Datasheet

LT3692A
22
3692afc
For more information www.linear.com/3692A
applicaTions inForMaTion
Soft-Start
The output of the LT3692A regulates to the lowest voltage
present at either the SS pin or an internal 0.806V reference.
A capacitor from the SS pin to ground is charged by an
internal 12µA current source resulting in a linear output
ramp from 0V to the regulated output whose duration is
given by:
t
RAMP
=
C
SS
• 0.806V
12µA
At power-up, a reset signal sets the soft-start latch and
discharges both SS pins to approximately 0V to ensure
proper start-up. When both SS pins are fully discharged
the latch is reset and the internal 12µA current source
starts to charge the SS pin.
When the SS pin voltage is below 115mV, the V
C
pin is
pulled low which disables switching. This allows the SS
pin to be used as an individual shutdown for each channel.
As the SS pin voltage rises above 90mV, the V
C
pin is re-
leased and the output is regulated to the SS voltage. When
the SS pin voltage exceeds the internal 0.806V reference,
the output is regulated to the reference. The SS pin voltage
will continue to rise until it is clamped at typically 2.15V
.
In the event of a V
IN1
undervoltage lockout, the soft-start latch
is set for both channels, triggering a full start-up sequence. If
a channel’s SHDN pin is driven below 1.32V, its overvoltage
lockout is enabled, or the internal die temperature for its
power switch exceeds its maximum rating during normal
operation, the soft-start latch is set for that channel.
In addition, if the load exceeds the maximum output switch
current, the output will start to drop causing the V
C
pin
clamp to be activated. As long as the V
C
pin is clamped,
the SS pin will be discharged. As a result, the output will
be regulated to the highest voltage that the maximum
output current can support. For example, if a 6V output is
loaded by 1Ω the SS pin will drop to 0.64V, regulating the
output at 4.8V (4.8A • 1Ω). Once the overload condition
is removed, the output will soft start from the temporary
voltage level to the normal regulation point.
Since the SS pin is clamped at typically 2.15V and has to
discharge to 0.806V before taking control of regulation,
momentary overload conditions will be tolerated without
a soft-start recovery. The typical time before the SS pin
takes control is:
t
SS(CONTROL)
=
C
SS
• 1.2V
1.4mA
Open-Collector Comparators
The CMPO pin is the open-collector output of an internal
comparator. The comparator compares the CMPI pin volt
-
age to 90% of the reference voltage (0.72V) with 60mV
of hysteresis.
The CMPO pin has a typical sink capability of 300µA when
the CMPI pin is below the threshold and can withstand 40V
when the threshold is exceeded. The CMPO pin is active
(sink capability is reduced in shutdown and undervoltage
lockout mode) as long as the V
IN1
pin voltage exceeds
typically 2.8V.
The comparators can be used to monitor input and output
voltages as well as die temperature. See the Typical Ap
-
plications circuit collection for examples.
Output Tracking/Sequencing
Complex output tracking and sequencing between channels
can be implemented using the LT3692A’s SS and CMPO
pins. Figure 14 shows several configurations for output
tracking/sequencing for a 3.3V and 1.8V application.
Independent soft-start for each channel is shown in Fig
-
ure16a. The output ramp time for each channel is set by
the soft-start capacitor as described in the soft-start section.
Ratiometric tracking is achieved
in Figure 14b by con
-
necting both SS pins together. In this configuration, the
SS pin source current is doubled (24µA) which must be
taken into account when calculating the output rise time.
By connecting a feedback network from V
OUT1
to the SS2
pin with the same ratio that sets V
OUT2
voltage, absolute
tracking shown in Figure 14c is implemented. The mini-
mum value of the top feedback resistor (R1) should be set
such that the SS pin can be driven all the way to ground
with 1.4mA of sink current when V
OUT1
is at its regulated
voltage. In addition, a small V
OUT2
voltage offset will be
present due to the SS2 12µA source current. This offset
can be corrected for by slightly reducing the value of R2.