Keep checking between old and new boards - read the data sheet and the app notes carefully ...!
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Two things from the data sheet:
1) if a PowerPath FET is not being used, such as with a lead-acid charging application, connect a 0.1nF capacitor from BGATE to CSN.
2) If the battery voltage is lower than the instant-on threshold (see VFBMIN), BGATE servos the PowerPath FET imped-ance such that a voltage drop between the CSN pin and the BAT pin is created while battery charging continues. If the VCSN – VBAT voltage exceeds 0.4V, maximum charge current is reduced to decrease power dissipation in the PowerPath FET. so at 10A and 0.38V the diss can be 3.8W - enough to kill a fet not heatsunk enough ...
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and: During the majority of a normal battery charge cycle, the LTC4020 makes a low impedance connection between the battery and the DC/DC converter output through the PowerPath FET, as in Figure 3. This PFET is controlled by the LTC4020 through modulation of the BGATE pin, which is connected to the FET gate. When charging is disabled, the FET is disabled, disconnecting the battery from the converter output by pulling the gate of the PowerPath FET high via the BGATE pin. The converter output is regulated by VFBMAX while the FET is disabled. When normal charger operation resumes, the gate is pulled low. As the BGATE pin is a slow-moving node, C/10 detection is disabled until the BGATE pin approaches its normal operating voltage, which prevents premature C/10 detection during reconnection of the battery. The slow movement of BGATE can also cause the converter output to regulate to VFBMAX for a short time during start-up until the FET is enabled. This FET is also linearly controlled during low battery conditions to enable the instant-on function, where the converter output can be separated from a heavily discharged battery to power the rest of the system before the battery voltage responds to charging. C/10 detection is also disabled when the charger is operating in instant-on mode.
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and: In CC mode, the LTC4020 will maintain full programmed charge current capability for the duration of the timer period. The trickle charge function is disabled, although maximum charge current will be reduced during lower deck operation if there is excessive voltage (>0.3V) imposed across the PowerPath FET. The charger will terminate the charge cycle and the PowerPath FET will become high-impedance once timer EOC is reached.
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and for the IC: The LTC4020 is typically biased directly from the charger input supply through the PVIN and SENSVIN pins. This supply provides large switched currents, so a high quality, low ESR decoupling capacitor is recommended to mini-mize voltage glitches on the VIN supply. Placing a smaller ceramic capacitor (0.1μF to 10μF) close to the IC in parallel with the input decoupling capacitor is also recommended for high frequency noise reduction. T
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and for the o/p:
Output Decoupling During periods when the LTC4020 DC/DC converter output is not connected to the battery through the PowerPath FET, the system load is driven directly by the converter. The converter creates large switched currents, so a high quality, low ESR decoupling capacitor is recommended to minimize voltage glitches on the VOUT supply. Placing a smaller ceramic decoupling capacitor (0.1μF to 10μF) in parallel with the output decoupling capacitor is also recommended for high frequency noise reduction. The VOUT decoupling capacitor (CVOUT) absorbs the majority of LTC4020244020fdFor more information
www.linear.com/LTC4020the converter ripple current, so it must have an adequate ripple current rating. RMS ripple current (IΔRMS) is highest during step up operation, and follows the relation: ...
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and again Bgate: If the battery voltage is below a programmed minimum operational output volt-age, corresponding to VFBMIN = 2.125V, the PowerPath FET is configured as a linear regulator, allowing the DC/DC converter output to rise above the battery voltage while still providing charge current into the battery. During instant-on operation, the BGATE pin is driven by theLTC4020 to maintain the minimum programmed voltage on the PowerPath FET source, the FET acting as a high impedance current source, providing charge current to the battery, independent of the battery voltage.
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and: in certain applications, the PowerPath function is not required. For example, lead-acid chargers do not termi-nate (they remain in float charging mode indefinitely), so the battery need never be disconnected from the output, provided the instant-on feature is not desired. The PowerPath FET can be eliminated in these applica-tions by tying the CSN side of the sense resistor to BAT, connecting VFBMIN to ground, and connecting a 100pF capacitor from the BGATE pin to CSN. See Typical Ap-plication Circuits section.
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also see fig 20 of data sheet.