BQ24257 Datasheet, PDF (19/45 Pages) Texas Instruments – 2A Single Input I2C, Standalone Switch-Mode Li-Ion Battery Charger with Integrated Current Sense Resistor

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bq24257
bq24258
SLUSBG0B – FEBRUARY 2013 – REVISED JULY 2013
Figure 16 illustrates the behavior of the bq24257 when transitioning between i2c mode and stand alone mode
(except for the bq24257).
Battery or Input
is Inserted
No
VIN or VBAT GOOD?
Yes
No
i2c command received?
Yes
ILIM=Register Value
VDPM=Register Value
ISET=Register Value
ILIM=EN1/EN2
VDPM=External Default
ISET=External Default
No
Yes
32s Watchdog Expired?
Figure 16. I2C and Stand Alone Mode Handoff
Once the battery or input is inserted and above the good thresholds, the device will determines if an i2c
command has been received in order to discern whether to operate from the i2c registers or the external
settings. Note that the bq24257 does not have EN1/EN2 pins and therefore the input current limit will be based
on the D+/D– results. When in host mode (i2c operation), the device will enter stand alone operation once the
watchdog timer expires.
External settings: ISET, ILIM and VIN_DPM
The fast charge current resistor (R_ISET) can be set by using the following formula:
R_ISET
K_ISET
= _IFC
=
250
I_FC
(1)
Where I_FC is the desired fast charge current setting in Amperes.
The input current limit resistor (R_ILIM) can be set by using the following formula:
R_ILIM
=
K_ILIM
I_IC
=
270
I_IC
(2)
Where _IIC is the desired input current limit in Amperes.
Based on the application diagram reference designators, the resistor R₁ and R₂ can be calculated as follows to
set V_{IN_DPM}:
V_{IN _ DPM}
=
V_{REF _ DPM}
´
R₁ + R₂
R₂
= 1.2 ´ ^{R1 + R2}
R₂
(3)
V_{IN_DPM} should be chosen first along with R₁. Choosing R₁ first will ensure that R₂ will be greater than the
resistance chosen. This is the case since V_{IN_DPM} should be chosen to be greater than 2x V_{REF_DPM}.
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