LTC1909-8 LINER [Linear Technology], LTC1909-8 Datasheet - Page 9

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

PI FU CTIO S
RUN/SS (Pin 1): Run Control and Soft-Start Input. A
capacitor to ground at this pin sets the ramp time to full
output current (approximately 3s/ F) and the time delay
for overcurrent latchoff (see Applications Information).
Forcing this pin below 0.8V shuts down the device.
V
the on-time comparator. Tying this pin to the output
voltage makes the on-time proportional to V
comparator input defaults to 0.7V when the pin is grounded
and 2.4V when the pin is tied to INTV
PGOOD (Pin 3): Power Good Output. Open-drain logic
output that is pulled to ground when the output voltage is
not within 7.5% of the regulation point.
V
this pin is ten times the nominal sense voltage at maxi-
mum output current and can be set from 0.5V to 2V by a
resistive divider from INTV
defaults to 70mV when this pin is tied to ground, 140mV
when tied to INTV
FCB (Pin 5): Forced Continuous Input. Tie this pin to
ground to force continuous synchronous operation at low
load, to INTV
at low load or to a resistive divider from a secondary output
when using a secondary winding.
I
Compensation Point. The current comparator threshold
increases with this control voltage. The voltage ranges
from 0V to 2.4V with 0.8V corresponding to zero sense
voltage (zero current).
SGND (Pin 7): Signal Ground. All small-signal compo-
nents and compensation components should connect to
this ground, which in turn connects to PGND at one point.
I
to this pin to set the one-shot timer current and thereby set
the switching frequency.
V
connects to the error amplifier input to the center tap of the
SMBus programmable divider at the FB pin (Pin 17).
TH
ON
ON
RNG
FB
U
(Pin 6): Current Control Threshold and Error Amplifier
(Pin 8): On-Time Current Input. Tie a resistor from V
(Pin 2): On-Time Voltage Input. Voltage trip point for
(Pin 9): Error Amplifier Feedback Input. This pin
(Pin 4): Sense Voltage Range Input. The voltage at
U
CC
to enable discontinuous mode operation
CC
U
.
CC
. The nominal sense voltage
CC
.
OUT
. The
IN
SEL (Pin 10): Register Select Input. A TTL compatible
logic input pin that is used to select 1 of 2 resistor divider
settings. SEL selects the setting in Register 0 if pulled low
and the setting in Register 1 if pulled high.
SDA (Pin 11): SMBus Data Input/Output. SDA is a high
impedance input when address, command or data bits are
shifted into the SMBus interface. It is an open-drain
N-channel output when acknowledging or sending data
back to the microprocessor during read back. It requires
a pull-up resistor or current source to V
SCL (Pin 12): SMBus Clock Input. Data at the SDA pin is
latched into the LTC1909-8 SMBus interface at the rising
edge of the clock and is shifted out of the SDA pin at the
falling edge of the clock. SCL is a high impedance input
pin. It is driven by the open collector output of a micropro-
cessor and requires a pull-up resistor or current source to
V
VRON (Pin 13): Global Control Input. This TTL compatible
input pin is pulled up internally by a 2.5 A current source.
Pulling VRON low forces the open-drain output pins
(CPUON and PGTMR) to pull to ground. If the LTC1909-8
is programmed to turn on a DC/DC converter, pulling
VRON high three-states the CPUON pin and allows the
switching regulator to soft-start if CPUON is tied to the
RUN/SS pin.
PGTMR (Pin 14): Power Good Timer Output. This open-
drain output is pulled low for 50 s each time the switching
regulator is turned on or SEL is toggled to select a new
code. PGTMR may be connected to the FCB pin to force the
converter into continuous mode operation. This reduces
the time needed for the converter output to settle to a lower
output voltage under light load conditions if the SEL pin is
toggled to select a lower output voltage.
CPUON (Pin 15): CPU DC/DC Converter Control. Open-
drain output, usually connected to the RUN/SS pin. It pulls
low to shut down the converter or becomes high imped-
ance to allow the converter to soft-start.
V
SMBus programmable resistor divider that is connected
directly to the regulated output voltage node.
CC
OSENSE
.
(Pin 16): Sense Input. Upper terminal of the
LTC1909-8
CC
.
19098f
9