ltc1709-85 Linear Technology Corporation, ltc1709-85 Datasheet - Page 17

• Manufacturer Part Number: ltc1709-85
• Description: 2-phase, 5-bit Vid, Current Mode, High Efficiency, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Output Voltage Programming
The output voltage is digitally programmed as defined in
Table 1 using the VID25mV to VID3 logic input pins. The
VID logic inputs program a precision, 0.25% internal
feedback resistive divider. The LTC1709-85 has an output
voltage range of 1.05V to 1.825V in 25mV steps.
Between the ATTENOUT pin and ground is a variable
resistor, R1, whose value is controlled by the five VID input
pins (VID25mV to VID3). Another resistor, R2, between
the ATTENIN and the ATTENOUT pins completes the
resistive divider. The output voltage is thus set by the ratio
of (R1 + R2) to R1.
Each VID digital input is pulled up by a 40k resistor in
series with a diode from V
grounded to get a digital low input, and can be either
floated or connected to V
series diode is used to prevent the digital inputs from
being damaged or clamped if they are driven higher than
V
V
normally connected to INTV
other sources. If it is driven from another source, that
source must be in the range of 2.7V to 5.5V and must be
alive prior to enabling the LTC1709-85.
Soft-Start/Run Function
The RUN/SS pin provides three functions: 1) Run/Shut-
down, 2) soft-start and 3) a defeatable short-circuit latchoff
timer. Soft-start reduces the input power sources’ surge
currents by gradually increasing the controller’s current
limit I
extreme load transients from tripping the overcurrent
latch. A small pull-up current (>5 A) supplied to the RUN/
SS pin will prevent the overcurrent latch from operating.
The following explanation describes how the functions
operate.
An internal 1.2 A current source charges up the soft-start
capacitor, C
1.5V, the controller is permitted to start operating. As the
voltage on RUN/SS increases from 1.5V to 3.0V, the
internal current limit is increased from 25mV/R
75mV/R
BIAS
BIAS
. The digital inputs accept CMOS voltage levels.
TH(MAX)
is the supply voltage for the VID section. It is
SENSE
SS
. The latchoff timer prevents very short,
. The output current limit ramps up slowly,
. When the voltage on RUN/SS reaches
U
BIAS
U
to get a digital high input. The
BIAS
CC
but can be driven from
. Therefore, it must be
W
U
SENSE
to
taking an additional 1.4s/ F to reach full current. The
output current thus ramps up slowly, reducing the starting
surge current required from the input power supply. If
RUN/SS has been pulled all the way to ground there is a
delay before starting of approximately:
Table 1. VID Output Voltage Programming
V
1.050
1.075
1.100
1.125
1.150
1.175
1.200
1.225
1.250
1.275
1.300
1.325
1.350
1.375
1.400
1.425
1.450
1.475
1.500
1.525
1.550
1.575
1.600
1.625
1.650
1.675
1.700
1.725
1.750
1.775
1.800
1.825
SENSE
t
DELAY
VID3
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
1 2
1 5
.
.
V
A
C
VID2
SS
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1 25
VID1
.
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
s F C
/
LTC1709-85
VID0
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
SS
VID25mV
17
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
170985f