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LTC4057 Datasheet(PDF) 7 Page - Linear Technology |
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LTC4057 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LTC4057-4.2 7 4057f Stability Considerations The constant-voltage mode feedback loop is stable with- out an output capacitor provided a battery is connected to the charge output. When an output capacitor is used, especially high value low ESR ceramic types, it is recom- mended that a 1 Ω resistor be placed in series with the capacitor to stabilize the voltage loop. The loop stability is determined by the bypass capacitor as well as the effective series resistance of the battery. When the battery is disconnected and the LTC4057 is still powered, the voltage regulation loop should be compen- sated by placing a capacitor greater than 1 µF from the BAT pin to ground with a 1 Ω to 2Ω resistor in series with this capacitor. Alternatively, powering down the LTC4057 or placing it into shutdown mode when the battery is discon- nected avoids this problem. 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 value. The pole fre- quency 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 ≤ 1 2105 π •• Average, rather than instantaneous, battery 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 1. A 10k resistor has been added between the PROG pin and the filter capacitor to ensure stability. APPLICATIO S I FOR ATIO Power Dissipation The conditions that cause the LTC4057 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 LTC4057 operating from a 4.5V USB supply is programmed to supply 600mA full-scale current to a discharged Li-Ion battery with a voltage of 3.7V. Assuming θJA is 150°C/W (see Board Layout Considerations), the ambient temperature at which the LTC4057 will begin to reduce the charge current is approximately: TA = 120°C – (4.5V – 3.7V) • (600mA) • 150°C/W TA = 120°C – 0.48W • 150°C/W = 120°C – 72°C TA = 48°C The LTC4057 can be used above 48 °C ambient, but the charge current will be reduced from 600mA. The approxi- mate current at a given ambient temperature can be approximated by: I CT VV BAT A CC BAT JA = ° − 120 – ()• θ LTC4057-4.2 PROG GND 10k RPROG CFILTER CHARGE CURRENT MONITOR CIRCUITRY 4057 F01 Figure 1. Isolating Capacitive Load on PROG Pin and Filtering |
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Similar Description - LTC4057 |
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