Datasheet

LTC3707-SYNC
19
3707sfa
the V
OUT
resistive divider to compensate for the current
comparator’s negative input bias current. The maximum
current fl owing out of each pair of SENSE pins is:
I
SENSE
+
+ I
SENSE
–
= (2.4V – V
OUT
)/24k
Since V
OSENSE
is servoed to the 0.8V reference voltage,
we can choose R1 in Figure 2 to have a maximum value
to absorb this current.
Rk
V
VV
MAX
OUT
124
08
24
()
.
.–
=
⎛
⎝
⎜
⎞
⎠
⎟
for V
OUT
< 2.4V
Regulating an output voltage of 1.8V, the maximum value
of R1 should be 32k. Note that for an output voltage above
2.4V, R1 has no maximum value necessary to absorb
the sense currents; however, R1 is still bounded by the
V
OSENSE
feedback current.
Soft-Start/Run Function
The RUN/SS1 and RUN/SS2 pins are multipurpose pins
that provide a soft-start function and a means to shut down
the IC. Soft-start reduces the input power source’s surge
currents by gradually increasing the controller’s current
limit (proportional to V
ITH
). This pin can also be used for
power supply sequencing.
An internal 1.2μA current source charges up the C
SS
capacitor. When the voltage on RUN/SS1 (RUN/SS2)
reaches 1.5V, the particular 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 1.25s/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
µA
C s µF C
D ELAY SS SS
==
()
15
12
125
.
.
./
t
VV
µA
C s µF C
IRAMP SS SS
=
−
=
()
315
12
125
.
.
./
APPLICATIONS INFORMATION
By pulling both RUN/SS pins below 1V, the IC is put into
low current shutdown (I
Q
= 20μA). The RUN/SS pins
can be driven directly from logic as shown in Figure 7.
Diode D1 in Figure 7 reduces the start delay but allows
C
SS
to ramp up slowly providing the soft-start function.
Each RUN/SS pin has an internal 6V zener clamp (See
Functional Diagram).
Fault Conditions: Overcurrent Latchoff
The RUN/SS pins also provide the ability to latch off the
controller(s) when an overcurrent condition is detected.
The RUN/SS capacitor, C
SS
, is used initially to turn on
and limit the inrush current. After the controller has been
started and been given adequate time to charge up the
output capacitor and provide full load current, the RUN/SS
capacitor is used for a short-circuit timer. If the regulator’s
output voltage falls to less than 70% of its nominal value
after C
SS
reaches 4.1V, C
SS
begins discharging on the as-
sumption 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
and the specifi ed discharge current,
the controller will be shut down until the RUN/SS pin volt-
age is recycled. If the overload occurs during start-up, the
time can be approximated by:
t
LO1
≈ [C
SS
(4.1 – 1.5 + 4.1 – 3.5)]/(1.2μA)
= 2.7 • 10
6
(C
SS
)
If the overload occurs after start-up the voltage on C
SS
will
begin discharging from the zener clamp voltage:
t
LO2
≈ [C
SS
(6 – 3.5)]/(1.2μA) = 2.1 • 10
6
(C
SS
)
Figure 7. RUN/SS Pin Interfacing
3.3V OR 5V RUN/SS
V
IN
INTV
CC
RUN/SS
D1
C
SS
R
SS
*
C
SS
R
SS
*
3707 F07
(a) (b)
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF