TPS51216
SLUSAB9 –NOVEMBER 2010
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External Components Selection
The external components selection is simple in D-CAP™ mode.
1. DETERMINE THE VALUE OF R1 AND R2
The output voltage is determined by the value of the voltage-divider resistor, R1 and R2 as shown in
Figure 30. R1 is connected between VREF and REFIN pins, and R2 is connected between the REFIN pin
and GND. Setting R1 as 10-kΩ is a good starting point. Determine R2 using Equation 6.
R1
R2 =
æ
ç
ç
ç
ç
ö
÷
÷
÷
÷
1.8
-1
I
´ESR
)
æ
ç
ç
è
ö
÷
÷
ø
IND ripple
(
V
-
ç
ç
è
÷
÷
ø
OUT
2
(6)
2. CHOOSE THE INDUCTOR
The inductance value should be determined to yield a ripple current of approximately ¼ to ½ of maximum
output current. Larger ripple current increases output ripple voltage and improves the signal-to-noise ratio
and helps stable operation.
V
(
IN
max
(
- V
´ V
)
V
- V
max
)
´ V
OUT
OUT
(
´
IN
OUT OUT
)
)
(
1
3
L
=
´
=
X
I
´ f
V
I
´ f
V
IN
(
SW
IN
max
(
O
SW
IND ripple
(
max
(
max
)
)
)
)
(7)
The inductor needs a low direct current resistance (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 8.
V
(
´
- V
´ V
OUT OUT
IN
max
(
)
)
V
1
TRIP
I
=
+
IND peak
(
)
8´R
L ´ f
V
IN
(
X
SW
DS on
max
)
( )
(8)
3. CHOOSE THE OCL SETTING RESISTANCE, RTRIP
Combining Equation 4 and Equation 5, RTRIP can be obtained using Equation 9.
æ
ö
æ
ö
V
IN - VOUT
(
)
VOUT
ç
÷
÷
ø
8´ IOCL
-
´
´RDS(on)
ç
ç
è
÷
÷
ø
ç
è
2´L
f
(
´ V
)
IN
(
)
X
SW
RTRIP
=
ITRIP
4. CHOOSE THE OUTPUT CAPACITORS
(9)
Organic semiconductor capacitor(s) or specialty polymer capacitor(s) are recommended. Determine ESR to
meet small signal stability and recommended ripple voltage. A quick reference is shown in Equation 10 and
Equation 11.
f
1
SW
£
2p´ESR´ C
3
OUT
(10)
(11)
V
´ESR
OUT
³ 20mV
f
´L
SW
X
20
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