BQ24157S Datasheet, PDF (30/45 Pages) Texas Instruments – Full USB Compliance and USB-OTG Support

Not Recommended for New Designs
bq24157S
SLUSB76B – FEBRUARY 2013 – REVISED MAY 2015
9.2.2 Detailed Design Procedure
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Systems design specifications:
• VBUS = 5 V
• V_BAT = 4.2 V (1 cell)
• I_(charge) = 1.25 A
• Inductor ripple current = 30% of fast charge current
SPACE
1. Determine the inductor value (L_OUT) for the specified charge current ripple:
L_OUT =
VBAT ´ (VBUS - VBAT)
VBUS ´ f ´ DI_L
, the worst case is when battery voltage is as close as to half of the input
voltage.
L_OUT
=
5
´
2.5 ´ (5 - 2.5)
(3 ´ 10⁶ ) ´ 1.25
´
0.3
(1)
L_OUT = 1.11 μH
Select the output inductor to standard 1 μH. Calculate the total ripple current with using the 1-μH inductor:
DI_L
=
VBAT ´ (VBUS - VBAT)
VBUS ´ f ´ L_OUT
(2)
DI_L
=
5
´
2.5 ´ (5 -
(3 ´ 106 ) ´
2.5)
(1 ´
10^-6 )
(3)
ΔI_L = 0.42 A
Calculate the maximum output current:
I_LPK
= I_OUT
+
DI_L
2
(4)
0.42
I_LPK = 1.25 + ₂
(5)
I_LPK = 1.46 A
Select 2.5-mm by 2-mm, 1-μH, 1.5-A surface mount multi-layer inductor. The suggested inductor part
numbers are shown in Table 11.
Table 11. Inductor Part Numbers
Part Number
Inductance
Size
Manufacturer
LQM2HPN1R0MJ0
1 μH
2.5 × 2.0 mm
Murata
MIPS2520D1R0
1 μH
2.5 × 2.0 mm
FDK
MDT2520-CN1R0M
1 μH
2.5 × 2.0 mm
TOKO
CP1008
1 μH
2.5 × 2.0 mm
Inter-Technical
2. Determine the output capacitor value (C_OUT) using 40 kHz as the resonant frequency:
1
f_o =
2p ´ L_OUT ´ C_OUT
(6)
1
C_OUT = _{4p2 ´ f02 ´ LOUT}
(7)
C_OUT
=
1
4p² ´ (40 ´ 10³ )² ´ (1 ´ 10^-6 )
(8)
30
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