Datasheet

( )
=
æ ö
ç ÷
ç ÷
ç ÷
-
´
æ ö
ç ÷
ç ÷
-
ç ÷
ç ÷
ç ÷
è ø
è ø
IND ripple
OUT
R1
R2
2.0
1
I ESR
V
2
( )
( )
(
)
( ) ( )
( )
(
)
( )
- ´ - ´
= ´ = ´
´ ´
IN OUT OUT IN OUT OUT
max max
X
SW IN O SW IN
IND ripple max max max
V V V V V V
1 3
L
I f V I f V
( )
( )
( )
(
)
( )
- ´
= + ´
´ ´
IN OUT OUT
max
TRIP
IND peak
X SW IN
DS on max
V V V
V
1
I
8 R L f V
( )
( )
( )
æ ö
æ ö
-
ç ÷
´ - ´ ´
ç ÷
ç ÷
ç ÷
´
´
è ø
è ø
=
IN OUT
OUT
OCL DS(on)
X
SW IN
TRIP
TRIP
V V
V
8 I R
2 L
f V
R
I
=
æ ö
-
- ´
ç ÷
´ ´
è ø
EXT
IN OUT OUT
OCL
X SW IN
25mV
R
V V V
I
2 L f V
´ =
X X
EXT
ESL
C R
R
TPS51219
www.ti.com SLUSAG1B –MARCH 2011– REVISED OCTOBER 2011
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 25.
R1 is connected between VREF and REFIN pins, and R2 is connected between the REFIN pin and GSNS.
Setting R1 as 10-kΩ is a good starting point. Determine R2 using Equation 10.
(10)
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.
(11)
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 12.
(12)
3. CHOOSE THE OCL SETTING RESISTANCE
R
TRIP
for R
DS(on)
Sensing
Combining Equation 7 and Equation 8, R
TRIP
can be obtained using Equation 13.
(13)
R
EXT
for Resistor Setting
Combining Equation 7 and Equation 9, R
EXT
can be obtained using Equation 14.
(14)
For more accurate current sensing with an external resistor, the following technique is recommended. Adding an
RC filter to cancel the parasitic inductance (ESL) of resistor, this filter value is calculated using Equation 15.
(15)
The time-constant of C
X
and R
X
should match the one of ESL and R
EXT
. Even when C
X
is not used, an R
X
of
100 Ω is recommended for noise suppression.
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