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| BQ24157 Datasheet, PDF (31/39 Pages) Texas Instruments – Fully Integrated Switch-Mode Charger With USB Compliance and USB-OTG Support | |||
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bq24157
SLUSB80 â SEPTEMBER 2012
DIL
=
VBAT ´ (VBUS - VBAT)
VBUS ´ f ´ LOUT
(5)
DIL
=
5
´
2.5 ´ (5 -
(3 ´ 106 ) ´
2.5)
(1 ´
10-6 )
(6)
ÎIL = 0.42 A
Calculate the maximum output current:
ILPK
= IOUT
+
DIL
2
(7)
0.42
ILPK = 1.25 + 2
(8)
ILPK = 1.46 A
Select 2.5mm by 2mm 1-μH 1.5-A surface mount multi-layer inductor. The suggested inductor part numbers
are shown as following.
Table 12. Inductor Part Numbers
PART NUMBER
LQM2HPN1R0MJ0
MIPS2520D1R0
MDT2520-CN1R0M
CP1008
1 μH
1 μH
1 μH
1 μH
INDUCTANCE
SIZE
2.5 x 2.0 mm
2.5 x 2.0 mm
2.5 x 2.0 mm
2.5 x 2.0 mm
MANUFACTURER
Murata
FDK
TOKO
Inter-Technical
2. Determine the output capacitor value (COUT) using 40 kHz as the resonant frequency:
1
fo =
2p ´ LOUT ´ COUT
(9)
1
COUT = 4p2 ´ f02 ´ LOUT
(10)
COUT
=
1
4p2 ´ (40 ´ 103 )2 ´ (1 ´ 10-6 )
(11)
COUT = 15.8 μF
Select two 0603 X5R 6.3V 10-μF ceramic capacitors in parallel i.e., Murata GRM188R60J106M.
3. Determine the sense resistor using the following equation:
R(SNS)
=
V(RSNS)
I(CHARGE)
(12)
The maximum sense voltage across the sense resistor is 85 mV. In order to get a better current regulation
accuracy, V(RSNS) should equal 85mV, and calculate the value for the sense resistor.
R(SNS)
=
85mV
1.25A
(13)
R(SNS) = 68 mâ¦
This is a standard value. If it is not a standard value, then choose the next close value and calculate the real
charge current. Calculate the power dissipation on the sense resistor:
P(RSNS) = I(CHARGE) 2 Ã R(SNS)
P(RSNS) = 1.252 Ã 0.068
P(RSNS) = 0.106 W
Copyright © 2012, Texas Instruments Incorporated
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