LTC3728LX Linear Technology, LTC3728LX Datasheet - Page 19

• Manufacturer Part Number: LTC3728LX
• Description: 2-Phase Synchronous Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Since V
can choose R1 in Figure 2 to have a maximum value to
absorb this current.
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
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 LTC3728L/LTC3728LX. Soft-start reduces the
input power source’s surge currents by gradually increas-
ing the controller’s current limit (proportional to V
This pin can also be used for power supply sequencing.
An internal 1.2 A current source charges up the C
capacitor
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
up slowly, taking an additional 1.25s/ F to reach full
current. The output current thus ramps up slowly, reduc-
ing 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:
By pulling both RUN/SS pins below 1V, the IC is put into
low current shutdown (I
be driven directly from logic as shown in Figure 7. Diode
D1 in Figure 7 reduces the start delay but allows C
OSENSE
SENSE
for V
R
t
t
IRAMP
DELAY
1
(
MAX
OSENSE
OUT
to 75mV/R
feedback current.
.
)
When the voltage on RUN/SS1 (RUN/SS2)
< 2.4V
1 2
3
1 5
24
.
V
is servoed to the 0.8V reference voltage, we
1 2
.
.
k
V
A
1 5
SENSE
2 4
U
A
C
.
.
SS
V
V V
0 8
C
. The output current limit ramps
Q
.
–
SS
U
= 20 A). The RUN/SS pins can
V
1 25
OUT
.
1 25
s
.
/
W
s
F C
/
F C
SS
SS
U
SS
ITH
SS
to
).
ramp up slowly providing the soft-start function. Each
RUN/SS pin has an internal 6V zener clamp (See Func-
tional 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
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
assumption that the output is in an overcurrent condition.
If the condition lasts for a long enough period as deter-
mined by the size of the C
current, 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:
If the overload occurs after start-up the voltage on C
begin discharging from the zener clamp voltage:
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor to the RUN/SS pin as shown
in Figure 7. This resistance shortens the soft-start period
and prevents the discharge of the RUN/SS capacitor
during an over current condition. Tying this pull-up resis-
tor to V
t
t
LO1
LO2
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
3.3V OR 5V
SS
IN
= 2.7 • 10
[C
, as in Figure 7a, defeats overcurrent latchoff.
reaches 4.1V, C
[C
SS
SS
D1
LTC3728L/LTC3728LX
Figure 7. RUN/SS Pin Interfacing
(6 – 3.5)]/(1.2 A) = 2.1 • 10
(4.1 – 1.5 + 4.1 – 3.5)]/(1.2 A)
(a)
V
IN
6
R
(C
SS
*
RUN/SS
SS
)
SS
SS
SS
C
, is used initially to turn on and
SS
and the specified discharge
begins discharging on the
w w w . D a t a S h e e t 4 U . c
INTV
CC
R
SS
(b)
6
*
(C
RUN/SS
SS
19
)
3728 F07
SS
C
SS
3728lxfa
will