LTC3727-1_15 Datasheet, PDF (19/32 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators

LTC3727/LTC3727-1
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_OSENSE feedback current.
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 LTC3727. Soft-start reduces the input power
source’s surge currents by gradually increasing the
controller’s current limit (proportional to V_ITH). This pin
can also be used for power supply sequencing.
An internal 1.2μA current source charges up the C_SS
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 1.5V
to 3.0V, the internal current limit is increased from 45mV/
R_SENSE to 135mV/R_SENSE. The output current limit ramps
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:
t_DELAY
=
1.5V
1.2μA
C_SS
=
(^1.25s
/
μF₎C_SS
t_IRAMP
=
3V − 1.5V
1.2μA
C_SS
=
(^1.25s
/
μF₎C_SS
By pulling both RUN/SS pins below 1V, the LTC3727 is
put into low current shutdown (I_Q = 20μA). The RUN/SS
pins can be driven directly from logic as shown in Fig-
ure 7. Diode D1 in Figure 7 reduces the start delay but
allows C_SS to ramp up slowly providing the soft-start
function. Each RUN/SS pin has an internal 6V zener clamp
(See Functional Diagram).
Fault Conditions: Overcurrent Latchoff (LTC3727 Only)
The RUN/SS pins also provide the ability to latch off the
controller(s) when an overcurrent condition is detected.
V_IN
3.3V OR 5V
RUN/SS
D1
R_SS*
C_SS
*OPTIONAL TO DEFEAT OVERCURRENT
LATCHOFF (NOT NEEDED WITH THE LTC3727-1)
(7a)
INTV_CC
R_SS*
RUN/SS
(7b)
C_SS
3727 F07
Figure 7. RUN/SS Pin Interfacing
The RUN/SS capacitor, C_SS, is used initially to turn on and
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_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 deter-
mined by the size of the C_SS 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 approximated by:
t_LO1 ≅ [C_SS(4.1 – 1.5 + 4.1 – 3.5)]/(1.2μA)
= 2.7 • 10⁶ (C_SS)
If the overload occurs after start-up the voltage on C_SS will
begin discharging from the zener clamp voltage:
t_LO2 ≅ [C_SS (6 – 3.5)]/(1.2μA) = 2.1 • 10⁶ (C_SS)
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_IN, as in Figure 7a, defeats overcurrent latchoff.
Diode-connecting this pull-up resistor to INTV_CC, as in
Figure 7b, eliminates any extra supply current during
controller shutdown while eliminating the INTV_CC loading
from preventing controller start-up. This pull-up resistor
is not needed in LTC3727-1 designs.
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