LTC1706-82
6
170682fa
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 when it is driven higher than V
CC
.
Voltage Sensing and Feedback Pins
The FB pin is a high impedance node that requires mini-
mum layout distance to reduce extra loading and un-
wanted stray pickup.
When used with the LTC1629, the LTC1706-82’s FB,
SENSE, V
CC
and GND pins should be connected, respec-
tively, with the EAIN, V
DIFFOUT
, INTV
CC
, and SGND pins of
the LTC1629. The result of this application is a precisely
controlled, multiphase, variable output voltage supply to
any low voltage, high current system such as a powerful
personal computer, workstation or network server. True
remote sense capability of the LTC1629 is also retained in
this case.
VID Input Characteristics
The VID inputs should be driven with a maximum V
IL
of
0.4V and a minimum V
IH
of 1.6V. However, the VID input
range is not limited to values less than V
CC
. Because of the
internal diode between V
CC
and the pull-up resistor, the
inputs can go higher than V
CC
without being clamped to
V
CC
or damaging the input.
This allows the LTC1706-82 to be fully logic compatible
and operational over a higher input voltage range (less
than the 7V absolute maximum rating).
When a VID input is grounded, there will be a higher
quiescent current flow from Vcc because of a resistor from
Vcc through a series diode to each one of the VID inputs.
This increase in quiescent current is calculated from
I
Q
= N(V
CC
– V
DIODE
)/R
PULLUP
N is the number of grounded VID inputs. V
DIODE
is typically
0.6V while R
PULLUP
has a typical pullup resistance of 40k.
In other words, each VID input has a typical pull up current
of (V
CC
– 0.6)/40K, which is approximately 68µA for a 3.3V
system.
OPERATIO
U
Table 1. VID Inputs and Corresponding Output Voltage
CODE VID4 VID3 VID2 VID1 VID0 OUTPUT
00000 GND GND GND GND GND 1.850
00001 GND GND GND GND Float 1.825
00010 GND GND GND Float GND 1.800
00011 GND GND GND Float Float 1.775V
00100 GND GND Float GND GND 1.750V
00101 GND GND Float GND Float 1.725V
00110 GND GND Float Float GND 1.700V
00111 GND GND Float Float Float 1.675V
01000 GND Float GND GND GND 1.650V
01001 GND Float GND GND Float 1.625V
01010 GND Float GND Float GND 1.600V
01011 GND Float GND Float Float 1.575V
01100 GND Float Float GND GND 1.550V
01101 GND Float Float GND Float 1.525V
01110 GND Float Float Float GND 1.500V
01111 GND Float Float Float Float 1.475V
10000 Float GND GND GND GND 1.450V
10001 Float GND GND GND Float 1.425V
10010 Float GND GND Float GND 1.400V
10011 Float GND GND Float Float 1.375V
10100 Float GND Float GND GND 1.350V
10101 Float GND Float GND Float 1.325V
10110 Float GND Float Float GND 1.300V
10111 Float GND Float Float Float 1.275V
11000 Float Float GND GND GND 1.250V
11001 Float Float GND GND Float 1.225V
11010 Float Float GND Float GND 1.200V
11011 Float Float GND Float Float 1.175V
11100 Float Float Float GND GND 1.150V
11101 Float Float Float GND Float 1.125V
11110 Float Float Float Float GND 1.100V
11111 Float Float Float Float Float NO_CPU
(1.075V)
