LTC1909-8 LINER [Linear Technology], LTC1909-8 Datasheet - Page 17

• Manufacturer Part Number: LTC1909-8
• Description: Wide Operating Range,No RSENSE TM Step-Down DC/DC Controller with SMBus Programming
• Manufacturer: LINER [Linear Technology]

OPERATIO
long time for the output of the DC/DC converter to de-
crease to the new voltage under light load conditions. To
reduce this time needed, the PGTMR pin can be connected
to the FCB (force continuous bar) pin of the converter.
When the SEL pin is toggled to select a new code, the FCB
pin is forced low for 50 s. This forces the DC/DC converter
out of Burst Mode
The PGTMR pin may be tied to the same pull-up resistor
as the PGOOD pin.
SMBus Controller Supply
If the EXTV
SMBus controller should be tied to an external 5V supply.
It should not be tied to the INTV
5V regulator at the INTV
is below 1.5V and the SMBus controller will not be pow-
ered up. If the EXTV
APPLICATIO S I FOR ATIO
The basic LTC1909-8 application circuit is shown in
Figure 1. External component selection is primarily deter-
mined by the maximum load current and begins with the
selection of the sense resistance and power MOSFET
switches. The LTC1909-8 uses either an external sense
resistor or the on-resistance of the synchronous power
MOSFET for determining the inductor current. The desired
amount of ripple current and operating frequency largely
determines the inductor value. Finally, C
its ability to handle the large RMS current into the con-
verter and C
the output voltage ripple and transient specification.
Maximum Sense Voltage and V
Inductor current is determined by measuring the voltage
across a sense resistance that appears between the PGND
and SENSE
the voltage applied to the V
approximately (0.133) • V
loop will not allow the inductor current valleys to exceed
(0.133) • V
margin for variations in the LTC1909-8 and external com-
ponent values and a good guide for selecting the sense
resistance is:
RNG
+
CC
OUT
pins. The maximum sense voltage is set by
/R
pin is tied to ground, the V
is chosen with low enough ESR to meet
SENSE
U
TM
CC
U
operation and into continuous mode.
(Refer to Functional Diagram)
pin is tied to an external 5V supply,
. In practice, one should allow some
CC
U
RNG
pin is shut down while V
. The current mode control
CC
RNG
RNG
pin because the internal
W
pin and is equal to
Pin
IN
is selected for
CC
U
pin of the
RUN/SS
An external resistive divider from INTV
set the voltage of the V
resulting in nominal sense voltages of 50mV to 200mV.
Additionally, the V
in which case the nominal sense voltage defaults to 70mV
or 140mV, respectively. The maximum allowed sense
voltage is about 1.33 times this nominal value.
Connecting the SENSE
The LTC1909-8 can be used with or without a sense
resistor. When using a sense resistor, it is placed between
the source of the bottom MOSFET M2 and ground. Con-
nect the SENSE
so that the resistor appears between the SENSE
PGND pins. Using a sense resistor provides a well defined
current limit, but adds cost and reduces efficiency. Alter-
natively, one can eliminate the sense resistor and use the
bottom MOSFET as the current sense element by simply
connecting the SENSE
drain of the bottom MOSFET. This improves efficiency, but
the V
pin since the INTV
an internal switch when V
V
rating of 7V.
Power-Up Reset
On power-up, the internal POR circuit generates a low
reset pulse, which stays low until V
mately 2.2V. The reset pulse forces the SMBus interface
into an idle state in which it listens for a start bit. At the
same time the outputs of both Register 0 and Register 1
are set to 11111B. The DCON bit is pulled high so that the
CPUON pin is pulled low to shut down the DC/DC con-
verter. PGTMR is also pulled low as the converter is shut
down and therefore not in regulation.
Burst Mode is a trademark of Linear Technology Corporation.
CC
R
voltages should be kept below the absolute maximum
SENSE
CC
pin can be tied to the same supply or to the INTV
10 •
+
pin to the source of the bottom MOSFET
CC
I
V
RNG
OUT MAX
RNG
pin is connected to the EXTV
(
pin can be tied to SGND or INTV
+
+
pin to the switch node SW at the
Pin
RNG
EXTVCC
)
pin between 0.5V and 2V
>4.7V. The EXTV
CC
rises above approxi-
LTC1909-8
CC
can be used to
CC
17
CC
pin by
+
19098f
and
and
CC
CC