LTC3728 Linear Technology, LTC3728 Datasheet - Page 19

• Manufacturer Part Number: LTC3728
• Description: 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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 LTC3728. Soft-start reduces the input power
source’s surge currents by gradually increasing the
controller’s current limit (proportional to V
can also be used for power supply sequencing.
An internal 1.2µA 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/µF 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 LTC3728 is
put into low current shutdown (I
pins can be driven directly from logic as shown in Figure
7. Diode D1 in Figure 7 reduces the start delay but allows
C
Each RUN/SS pin has an internal 6V zener clamp (See
Functional Diagram).
OSENSE
SS
SENSE
for V
R
t
t
IRAMP
DELAY
1
to ramp up slowly providing the soft-start function.
(
MAX
OUT
to 75mV/R
feedback current.
.
)
=
=
When the voltage on RUN/SS1 (RUN/SS2)
=
< 2.4V
1 2
3
1 5
24
.
V
1 2
.
µ
.
k
−
V
⎛
⎜
⎝
A
µ
1 5
SENSE
2 4
U
C
.
A
.
SS
V
V V
0 8
C
.
. The output current limit ramps
–
=
SS
U
V
(
1 25
OUT
=
.
(
1 25
⎞
⎟
⎠
s
.
Q
/
= 20µA). The RUN/SS
W
µ
s
F C
/
)
µ
F C
SS
)
ITH
SS
). This pin
U
SS
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
Diode-connecting this pull-up resistor to INTV
Figure 7b, eliminates any extra supply current during
controller shutdown while eliminating the INTV
from preventing controller start-up.
t
t
LO1
LO2
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
3.3V OR 5V
SS
≈ [C
IN
≈ [C
= 2.7 • 10
reaches 4.1V, C
as in Figure 7a, defeats overcurrent latchoff.
SS
SS
D1
Figure 7. RUN/SS Pin Interfacing
(6 – 3.5)]/(1.2µA) = 2.1 • 10
(4.1 – 1.5 + 4.1 – 3.5)]/(1.2µA)
(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
LTC3728
CC
R
SS
(b)
6
*
(C
RUN/SS
CC
SS
CC
19
)
loading
3728 F07
, as in
SS
C
SS
3728fb
will