LTC1706-85 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1706-85' PDF : 8 Pages View PDF
U
OPERATIO
The LTC1706-85 is a precision programmable resistive
divider designed specifically for use with an entire family
of Linear Technology Corporation DC/DC switching regu-
lators with 0.8V internal reference and feedback voltages.
The LTC1706-85 programs an output voltage ranging
from 1.050V to 1.825V in 25mV steps, depending on the
state of the VID input pins. The LTC1706-85 in conjunc-
tion with a Linear Technology DC/DC switching regulator
can be used to create a high performance voltage regu-
lator meeting all the requirements of the Intel VRM 8.5
specification.
Voltage Sensing and Feedback Pins
The LTC1706-85 operates by closing the loop between
the output node and the feedback node of the regulator
with an appropriate resistive divider network. The “top”
feedback resistor, RFB1, connected between SENSE and
FB, is a fixed value of typically 10k. The “bottom” feedback
resistor, RFB2, is set by the five VID inputs to generate the
desired regulator output voltage. Feedback resistors RFB1
and RFB2 are matched and temperature stable in order to
provide a highly accurate output voltage.
The FB pin is a sensitive node in the circuit. Care should
be taken to minimize the layout distance between the
LTC1706-85 FB node and the regulator feedback node. In
addition, it is important to keep tight ground connections
between the two chips.
VID Inputs
The desired output voltage is obtained by applying the
proper voltage or float condition to the five digital VID
inputs. Table␣ 1 shows the translation table with each in-
put state and the corresponding regulator output voltage.
This translation is derived from and adheres to the Intel
VRM 8.5 specification.
Each VID input is pulled up by a 40k resistor in series with
a diode connected to VCC. To produce a digital low a VID
LTC1706-85
input should be grounded or driven to a low state. The
VID inputs must be driven with a maximum VIL of 0.8V
(VCC = 3.3V).
When a VID input is grounded or pulled low with a logic
gate, the power supply current will increase because of
the resistor from VCC through the series diode to the
input. This increase in current is calculated from:
IQ = N • (VCC – VDIODE)/RPULLUP
where N is the number of grounded VID inputs. With
typical values of VCC = 3.3V, VDIODE = 0.6V and RPULLUP
= 40k, each grounded VIN input will sink approximately
68µA.
To apply a digital high state the input can be either floated,
connected to VCC or driven by a logic gate. The VID inputs
should be driven with a minimum VIH of 2V (VCC = 3.3V).
Because of the diode between VCC and the pull-up resis-
tor, the maximum VIH is not limited to VCC. The VID inputs
can be driven higher than VCC without being clamped or
damaged. This allows the LTC1706-85 to be fully logic
compatible and operational over a wide input voltage
range, up to the 7V absolute maximum rating.
When used with the LTC1629 and LTC1929, the
LTC1706-85’s FB, SENSE, VCC and GND pins should be
connected respectively to the EAIN, VDIFFOUT, INTVCC and
SGND pins of the LTC1629 and LTC1929. The result of this
application is a precisely controlled, multiphase, variable
output voltage supply applicable to any low output voltage
system such as a personal computer, workstation or
network server.
In addition to the LTC1629 and LTC1929, the LTC1706-85
also programs a whole family of LTC DC/DC converters
that have an onboard 0.8V reference. The LTC1628,
LTC1735 and LTC1622 are just a few of the high efficiency
step-down switching regulators that will work equally well
with the LTC1706-85. Contact LTC Marketing for a more
complete listing of compatible DC/DC regulators.
170685f
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