LTC3728 Datasheet, PDF (19/32 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
APPLICATIO S I FOR ATIO
R1_(MAX)
=
⎛
24k_⎝⎜
0.8V
2.4V – V_OUT
⎞
⎠⎟
for V_OUT < 2.4V
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 LTC3728. 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 25mV/
R_SENSE to 75mV/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:
( ) ^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 1V, the LTC3728 is
put into low current shutdown (I_Q = 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
C_SS to ramp up slowly providing the soft-start function.
Each 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
C_SS
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
(a)
(b)
3728 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, 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.
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