ltc1709-7 Linear Technology Corporation, ltc1709-7 Datasheet - Page 18

• Manufacturer Part Number: ltc1709-7
• Description: 2-phase, 5-bit Vid, Current Mode, High Efficiency, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC1709-7
Table 1. VID Output Voltage Programming
*Represents codes without a defined output voltage as specified in Intel
specifications. The LTC1709-7 interprets these codes as a valid input and
produces an output voltage as follows:
18
VID4
(11111) = 0.900V
(01111) = 1.250V
0
0
0
0
0
0
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
VID3
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
1
U
VID2
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
0
0
1
1
1
1
U
VID1
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
W
VID0
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
Shutdown*
LTC1709-7
No_CPU/
U
2.000V
1.950V
1.900V
1.850V
1.800V
1.750V
1.700V
1.650V
1.600V
1.550V
1.500V
1.450V
1.400V
1.350V
1.300V
1.275V
1.250V
1.225V
1.200V
1.175V
1.150V
1.125V
1.100V
1.075V
1.050V
1.025V
1.000V
0.975V
0.950V
0.925V
*
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
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:
The time for the output current to ramp up is then:
By pulling the RUN/SS pin below 0.8V the LTC1709-7 is
put into low current shutdown (I
pins can be driven directly from logic as shown in Figure
6. Diode D1 in Figure 6 reduces the start delay but allows
C
The RUN/SS pin has an internal 6V zener clamp (see
Functional Diagram).
SS
t
t
IRAMP
DELAY
to ramp up slowly providing the soft-start function.
TH(MAX)
SENSE
SS
1 2
3
1 5
. The latchoff timer prevents very short,
. The output current limit ramps up slowly,
.
V
. When the voltage on RUN/SS reaches
1 2
.
.
V
A
1 5
C
.
A
SS
V
C
SS
1 25
.
1 25
s F C
.
Q
/
< 40 A). The RUN/SS
s F C
/
SS
SS
SENSE
to