Datasheet

www.ti.com
Peak Current Rating of Inductor
I
CHARGE
+
V
OUT
C
OUT
T
SS
(9)
L
L(peak)
+ I
OUT(max)
)
1
2
ǒ
I
RIPPLE
Ǔ
) I
CHARGE
(10)
Input Capacitor Selection (C1 & C2)
C
IN(min)
+
I
LOAD
V
OUT
V
RIPPLE(cap)
V
IN
F
SW
(11)
ESR
MAX
+
V
RIPPLE(ESR)
I
LOAD
)
1
2
ǒ
I
RIPPLE
Ǔ
(12)
I
RMS(cin)
+ I
IN(rms)
* I
IN(avg)
+
ƪ
ǒ
I
OUT
Ǔ
2
)
1
12
ǒ
I
RIPPLE
Ǔ
2
ƫ
V
OUT
V
IN
Ǹ
*
V
OUT
I
OUT
V
IN
(13)
TPS40040 , TPS40041
SLUS700D – MARCH 2006 – REVISED DECEMBER 2007
With output capacitance known, it is now possible to calculate the charging current during start-up and determine
the minimum saturation current rating for the inductor. The start-up charging current is approximated by:
Using the TPS40041 ’ s fixed 4.5-ms soft-start time, C
OUT
= 200 µ F and V
OUT
= 1.8 V, I
CHARGE
is found to be 80
mA. The peak current rating of the inductor is now found by:
The inductor requirements are summarized in the table below.
Inductor Requirements
PARAMETER SYMBOL VALUE UNITS
Inductance L 1.0 µ H
RMS current (thermal rating) I
L(rms)
6.03 A
Peak current (saturation rating) I
L(peak)
7.08
A PG0083.102, 1.0 µ H is selected for its small size, low DCR and high current handling capability.
The input voltage ripple is divided between capacitance and ESR. For this design, V
RIPPLE(CAP)
= 50 mV and
V
RIPPLE(ESR)
= 25 mV. The minimum capacitance and maximum ESR are estimated by:
For this design, C
IN
> 80 µ F and ESR < 3.5 m Ω . The RMS current in the input capacitors is estimated by:
With V
IN
= V
IN(max)
, the input capacitors must support a ripple current of 1.56 A
RMS
. Two 1206, 100- µ F, X5R
ceramic capacitors with about 5-m Ω ESR and a 2-A RMS current rating are selected. It is important to check the
dc bias voltage derating curves to ensure the capacitors provide sufficient capacitance at the working voltage.
Copyright © 2006 – 2007, Texas Instruments Incorporated 21
Product Folder Link(s): TPS40040 TPS40041