Datasheet

( )
-
= ´ ´
´
IN OUT
OUT
LOAD(LL)
X IN SW
V V
V
1
I
2 L V f
æ ö
= ´
ç ÷
è ø
TRIP
OCTRIP TRIP
I
V R
8
( ) ( )
æ ö æ ö
-
ç ÷ ç ÷
= + = + ´ ´
ç ÷ ç ÷
´
è ø è ø
IND(ripple)
OCTRIP OCTRIP IN OUT OUT
OCL
X SW IN
DS on DS on
I
V V V V V
1
I
R 2 R 2 L f V
TPS51219
www.ti.com SLUSAG1B –MARCH 2011– REVISED OCTOBER 2011
Light-Load Operation
In auto-skip mode, the TPS51219 SMPS control logic automatically reduces its switching frequency to improve
light-load efficiency. To achieve this intelligence, a zero cross detection comparator is used to prevent negative
inductor current by turning off the low-side MOSFET. Equation 6 shows the boundary load condition of this skip
mode and continuous conduction operation.
(6)
Current Sensing
In order to provide both cost effective solution and good accuracy, TPS51219 supports both of MOSFET R
DS(on)
sensing and external resistor sensing. For R
DS(on)
sensing scheme, TRIP pin should be connected to GND
through the trip voltage setting resistor, R
TRIP
. In this scheme, TRIP terminal sources 10µA of I
TRIP
current and
the trip level is set to 1/8 of the voltage across the R
TRIP
. The inductor current is monitored by the voltage
between the PGND pin and the SW pin so that the SW pin is connected to the drain terminal of the low-side
MOSFET. I
TRIP
has a 4700ppm/°C temperature slope to compensate the temperature dependency of the R
DS(on)
.
For resistor sensing scheme, an appropriate current sensing resistor should be connected between the source
terminal of the low-side MOSFET and PGND. The TRIP pin is connected to the MOSFET source terminal node.
The inductor current is monitored by the voltage between PGND pin and TRIP pin. In either scheme, PGND is
used as the positive current sensing node so that PGND should be connected to the proper current sensing
device, i.e. the sense resistor or the source terminal of the low-side MOSFET.
Overcurrent Protection
TPS51219 has cycle-by-cycle overcurrent limiting protection. The inductor current is monitored during the
off-state and the controller maintains the off-state when the inductor current is larger than the overcurrent trip
level. The trip level and current sense operation are determined by the MODE pin setting and TRIP pin
connection (See Table 2 and Current Sensing section). For R
DS(on)
sensing scheme, TRIP terminal sources
10 µA and the trip level is set to 1/8 of the voltage across this R
TRIP
resistor. The overcurrent trip level, V
OCTRIP
,
is determined by Equation 7.
(7)
For a resistor sensing scheme, the trip level, V
OCTRIP
, is a fixed value of 25 mV.
Because the comparison is made during the off-state, V
OCTRIP
sets the valley level of the inductor current. The
load current OCL level, I
OCL
, can be calculated by considering the inductor ripple current.
Overcurrent limiting using R
DS(on)
sensing is shown in Equation 8.
where
• I
IND(ripple)
is inductor ripple current (8)
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