BQ2057DGKR Datasheet, PDF (17/24 Pages) Texas Instruments – ADVANCED LINEAR CHARGE MANAGEMENT IC FOR SINGLE AND TWO CELL LITHIUM-POLYMER

APPLICATION INFORMATION
bq2057, bq2057C
bq2057T, bq2057W
SLUS025F − MAY 2001 − REVISED JULY 2002
selecting input capacitor
In most applications, all that is needed is a high-frequency decoupling capacitor. A 0.1 µF ceramic, placed in
proximity to VCC and VSS pins, works well. The bq2057 works with both regulated and unregulated external
dc supplies. If a non-regulated supply is chosen, the supply unit should have enough capacitance to hold up
the supply voltage to the minimum required input voltage at maximum load. If not, more capacitance must be
added to the input of the charger.
selecting output capacitor
The bq2057 does not require any output capacitor for loop stability. The user can add output capacitance in order
to control the output voltage when a battery is not present. The charger quickly charges the output capacitor
to the regulation voltage, but the output voltage decays slowly, because of the low leakage current on the BAT
pin, down to the recharge threshold. Addition of a 0.1µF ceramic capacitor, for instance, results in a 100 mV(pp)
ripple waveform, with an approximate frequency of 25Hz. Higher capacitor values can be used if a lower
frequency is desired.
automatic charge-rate compensation
To reduce charging time, the bq2057 uses the proprietary AutoComp technique to compensate safely for
internal impedance of the battery pack. The AutoComp feature is disabled by connecting the COMP pin to VCC
in high-side current-sensing configuration, and to VSS in low-side current-sensing configuration. The COMP
pin must not be left floating.
Figure 12 outlines the major components of a single-cell Li-Ion battery pack. The Li-Ion battery pack consists
of a cell, protection circuit, fuse, connector, current sense-resistors, and some wiring. Each of these components
contains some resistance. Total impedance of the battery pack is the sum of the minimum resistances of all
battery-pack components. Using the minimum resistance values reduces the odds for overcompensating.
Overcompensating may activate the safety circuit of the battery pack.
BAT+
Terminal
Fuse
Protection
Controller
Wire
Cell
BAT− _Wire
Terminal
Discharge
Wire
Wire
Charge
Figure 12. Typical Components of a Single-Cell Li-Ion Pack
Compensation is achieved through input pin COMP (Figure 13). A portion of the current-sense voltage,
presented through this pin, is scaled by a factor of G_(COMP) and summed with the regulation threshold, V_O(REG).
This process increases the output voltage to compensate for the battery pack’s internal impedance and for
undesired voltage drops in the circuit.
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