lt3692a Linear Technology Corporation, lt3692a Datasheet - Page 20

• Manufacturer Part Number: lt3692a
• Description: Monolithic Dual Tracking 3.5a Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

LT3692A
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 source resistance. A switching regulator
draws constant power from the source, 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.
The overvoltage lockout threshold is typically 39V. Each
channel of the LT3692A is forced into shutdown when its
input voltage exceeds 39V, and will survive voltages as
high as 60V. When the input voltage drops back below
39V, the LT3692A goes through a POR cycle and the output
soft-starts from its existing level to its regulation point.
Additionally, an internal comparator will force both chan-
nels into shutdown below the minimum V
This feature can be used to prevent excessive discharge
of battery-operated systems. In addition to the V
dervoltage lockout, both channels will be disabled when
SHDN1 is less than 1.32V.
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 1.32V, its respective
channel is allowed to operate. When the SHDN pin is
driven below 1.32V, its channel is placed in a low quiescent
current state. There is no hysteresis on the SHDN pins.
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.
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
APPLICATIONS INFORMATION
20
IN1
of 2.8V.
IN1
un-
ramp from 0V to the regulated output whose duration is
given by:
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
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
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
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
clamp to be activated. As long as the V
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
t
RAMP
SS(CONTROL)
=
C
SS
IN1
12μA
• 0.806V
=
undervoltage lockout, the soft-start latch
C
SS
1.4mA
• 1.2V
C
pin is clamped,
C
pin is re-
C
pin is
C
3692af
pin