FSP4054 参数 Datasheet PDF下载

Standalone Linear Li-ion Battery Charger with Thermal Regulation

FSP4054

APPLICATION INFORMATION

Stability Considerations

The constant-voltage mode feedback loop is stable without 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 recommended to add a 1W 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.

However, 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, C

PROG

, the

following equation can be used to calculate the maximum resistance value for R

PROG

:

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.

Figure 2: Isolating capacitive load on PROG pin and filtering

Power Dissipation

The conditions that cause the FSP4054 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:

where P

D

is the power dissipated, VCC is the input supply voltage, V

BAT

is the battery voltage and I

BAT

is the charge

current. The approximate ambient temperature at which the thermal feedback begins to protect the IC is:

Example: An FSP4054 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. Assuming

θ

JA

is 150°C/W, the ambient temperature at which

the FSP4054 will begin to reduce the charge current is approximately:

The FSP4054 can be used above 45°C ambient, but the charge current will be reduced from 400mA. The

approximate current at a given ambient temperature can be approximated by:

Using the previous example with an ambient temperature of 60°C, the charge current will be reduced to

approximately:

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2007-7-3