Datasheet

LTC3727A-1
19
3727a1fa
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
LTC3727A-1. 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, 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:
t
V
μA
C s μF C
t
V
DELAY SS SS
IRAMP
==
()
=
−
15
12
125
3
.
.
./
115
12
125
.
.
./
V
μA
C s μF C
SS SS
=
()
By pulling both RUN/SS pins below 1V, the LTC3727A-1
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).
Fault Conditions: Current Limit and Current Foldback
The LTC3727A-1 current comparator has a maximum
sense voltage of 135mV resulting in a maximum MOSFET
APPLICATIONS INFORMATION
current of 135mV/R
SENSE
. The maximum value of current
limit generally occurs with the largest V
IN
at the highest
ambient temperature, conditions that cause the highest
power dissipation in the top MOSFET.
The LTC3727A-1 includes current foldback to help further
limit load current when the output is shorted to ground.
The foldback circuit is active even when the overload
shutdown latch described above is overridden. If the
output falls below 70% of its nominal output level, then
the maximum sense voltage is progressively lowered from
135mV to 45mV. Under short-circuit conditions with very
low duty cycles, the LTC3727A-1 will begin cycle skipping
in order to limit the short-circuit current. In this situation
the bottom MOSFET will be dissipating most of the power
but less than in normal operation. The short-circuit ripple
current is determined by the minimum on-time t
ON(MIN)
of the LTC3727A-1 (less than 200ns), the input voltage
and inductor value:
ΔI
L(SC)
= t
ON(MIN)
(V
IN
/L)
The resulting short-circuit current is:
I
mV
R
I
SC
SENSE
LSC
=+
45 1
2
Δ
()
Fault Conditions: Overvoltage Protection (Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the regulator rises
much higher than nominal levels. The crowbar causes huge
currents to fl ow, that blow the fuse to protect against a
shorted top MOSFET if the short occurs while the controller
is operating.
Figure 7
3.3V OR 5V RUN/SS
RUN/SS
D1
C
SS
C
SS
3727 F07
(7a) (7b)