BQ24157S Datasheet, PDF (32/45 Pages) Texas Instruments – Full USB Compliance and USB-OTG Support

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bq24157S
SLUSB76B – FEBRUARY 2013 – REVISED MAY 2015
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9.2.2.1 Charge Current Sensing Resistor Selection Guidelines
Both the termination current range and charge current range depend on the sensing resistor (R_SNS). The
termination current step (I_{OTERM_STEP}) can be calculated using Equation 11.
I_{O(TERM_STEP)}
=
V_I(TERM0)
R_(SNS)
(11)
Table 12 shows the termination current settings for three sensing resistors.
Table 12. Termination Current Settings for 55-mΩ, 68-mΩ, and 100-mΩ Sense Resistors
BIT
V_I(TERM2)
V_I(TERM1)
V_I(TERM0)
Offset
V_I(TERM) (mV)
13.6
6.8
3.4
3.4
I_(TERM) (mA)
R_(SNS) = 55 mΩ
247
124
62
62
I_(TERM) (mA)
R_(SNS) = 68 mΩ
200
100
50
50
I_(TERM) (mA)
R_(SNS) = 100 mΩ
136
68
34
34
For example, with a 68-mΩ sense resistor, V_(ITERM2) = 1, V_(ITERM1) = 0, and V_(ITERM0) = 1, I_TERM = [(13.6 mV × 1) +
(6.8 mV × 0) + (3.4 mV × 1) + 3.4 mV] / 68 mΩ = 200 mA + 0 + 50 mA + 50 mA = 300 mA.
The charge current step (I_{O(CHARGE_STEP)}) is calculated using Equation 12.
I_{O(CHARGE_STEP)}
=
V_I(CHRG0)
R_(SNS)
(12)
Table 13 shows the charge current settings for three sensing resistors.
Table 13. Charge Current Settings for 55-mΩ, 68-mΩ, and 100-mΩ Sense Resistors
BIT
V_I(CHRG3)
V_I(CHRG2)
V_I(CHRG1)
V_I(CHRG0)
Offset
V_I(REG) (mV)
54.4
27.2
13.6
6.8
37.4
I_O(CHARGE) (mA)
R_(SNS) = 55 mΩ
989
495
247
124
680
I_O(CHARGE) (mA)
R_(SNS) = 68 mΩ
800
400
200
100
550
I_O(CHARGE) (mA)
R_(SNS) = 100 mΩ
544
272
136
68
374
For example, with a 68-mΩ sense resistor, V_(CHRG3) = 1, V_(CHRG2) = 0, V_(ICHRG1) = 0, and V_(ICHRG0) = 1, I_TERM =
[(54.4 mV × 1) + (27.2 mV × 0) + (13.6 mV × 0) + (6.8 mV × 1) + 37.4 mV] / 68 mΩ = 800 mA + 0 + 0 + 100 mA
= 900 mA.
9.2.2.2 Output Inductor and Capacitance Selection Guidelines
The IC provides internal loop compensation. With the internal loop compensation, the highest stability occurs
when the LC resonant frequency, ƒ_o, is approximately 40 kHz (20 to 80 kHz). Equation 13 can be used to
calculate the value of the output inductor, L_OUT, and output capacitor, C_OUT.
1
f_o =
2p ´ L_OUT ´ C_OUT
(13)
To reduce the output voltage ripple, TI recommends a ceramic capacitor with the capacitance between 4.7 to 47
μF for C_OUT. See previous sections in the Detailed Design Procedure for components selection.
32
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