LTC1709 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1709
Linear Technology
'LTC1709' PDF : 28 Pages View PDF
LTC1709
APPLICATIO S I FOR ATIO
floated or connected to VBIAS to get a digital high input. The
series diode is used to prevent the digital inputs from
being damaged or clamped if they are driven higher than
VBIAS. The digital inputs accept CMOS voltage levels.
VBIAS is the supply voltage for the VID section. It is
normally connected to INTVCC but can be driven from
other sources. If it is driven from another source, that
source MUST be in the range of 2.7V to 5.5V and MUST be
alive prior to enabling the LTC1709.
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 ITH(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, CSS. 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/RSENSE to
75mV/RSENSE. 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:
tDELAY
=
1.5V
1.2µA
CSS
=
(1.25s
/
µF)CSS
The time for the output current to ramp up is then:
tRAMP
=
3V − 1.5V
1.2µA
CSS
=
(1.25s
/
µF)CSS
By pulling the RUN/SS pin below 0.8V the LTC1709 is put
into low current shutdown (IQ < 40µA). The RUN/SS pins
can be driven directly from logic as shown in Figure 6.
Diode D1 in Figure 6 reduces the start delay but allows CSS
to ramp up slowly providing the soft-start function. The
RUN/SS pin has an internal 6V zener clamp (see Func-
tional Diagram).
VIN
3.3V OR 5V
RUN/SS
D1
RSS*
CSS
INTVCC
RSS*
RUN/SS
D1*
CSS
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
1709 F06
Figure 6. 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, CSS, 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 CSS
reaches 4.1V, CSS 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 CSS, 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:
tLO1 ≈ (CSS • 0.6V)/(1.2µA) = 5 • 105 (CSS)
If the overload occurs after start-up, the voltage on CSS will
continue charging and will provide additional time before
latching off:
tLO2 ≈ (CSS • 3V)/(1.2µA) = 2.5 • 106 (CSS)
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor, RSS, to the RUN/SS pin as
shown in Figure 6. 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 con-
dition. When deriving the 5µA current from VIN as in the
figure, current latchoff is always defeated. Diode connect-
ing this pull-up resistor to INTVCC, as in
18
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