5
LTC1706-19
OPERATIO
U
The LTC1706-19 is a precision resistive divider designed
specifically for use with an entire family of Linear Technol-
ogy Corporation DC/DC switching regulators with 1.19V
internal reference and feedback voltages. The LTC1706-19
produces an output voltage ranging from 1.3V to 2V in
50mV steps by closing the loop between the output
voltage sense and the feedback input of the regulator with
the appropriate resistive divider network.
The “top” feedback resistor, R
FB1
, connected between
SENSE and FB, is typically 15k and is not modified by the
state of the VID program inputs. However, the “bottom”
feedback resistor, R
FB2
, is modified by the four VID inputs
and is precisely ratioed to R
FB1
.
VID Programming
Programming is accomplished by applying the proper
voltage (or float condition) on the four digital VID inputs.
VID3 is the most significant bit (MSB), and VID0 is the
least significant bit (LSB). When all four inputs are low, or
grounded, the regulator output voltage is set to 2V. Each
increasing binary count is equivalent to a decrease of
50mv in the output voltage. Therefore, to obtain a 1.3V
output, the three MSBs are left floating, or high, while only
the LSB, VID0, is grounded. A list of programmed inputs
and their corresponding output voltages is shown in
Table 1.
When all four VID inputs are high or floating (1111), such
as when no CPU is present in a system, a regulated 1.25V
output is generated at V
SENSE
.
Each VID
input pin is pulled up by a 40k resistor in series
with a diode connected to V
CC
. Therefore, it should be
grounded (or driven low) to produce a digital low input. It
can be either floated or connected to V
CC
to get a digital
high input. The series diode is included to prevent the
input from being damaged or clamped if it is driven higher
than V
CC
.
Table 1. VID Inputs and Corresponding Output Voltage
CODE VID3 VID2 VID1 VID0 OUTPUT
0000 GND GND GND GND 2.00V
0001 GND GND GND Float 1.95V
0010 GND GND Float GND 1.90V
0011 GND GND Float Float 1.85V
0100 GND Float GND GND 1.80V
0101 GND Float GND Float 1.75V
0110 GND Float Float GND 1.70V
0111 GND Float GND GND 1.65V
1000 Float GND GND GND 1.60V
1001 Float GND GND Float 1.55V
1010 Float GND Float GND 1.50V
1011 Float GND Float Float 1.45V
1100 Float Float GND GND 1.40V
1101 Float Float GND Float 1.35V
1110 Float Float Float GND 1.30V
Voltage Sensing and Feedback Pins
The FB pin is a high impedance node that requires mini-
mum layout distance to reduce extra loading and
unwanted stray pickup.
When used with the LTC1435A, the LTC1706-19’s FB,
SENSE, V
CC
and GND pins should be connected, respec-
tively, with the V
OSENSE
, V
OUT
, INTV
CC
and SGND pins of
the LTC1435A. The result of this application is a precisely
controlled variable voltage supply for any low voltage
system such as a palmtop or a laptop computer.
