LTC1876 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1876
Linear Technology
'LTC1876' PDF : 36 Pages View PDF
LTC1876
APPLICATIO S I FOR ATIO
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 LTC1876 step-down controllers. Soft-start re-
duces the input power source’s surge currents by gradu-
ally increasing the controller’s current limit (proportional
to VITH). This pin can also be used for power supply
sequencing.
An internal 1.2µA current source charges up the CSS
capacitor. When the voltage on RUN/SS1 (RUN/SS2)
reaches 1.5V, the particular controller is permitted to start
operating. As the voltage on RUN/SS increases from 1.3V
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 1.2s/µF to reach full cur-
rent. 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
( ) tIRAMP
=
3V − 1.5V
1.2µA
C SS
=
1.25s / µF
C SS
By pulling both RUN/SS pins below 1.0V and/or pulling the
STBYMD pin below 0.2V, the controllers are put into low
current shutdown (IQ = 20µA). The RUN/SS pins can be
driven directly from logic as shown in Figure 7. Diode D1
in Figure 7 reduces the start delay but allows CSS to ramp
up slowly providing the soft-start function. Each RUN/SS
pin has an internal 6V Zener clamp (See Functional Dia-
gram).
3.3V OR 5V
D1
VIN
RUN/SS
RSS*
INTVCC
RSS*
RUN/SS
CSS
CSS
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
(a)
(b)
1876 F07
Figure 7. RUN/SS Pin Interfacing
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, CSS, is used initially to turn on and
limit the inrush current of the controller. After the control-
ler 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 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 the CSS and the
specified discharge 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 approxi-
mated by:
TLO1 ≅ [CSS(4.1 – 1.5 + 4.1 – 3.5)]/(1.2µA)
= 2.7 • 106 (CSS)
If the overload occurs after start-up the voltage on CSS will
begin discharging from the zener clamp voltage:
TLO2 ≅ [CSS (6 – 3.5)]/(1.2µA) = 2.1 • 106 (CSS)
If an overload occurs on one channel, it will also latch off
the other channel. 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 resistor to VIN as in Figure 7a, defeats overcurrent
latchoff. Diode-connecting this pull-up resistor to INTVCC,
as in Figure 7b, eliminates any extra supply current during
controller shutdown while eliminating the INTVCC loading
from preventing controller start-up.
Why should you defeat overcurrent latchoff? During the
prototype stage of a design, there may be a problem with
noise pickup or poor layout causing the protection circuit
to latch off. Defeating this feature will easily allow trouble-
shooting of the circuit and PC layout. The internal short-
circuit and foldback current limiting still remains active,
thereby protecting the power supply system from failure.
After the design is complete, a decision can be made
whether to enable the latchoff feature.
1876fa
21
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