Datasheet

LT3971/LT3971-3.3/LT3971-5
21
3971fd
APPLICATIONS INFORMATION
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 318
shows how to generate a bipolar output supply using a
buck regulator.
Table 5: Effects of Pin Shorts
PINS EFFECT
V
IN
-EN No effect. In most applications, EN is tied to V
IN
. If EN is driven with a logic signal, a series resistor is recommended to protect the circuit
generating the logic signal from the full V
IN
voltage.
SS-RT V
OUT
may fall below regulation voltage. The switching frequency will be increased and the current limit will be reduced.
RT-PG No effect if PG is floated.
V
OUT
will fall below regulation if PG is connected to the output with a resistor pull-up as long as the resister divider formed by the PG pin
pull-up and the R
T
resistor prevents the RT pin absolute maximum from being violated. (see discussion in Fault Tolerance section)
In both cases, the switching frequency will be significantly increased if the output goes below regulation, which may cause the LT3975 to
go into pulse-skipping mode if the minimum on-time is violated.
PG-SYNC No effect if PG is floated.
No effect if PG is connected to the output with a resistor pull-up as long as there is a resistor to GND on the SYNC pin or the SYNC pin is
tied to GND. This is to ensure that the resistor divider formed by the PG pin pull-up and the SYNC pin resistor to GND prevents the SYNC
pin absolute maximum from being violated. (see discussion in Fault Tolerance section)
SYNC-GND No effect. If the SYNC pin is driven with a clock, a series resistor is recommended to prevent the clock source from getting shorted out.
Table 6: Effects of Floating Pins
PIN EFFECT
SS No effect; soft-start feature will not function.
BD V
OUT
may fall below regulation voltage. With the BD pin disconnected, the boost capacitor cannot be charged and thus the power switch
cannot fully saturate, which increases power dissipation.
BOOST V
OUT
may fall below regulation voltage. With the BOOST pin disconnected, the boost capacitor cannot be charged and thus the power switch
cannot fully saturate, which increases power dissipation.
SW V
OUT
will fall below regulation voltage.
V
IN
V
OUT
will fall below regulation voltage.
EN V
OUT
may fall below regulation voltage. Part may work normally or be shutdown depending on how the application circuit couples to the
floating EN pin.
RT V
OUT
may fall below regulation voltage.
PG No effect.
SYNC No effect. The LT3971 may be in Burst Mode operation or pulse-skipping mode depending on how the application circuit couples to the
floating SYNC pin.
FB No effect; there are two FB pins.
GND No effect; there are two GND connections. If exposed pad is floated, thermal performance will be degraded.