6
LT3431
sn3431 3431fs
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PI FU CTIO S
GND (Pins 1, 8, 9, 16): The GND pin connections act as
the reference for the regulated output, so load regulation
will suffer if the “ground” end of the load is not at the same
voltage as the GND pins of the IC. This condition will occur
when load current or other currents flow through metal
paths between the GND pins and the load ground. Keep the
paths between the GND pins and the load ground short
and use a ground plane when possible. The FE package has
an exposed pad that is fused to the GND pins. The pad
should be soldered to the copper ground plane under the
device to reduce thermal resistance. (See Applications
Information—Layout Considerations.)
SW (Pins 2, 5): The switch pin is the emitter of the on-chip
power NPN switch. This pin is driven up to the input pin
voltage during switch on time. Inductor current drives the
switch pin voltage negative during switch off time. Nega-
tive voltage is clamped with the external catch diode.
Maximum negative switch voltage allowed is –0.8V.
V
IN
(Pins 3, 4): This is the collector of the on-chip power
NPN switch. V
IN
powers the internal control circuitry when
a voltage on the BIAS pin is not present. High dI/dt edges
occur on this pin during switch turn on and off. Keep the
path short from the V
IN
pin through the input bypass
capacitor, through the catch diode back to SW. All trace
inductance in this path creates voltage spikes at switch off,
adding to the V
CE
voltage across the internal NPN.
BOOST (Pin 6): The BOOST pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipo-
lar NPN power switch. Without this added voltage, the
typical switch voltage loss would be about 1.5V. The ad-
ditional BOOST voltage allows the switch to saturate and
voltage loss approximates that of a 0.1Ω FET structure.
NC (Pins 7, 13): No Connection.
B
IAS (Pin 10): The BIAS pin is used to improve efficiency
when operating at higher input voltages and light load
current. Connecting this pin to the regulated output volt-
age forces most of the internal circuitry to draw its oper-
ating current from the output voltage rather than the input
supply. This architecture increases efficiency especially
when the input voltage is much higher than the output.
Minimum output voltage setting for this mode of operation
is 3V.
V
C
(Pin 11) The V
C
pin is the output of the error amplifier
and the input of the peak switch current comparator. It is
normally used for frequency compensation, but can also
serve as a current clamp or control loop override. V
C
sits
at about 0.9V for light loads and 2.1V at maximum load. It
can be driven to ground to shut off the regulator, but if
driven high, current must be limited to 4mA.
FB (Pin 12): The feedback pin is used to set the output
voltage using an external voltage divider that generates
1.22V at the pin for the desired output voltage. Three
additional functions are performed by the FB pin. When the
pin voltage drops below 0.6V, switch current limit is
reduced and the external SYNC function is disabled. Below
0.8V, switching frequency is also reduced. See Feedback
Pin Functions in Applications Information for details.
SYNC (Pin 14): The SYNC pin is used to synchronize the
internal oscillator to an external signal. It is directly logic
compatible and can be driven with any signal between
10% and 90% duty cycle. The synchronizing range is
equal to initial operating frequency up to 700kHz. See
Synchronizing in Applications Information for details.
SHDN (Pin 15): The SHDN pin is used to turn off the
regulator and to reduce input drain current to a few
microamperes. This pin has two thresholds: one at 2.38V
to disable switching and a second at 0.4V to force com-
plete micropower shutdown. The 2.38V threshold func-
tions as an accurate undervoltage lockout (UVLO); some-
times used to prevent the regulator from delivering power
until the input voltage has reached a predetermined level.
If the SHDN pin functions are not required, the pin can
either be left open (to allow an internal bias current to lift
the pin to a default high state) or be forced high to a level
not to exceed 6V.
