Datasheet

( )
(
)
( )
( )
( )
(
)
( )
f f
OUT OUT OUT OUT
IN max IN max
IND(ripple) SW SW
IN max OUT max IN max
V V V V V V
1 3
L
I V I V
- ´ - ´
= ´ = ´
´ ´
( )
(
)
( )
f
OUT OUT
IN max
TRIP
IND(peak )
DS(on) SW
IN max
V V V
V
1
I
8 R L V
- ´
= + ´
´ ´
( )
f
f
OUT SW
SW
IND(ripple)
V 10 mV 1 D 10 mV L
L
ESR
0.7 V I 0.7 V 70
´ ´ - ´ ´
é ù é ù
´
ë û ë û
= = = W
é ù
ë û
´
é ù é ù
ë û ë û
t
SW
t – Time
0
V
VFB
– Feedback Voltage – mV
10
t
SW
x (1-D)
V
RIPPLE(FB)
TPS51218
SLUS935B – MAY 2009– REVISED FEBRUARY 2012 www.ti.com
EXTERNAL COMPONENTS SELECTION
Selecting external components is simple in D-CAP™ mode.
1. Choose the inductor.
The inductance value should be determined to give the ripple current of approximately 1/4 to 1/2 of maximum
output current. Larger ripple current increases output ripple voltage and improves S/N ratio and helps stable
operation.
(6)
The inductor also needs to have low DCR to achieve good efficiency, as well as enough room above peak
inductor current before saturation. The peak inductor current can be estimated in Equation 7.
(7)
2. Choose the output capacitor(s).
Organic semiconductor capacitor(s) or specialty polymer capacitor(s) are recommended. For loop stability,
capacitance and ESR should satisfy Equation 2. For jitter performance, Equation 8 is a good starting point to
determine ESR.
where
• D is the duty ratio
• the output ripple down slope rate is 10 mV/t
SW
in terms of VFB terminal voltage as shown in Figure 18
• t
SW
is the switching period (8)
Figure 18. Ripple Voltage Down Slope
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