur- rent by pulling up the gate of the charger FET as much as necessary to maintain a 15mV delta across the FET (see Figure 2). As a result, VOUT can only be charged to 15mV below VIN. In the event VIN suddenly drops below VOUT, the controller will quickly turn the FET completely off to prevent any loss of charge due to the reverse flow of charge from the supercap back to the supply. |
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