LTC3819 Datasheet, PDF (19/32 Pages) Linear Technology – 2-Phase, High Efficiency, Step-Down Controller for Sun Server CPUs

LTC3819
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_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:
( ) ^tDELAY
=
1.5V
1.2µA
C _SS
=
1.25s/µF
C _SS
The time for the output current to ramp up is then:
( ) ^tIRAMP
=
3V − 1.5V
1.2µA
C _SS
=
1.25s/µF
C _SS
By pulling the RUN/SS pin below 0.8V the LTC3819 is put
into low current shutdown (I_Q < 40µA). The RUN/SS pins
can be driven directly from logic as shown in Figure 5.
Diode D1 in Figure 5 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
D1
V_IN
RUN/SS
R_SS*
C_SS
INTV_CC
R_SS*
RUN/SS
D1*
C_SS
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
3719 F05
Figure 5. RUN/SS Pin Interfacing
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 • 10⁵ (C_SS)
If the overload occurs after start-up, the voltage on C_SS will
continue charging and will provide additional time before
latching off:
t_LO2 ≈ (C_SS • 3V)/(1.2µA) = 2.5 • 10⁶ (C_SS)
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor, R_SS, to the RUN/SS pin as
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
3819f
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