LT1767/LT1767-1.8/
LT1767-2.5/LT1767-3.3/LT1767-5
6
1767fb
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pin FuncTions
FB: The feedback pin is used to set output voltage using
an external voltage divider that generates 1.2V at the pin
with the desired output voltage. The fixed voltage 1.8V,
2.5V, 3.3V and 5V versions have the divider network in
-
cluded internally
and the FB pin is connected directly to
the output. If required, the current limit can be reduced
during start up or short-circuit when the FB pin is below
0.5V (see the Current Limit Foldback graph in the Typical
Performance Characteristics section). An impedance of
less than 5kΩ (adjustable part only) at the FB pin is needed
for this feature to operate.
BOOST: The BOOST pin is used to provide a drive voltage,
higher than the input voltage, to the internal bipolar NPN
power switch. Without this added voltage, the typical
switch voltage loss would be about 1.5V. The additional
boost voltage allows the switch to saturate and voltage
loss approximates that of a 0.22Ω FET structure.
V
IN
: This is the collector of the on-chip power NPN switch.
This pin powers the internal circuitry and internal regu-
lator. At
NPN switch on and off, high dI/dt edges occur
on
this pin. Keep the external bypass capacitor and catch
diode
close to this pin. All trace inductance in this path
will create a voltage spike at switch off, adding to the V
CE
voltage across the internal NPN.
GND: The GND pin acts 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 pin of the
IC. This condition will occur when load current or other
currents flow through metal paths between the GND pin
and the load ground point. Keep the ground path short
between the GND pin and the load and use a ground plane
when possible. Keep the path between the input bypass
and the GND pin short. The GND pin of the MS8 package
is directly attached to the internal tab. This pin should
be attached to a large copper area to improve thermal
resistance. The
exposed pad of the MS8E package is also
connected to GND. This should be soldered to a large
copper area to improve its thermal resistance.
V
SW
: 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 negative during switch off time. Negative voltage must
be clamped with an external catch diode with a V
BR
<0.8V.
SYNC: The sync pin is used to synchronize the internal
oscillator to an external signal. It is directly logic compat
-
ible and can be driven with any signal between 20% and
80% duty cycle. The synchronizing range is equal to
initial
operating frequency, up to 2MHz. See Synchronization in
Applications Information section for details. When not in
use, this pin should be grounded.
SHDN: The shutdown pin is used to turn off the regulator and
to reduce input drain current to a few microamperes. The
1.33V threshold can function as an accurate undervoltage
lockout (UVLO), preventing the regulator from operating
until the input voltage has reached a predetermined level.
Float or pull high to put the regulator in the operating mode.
V
C
: 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 do double duty
as a current clamp or control loop override
. This pin sits
at about 0.35V for very light loads and 0.9V at maximum
load. It can be driven to ground to shut off the output.
