LTC3719EG Linear Technology, LTC3719EG Datasheet - Page 19

• Manufacturer Part Number: LTC3719EG
• Description: IC SW REG SYNC STEP-DOWN 36-SSOP
• Manufacturer: Linear Technology

Specifications of LTC3719EG

• Applications: Controller, AMD Opteron™
• Voltage - Input: 4 ~ 36 V
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 1.55 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
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:
The time for the output current to ramp up is then:
By pulling the RUN/SS pin below 0.8V the LTC3719 is put
into low current shutdown (I
can be driven directly from logic as shown in Figure 5.
Diode D1 in Figure 5 reduces the start delay but allows
C
The RUN/SS pin has an internal 6V zener clamp (see
Functional Diagram).
SS
t
t
IRAMP
DELAY
to ramp up slowly providing the soft-start function.
TH(MAX)
SENSE
SS
1 2
3
1 5
. The latchoff timer prevents very short,
. The output current limit ramps up slowly,
.
V
. When the voltage on RUN/SS reaches
1 2
.
.
V
A
1 5
U
C
.
A
SS
V
C
SS
U
1 25
Q
.
< 40 A). The RUN/SS pins
1 25
s F C
.
/
W
s F C
/
SS
SS
U
SENSE
to
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:
If the overload occurs after start-up, the voltage on C
continue charging and will provide additional time before
latching off:
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor, R
shown in Figure 5. This resistance shortens the soft-start
period and prevents the discharge of the RUN/SS capaci-
tor during a severe overcurrent and/or short-circuit
3.3V OR 5V
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
t
t
LO1
LO2
(C
(C
D1
SS
SS
Figure 5. RUN/SS Pin Interfacing
V
• 0.6V)/(1.2 A) = 5 • 10
• 3V)/(1.2 A) = 2.5 • 10
SS
IN
SS
R
, the controller will be shut down until the
SS
begins discharging on the assumption
*
RUN/SS
SS
, is used initially to limit the inrush
C
SS
SS
, to the RUN/SS pin as
D1*
INTV
5
6
CC
R
(C
(C
SS
LTC3719
*
SS
SS
RUN/SS
)
)
3719 F06
sn3719 3719fs
C
19
SS
SS
will
SS