BQ25040 Datasheet, PDF (18/25 Pages) Texas Instruments – 1.1A, Single-Input, Single Cell Li-Ion Battery Charger With 50mA LDO and 2.3A Production Test Support

bq25040
SLUS910B – MARCH 2009 – REVISED MARCH 2009 ...................................................................................................................................................... www.ti.com
APPLICATION INFORMATION
Selection of Input/Output Capacitors
In most applications, all that is needed is a high-frequency decoupling capacitor on the input power pin. For
normal charging applications, a 1µF ceramic capacitor, placed in close proximity to the IN pin and GND pad
works best. For Production Test mode applications, where the current is up to 2.3A, a 22µF input capacitor is
required. In some applications, depending on the power supply characteristics and cable length, it may be
necessary to increase the input filter capacitor to avoid exceeding the OVP voltage threshold during adapter hot
plug events where the ringing exceeds the deglitch time.
The charger in the bq25040 requires a capacitor from BAT to GND for loop stability. Connect a 1µF ceramic
capacitor from BAT to GND close to the pins for best results. For Production Test mode applications up to 2.3A,
a 40µF capacitor from BAT to GND is required. More output capacitance may be required to minimize the output
droop during large load transients.
The LDO also requires an output capacitor for loop stability. Connect at least a 1µF ceramic capacitor from LDO
to GND close to the pins. For improved transient response, this capacitor may be increased.
b25040 Charger Design Example
The following sections provide an example for determining the component values for use with the bq25040.
Requirements
Refer to Figure 1 for Schematics of the Design Example.
• Supply voltage = 5V
• Fast charge current of approximately 780 mA; ISET - pin 2
• Full Current Threshold = 10% of Fast Charge; IFULL – pin 5
Calculations
Program the Fast Charge Current (ISET)
R_ISET = K_ISET / I_CHG
K_ISET = 530AΩ from the electrical characteristics table.
R_ISET = 530AΩ/0.78A = 679Ω
Select the closest standard value, which for this case is 680Ω. Connect this resistor between ISET (pin 2) and
VSS.
Program the Charge Done Current (IFULL)
R_IFULL = K_IFULL × I_IFULL%
K_IFULL = 200Ω/%.
R_IFULL = 200Ω/% × 10% = 2kΩ.
Connect this resistor between IFULL (pin 5) and VSS.
Status Indicators (CHG and PG)
The STAT pins (PG and CHG) are open drain FETs (internal), if used, should be pulled up via a resistor and
possibly a LED to a power source. If monitored by a host, the host V_CC source should be used. The PG and
CHG are 7V devices. If used as a LED indicator, the BAT, LDO or IN could be used.
If the IN pin is used a 6.2V zener should be used to clamp the voltage if there is a possibility that the input
voltage could exceed 7V. If the BAT pin is used, as the battery voltage changes the intensity of the LED will
change. The brightness is greatly decreased for a battery voltage less than 3V. The LDO may be the best source
to power the LEDs from since it is a regulated source for high input voltages.
18
Submit Documentation Feedback
Product Folder Link(s) :bq25040
Copyright © 2009, Texas Instruments Incorporated