Datasheet

LT3692
20
3692fa
For more information www.linear.com/3692
applicaTions inForMaTion
Shutdown and Undervoltage/Overvoltage Lockout
Typically, undervoltage lockout (UVLO) is used in situa-
tions where the input supply is current limited, or has a
relatively high sour
ce resistance. A switching regulator
draws constant power from the sour
ce, so source cur
-
rent increases as source voltage drops. This looks like a
negative resistance load to the source and can cause the
source to current limit or latch low under low source voltage
conditions. UVLO prevents the regulator from operating
at source voltages where these problems might occur.
Overvoltage lockout (OVLO) is typically used to shut down
the switching regulator during potentially harmful input
voltage transients.
Referring to Figure 14, if the SHDN pin is connected to
V
IN
, then the overvoltage lockout threshold is set to the
typical maximum value of 38V.
Additionally, an internal comparator will force both chan
-
nels into shutdown below the minimum V
IN1
of 2.8V.
This feature can be used to prevent excessive discharge
of battery-operated systems. In addition to the V
IN1
un-
dervoltage lockout, both channels will be disabled when
SHDN1 is less than 1.3V.
Programmable
UVLO may be implemented using an input
voltage divider and one of the internal comparators (see
the Typical Applications section).
When the SHDN pin is taken above 1.3V, its respective
channel is allowed to operate. When the SHDN pin is driven
below 1.3V, its channel is disabled. Taking SHDN1 below
0.6V will place the LT3692 in a low quiescent current mode.
A graph of quiescent current vs SHDN1 voltage can be
found in the Typical Performance Characteristics section.
There is no hysteresis on the SHDN pins. If the SHDN pins
are not connected to V
IN
, then an internal clamp regulates
the SHDN pin voltage to 2.5V.
If the SHDN pin is driven by a logic signal greater than
2.5V, a series resistor is required to limit the current into
the SHDN pin to no more than 10µA.
Referring to Figure 15, a 249k resistor will suffice for
a typical logic-level signal. If the logic signal is 5V or
greater, choose a current limiting resistor equal to R
SHDN
=
(V
LOGIC
– 2.5V)/10µA. Place a small Schottky diode (such
as a BAT54) in parallel to the current-limiting resistor as
shown in Figure 15.
Keep the connections from any series resistors to the SHDN
pins short and make sure that the interplane or surface
capacitance to switching nodes is minimized.
Figure 14. Connect SHDN to V
IN
to Select Default OVLO and UVLO
Figure 15. External Control of the SHDN Pin
+
–
+
–
1.3V
38V
THERMAL
SHUTDOWN
SHDN1
V
INX
LT3692
3692 F15
CHANNEL
DISABLE
LT3692
3692 F15
SHDN
R
SHDN
+
–
SCHOTTKY
OPTIONAL
IF V
LOGIC
< 5V
V
LOGIC
Soft-Start
The output of the LT3692 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.