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LTC4058-4.2 Datasheet(PDF) 9 Page - Linear Technology |
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LTC4058-4.2 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC4058-4.2/LTC4058X-4.2 sn405842 405842fs APPLICATIO S I FOR ATIO Kelvin Sensing the Battery (BSENSE Pin) The internal P-channel MOSFET drain is connected to the BAT pin, while the BSENSE pin connects through an inter- nal precision resistor divider to the input of the constant- voltage amplifier. This architecture allows the BSENSE pin to Kelvin sense the positive battery terminal. This is espe- cially useful when the copper trace from the BAT pin to the Li-Ion battery is long and has a high resistance. High charge currents can cause a significant voltage drop be- tween the positive battery terminal and the BAT pin. In this situation, a separate trace from the BSENSE pin to the battery terminals will eliminate this voltage error and re- sult in more accurate battery voltage sensing. The BSENSE pin MUST be electrically connected to the BAT pin. Stability Considerations The constant-voltage mode feedback loop is stable with- out an output capacitor, provided a battery is connected to the charger output. With no battery present, an output capacitor on the BAT pin is recommended to reduce ripple voltage. When using high value, low ESR ceramic capaci- tors, it is recommended to add a 1 Ω resistor in series with the capacitor. No series resistor is needed if tantalum capacitors are used. In constant-current mode, the PROG pin is in the feedback loop, not the battery. The constant-current mode stability is affected by the impedance at the PROG pin. With no additional capacitance on the PROG pin, the charger is stable with program resistor values as high as 20k; how- ever, additional capacitance on this node reduces the maximum allowed program resistor. The pole frequency at the PROG pin should be kept above 100kHz. Therefore, if the PROG pin is loaded with a capacitance, CPROG, the following equation can be used to calculate the maximum resistance value for RPROG: R C PROG PROG ≤ π 1 2105 •• Average, rather than instantaneous charge current may be of interest to the user. For example, if a switching power supply operating in low current mode is connected in parallel with the battery, the average current being pulled out of the BAT pin is typically of more interest than the instantaneous current pulses. In such a case, a simple RC filter can be used on the PROG pin to measure the average battery current, as shown in Figure 2. A 10k resistor has been added between the PROG pin and the filter capacitor to ensure stability. LTC4058-4.2 GND PROG RPROG 10k CFILTER 405842 F02 CHARGE CURRENT MONITOR CIRCUITRY Figure 2. Isolating Capacitive Load on PROG Pin and Filtering Power Dissipation It is not necessary to design for worst-case power dissi- pation scenarios because the LTC4058 automatically re- duces the charge current during high power conditions. The conditions that cause the LTC4058 to reduce charge current through thermal feedback can be approximated by considering the power dissipated in the IC. Nearly all of this power dissipation is generated by the internal MOSFET—this is calculated to be approximately: PD = (VCC – VBAT) • IBAT where PD is the power dissipated, VCC is the input supply voltage, VBAT is the battery voltage and IBAT is the charge current. The approximate ambient temperature at which the thermal feedback begins to protect the IC is: TA = 120°C – PDθJA TA = 120°C – (VCC – VBAT) • IBAT • θJA Example: An LTC4058 operating from a 5V supply is programmed to supply 800mA full-scale current to a discharged Li-Ion battery with a voltage of 3.3V. Assuming θJA is 50°C/W (see Thermal Considerations), the ambient temperature at which the LTC4058 will begin to reduce the charge current is approximately: TA = 120°C – (5V – 3.3V) • (800mA) • 50°C/W TA = 120°C – 1.36W • 50°C/W = 120°C – 68°C TA = 52°C |
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