Datasheet

LT8640/LT8640-1
22
Rev.C
For more information www.analog.com
APPLICATIONS INFORMATION
Output Voltage Tracking and Soft-Start
T he LT8640/LT8640-1 allows the user to program its output
voltage ramp rate by means of the TR/SS pin. An internal
1.9µA pulls up the TR/SS pin to INTV
CC
. Putting an external
capacitor on TR/SS enables soft starting the output to pre-
vent current surge on the input supply. During the soft-start
ramp the output voltage will proportionally track the TR/SS
pin voltage. For output tracking applications, TR/SS can
be externally driven by another voltage source. From 0V to
0.97V, the TR/SS voltage will override the internal 0.97V
reference input to the error amplifier, thus regulating the
FB pin voltage to that of TR/SS pin. When TR/SS is above
0.97V, tracking is disabled and the feedback voltage will
regulate to the internal reference voltage. The TR/SS pin
may be left floating if the function is not needed.
An active pull-down circuit is connected to the TR/SS pin
which will discharge the external soft-start capacitor in
the case of fault conditions and restart the ramp when the
faults are cleared. Fault conditions that clear the soft-start
capacitor are the EN/UV pin transitioning low, V
IN
voltage
falling too low, or thermal shutdown.
Output Power Good
When the LT8640/LT8640-1’s output voltage is within the
±8% window of the regulation point, the output voltage
is considered good and the open-drain PG pin goes high
impedance and is typically pulled high with an external
resistor. Otherwise, the internal pull-down device will pull
the PG pin low. To prevent glitching both the upper and
lower thresholds include 0.2% of hysteresis.
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1V, INTV
CC
has fallen too
low, V
IN
is too low, or thermal shutdown.
Shorted and Reversed Input Protection
The LT8640/LT8640-1 will tolerate a shorted output.
Several features are used for protection during output
short-circuit and brownout conditions. The first is the
switching frequency will be folded back while the output
is lower than the set point to maintain inductor current
control. Second, the bottom switch current is monitored
such that if inductor current is beyond safe levels switch
-
ing of the top switch will be delayed until such time as the
inductor current falls to safe levels.
Frequency foldback behavior depends on the state of the
SYNC pin: If the SYNC pin is low the switching frequency
will slow while the output voltage is lower than the pro
-
grammed level. If the SYNC pin is connected to a clock
source, floated or tied high, the LT8640/LT8640-1 will stay
at the programmed frequency without foldback and only
slow switching if the inductor current exceeds safe levels.
There is another situation to consider in systems where
the output will be held high when the input to the LT8640/
LT8640-1 is absent. This may occur in battery charging
applications or in battery-backup systems where a battery
or some other supply is diode ORed with the LT8640/
LT8640-1’s output. If the V
IN
pin is allowed to float and the
EN pin is held high (either by a logic signal or because it is
tied to V
IN
), then the LT8640/LT8640-1’s internal circuitry
will pull its quiescent current through its SW pin. This is
acceptable if the system can tolerate several µA in this
state. If the EN pin is grounded the SW pin current will
drop to near 1µA. However, if the V
IN
pin is grounded while
the output is held high, regardless of EN, parasitic body
diodes inside the LT8640/LT8640-1 can pull current from
the output through the SW pin and the V
IN
pin, which may
damage the IC. Figure5 shows a connection of the V
IN
and
EN/UV pins that will allow the LT8640/LT8640-1 to run
only when the input voltage is present and that protects
against a shorted or reversed input.
Figure5. Reverse V
IN
Protection
V
IN
V
IN
D1
LT8640/
LT8640-1
EN/UV
8640 F05
GND
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