18
LTC1709-8/LTC1709-9
APPLICATIO S I FOR ATIO
WUU
U
Figure 6. RUN/SS Pin Interfacing
The time for the output current to ramp up is then:
t
VV
A
CsFC
IRAMP SS SS
=
−
µ
=µ
()
315
12
125
.
.
./
By pulling the RUN/SS pin below 0.8V the LTC1709 is put
into low current shutdown (I
Q
< 40µA). The RUN/SS pins
can be driven directly from logic as shown in Figure 6.
Diode D1 in Figure 6 reduces the start delay but allows
C
SS
to ramp up slowly providing the soft-start function.
The RUN/SS pin has an internal 6V zener clamp (see
Functional Diagram).
3.3V OR 5V RUN/SS
V
IN
INTV
CC
RUN/SS
D1
D1*
C
SS
R
SS
*
C
SS
R
SS
*
170989 F06
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
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
SS
, is used initially to limit the inrush
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
SS
reaches 4.1V, C
SS
begins discharging on the assumption
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
SS
, the controller will be shut down until the
RUN/SS pin voltage is recycled. If the overload occurs
during start-up, the time can be approximated by:
t
LO1
≈ (C
SS
• 0.6V)/(1.2µA) = 5 • 10
5
(C
SS
)
Each VID digital input is pulled up by a 40k resistor in
series with a diode from V
BIAS
. Therefore, it must be
grounded to get a digital low input, and can be either
floated or connected to V
BIAS
to get a digital high input. The
series diode is used to prevent the digital inputs from
being damaged or clamped if they are driven higher than
V
BIAS
. The digital inputs accept CMOS voltage levels.
V
BIAS
is the supply voltage for the VID section. It is
normally connected to INTV
CC
but can be driven from
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
TH(MAX)
. The latchoff timer prevents very short,
extreme load transients from tripping the overcurrent
latch. A small pull-up current (>5µA) supplied to the RUN/
SS pin will prevent the overcurrent latch from operating.
The following explanation describes how the functions
operate.
An internal 1.2µA current source charges up the soft-start
capacitor, C
SS
.
When the voltage on RUN/SS reaches
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
SENSE
to
75mV/R
SENSE
. The output current limit ramps up slowly,
taking an additional 1.4s/µF 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:
t
V
A
CsFC
DELAY SS SS
=
µ
=µ
()
15
12
125
.
.
./
