LTC1709-9 Linear Technology, LTC1709-9 Datasheet - Page 18

• Manufacturer Part Number: LTC1709-9
• Description: 2-Phase/ 5-Bit VID/ Current Mode/ High Efficiency/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1709-8/LTC1709-9
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.
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:
18
BIAS
BIAS
t
DELAY
. The digital inputs accept CMOS voltage levels.
TH(MAX)
is the supply voltage for the VID section. It is
SENSE
SS
1 2
1 5
. The latchoff timer prevents very short,
. The output current limit ramps up slowly,
.
. When the voltage on RUN/SS reaches
.
V
A
U
C
SS
BIAS
U
1 25
to get a digital high input. The
BIAS
.
CC
s F C
but can be driven from
. Therefore, it must be
/
W
SS
U
SENSE
to
The time for the output current to ramp up is then:
By pulling the RUN/SS pin below 0.8V the LTC1709 is put
into low current shutdown (I
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).
Fault Conditions: Overcurrent Latchoff
The RUN/SS pin also provides the ability to latch off the
controllers when an overcurrent condition is detected. The
RUN/SS capacitor, C
current of both controllers. After the controllers have been
started and been given adequate time to charge up the
output capacitors and provide full load current, the RUN/
SS capacitor is used for a short-circuit timer. If the output
voltage falls to less than 70% of its nominal value after C
reaches 4.1V, C
that the output is in an overcurrent condition. If the
condition lasts for a long enough period as determined by
the size of the C
RUN/SS pin voltage is recycled. If the overload occurs
during start-up, the time can be approximated by:
SS
t
t
LO1
IRAMP
3.3V OR 5V
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
to ramp up slowly providing the soft-start function.
(C
SS
D1
3
Figure 6. RUN/SS Pin Interfacing
V
• 0.6V)/(1.2 A) = 5 • 10
1 2
SS
SS
.
V
IN
, the controller will be shut down until the
1 5
begins discharging on the assumption
R
.
SS
A
SS
*
RUN/SS
V
, is used initially to limit the inrush
C
SS
C
SS
Q
< 40 A). The RUN/SS pins
1 25
.
D1*
s F C
/
INTV
5
(C
CC
R
SS
SS
SS
*
)
RUN/SS
170989 F06
C
SS
SS