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LTC4054X-4.2 Datasheet(PDF) 10 Page - Linear Technology |
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LTC4054X-4.2 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LTC4054-4.2/LTC4054X-4.2 405442xf APPLICATIO S I FOR ATIO 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 is recommended to reduce ripple voltage. When using high value, low ESR ceramic capacitors, it is recom- mended 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. Power Dissipation The conditions that cause the LTC4054 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 LTC4054 operating from a 5V USB supply is programmed to supply 400mA full-scale current to a discharged Li-Ion battery with a voltage of 3.75V. Assum- ing θJAis150°C/W(seeBoardLayoutConsiderations),the ambient temperature at which the LTC4054 will begin to reduce the charge current is approximately: TA = 120°C – (5V – 3.75V) • (400mA) • 150°C/W TA = 120°C – 0.5W • 150°C/W = 120°C – 75°C TA = 45°C PROG 10k RPROG CFILTER 405442 F02 CHARGE CURRENT MONITOR CIRCUITRY LTC4054 GND Figure 2. Isolating Capacitive Load on PROG Pin and Filtering |
Similar Part No. - LTC4054X-4.2 |
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Similar Description - LTC4054X-4.2 |
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