Datasheet

QFN I2C PART
WCSP I2C PART
AGNDBYP
BOOT
SW
SYS
SW
PMID
USB
BAT
SYS
PGND
BYP
PMID
USB
SW
SYS
SYS
BAT
PGND
BOOT
AGND
PGND
%
RIPPPLE
I = I × (1 + )
PEAK
LOAD(MAX)
2
bq24270
bq24271
SLUSB10 –JUNE 2012 www.ti.com
LOW_CHG Bit (Low Charge Mode Enable)
The LOW_CHG bit is used to reduce the charge current from the programmed value. This feature is used by
systems where battery NTC is monitored by the host and requires a reduced charge current setting or by
systems that need a “preconditioning” current for low battery voltages. Write a “1” to this bit to charge at half of
the programmed charge current. Write a “0” to this bit to charge at the programmed charge current.
APPLICATION INFORMATION
Output Inductor and Capacitor Selection Guidelines
When selecting an inductor, several attributes must be examined to find the right part for the application. First,
the inductance value should be selected. The bq24270 and bq24271 are designed to work with 1.5 µH to 2.2 µH
inductors. The chosen value will have an effect on efficiency and package size. Due to the smaller current ripple,
some efficiency gain is reached using the 2.2 µH inductor, however, due to the physical size of the inductor, this
may not be a viable option. The 1.5 µH inductor provides a good tradeoff between size and efficiency.
Once the inductance has been selected, the peak current must be calculated in order to choose the current
rating of the inductor. Use Equation 5 to calculate the peak current.
(5)
The inductor selected must have a saturation current rating less than or equal to the calculated I
PEAK
. Due to the
high currents possible with the devices, a thermal analysis must also be done for the inductor. Many inductors
have 40°C temperature rise rating. This is the DC current that will cause a 40°C temperature rise above the
ambient temperature in the inductor. For this analysis, the typical load current may be used adjusted for the duty
cycle of the load transients. For example, if the application requires a 1.5 A DC load with peaks at 2.5 A 20% of
the time, a Δ40°C temperature rise current must be greater than 1.7 A:
I
TEMPRISE
= I
LOAD
+ D × (I
PEAK
– I
LOAD
) = 1.5 A + 0.2 × (2.5 A – 1.5 A) = 1.7 A (6)
The devices provide internal loop compensation. Using this scheme, the bq24270 is stable with 10 µF to 200 µF
of local capacitance. The capacitance on the SYS rail can be higher if distributed amongst the rail. To reduce the
output voltage ripple, a ceramic capacitor with the capacitance between 10µF and 47µF is recommended for
local bypass to SYS.
PCB Layout Guidelines
It is important to pay special attention to the PCB layout. Figure 13 provides a sample layout for the high current
paths of the bq24270 and bq24271.
Figure 13. Recommended bq24270 and bq24271 PCB Layout
28 Submit Documentation Feedback Copyright © 2012, Texas Instruments Incorporated
Product Folder Link(s): bq24270 bq24271