LT1765-2.5 Linear Technology, LT1765-2.5 Datasheet - Page 6

• Manufacturer Part Number: LT1765-2.5
• Description: Monolithic 3A/ 1.25MHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

PI FU CTIO S
LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
6
FB: The feedback pin is used to set output voltage using
an external voltage divider (adjustable version) that gen-
erates 1.2V at the pin when connected to the desired
output voltage. The fixed voltage 1.8V, 2.5V, 3.3V and 5V
versions have the divider network included 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 Characteris-
tics section). An impedance of less than 5k
adjustable version 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.
V
This pin powers the internal circuitry and internal regula-
tor. 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 on this path will
create a voltage spike at switch off, adding to the V
voltage across the internal NPN. Both V
TSSOP package must be shorted together on the PC
board.
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 exposed GND pad and/or GND
pins of the package are directly attached to the internal tab.
These pins/pad should be attached to a large copper area
to reduce thermal resistance.
IN
U
: This is the collector of the on-chip power NPN switch.
U
U
IN
pins of the
on the
CE
V
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
Both V
together on the PC board.
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 from 1.6MHz
to 2MHz. See Synchronization section in Applications
Information 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 under-
voltage lockout (UVLO), preventing the regulator from
operating until the input voltage has reached a predeter-
mined level. Float or pull high to put the regulator in the
operating mode.
V
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 0.4V for very light loads and 0.9V at maximum
load. It can be driven to ground to shut off the output.
SW
C
: The V
: The switch pin is the emitter of the on-chip power
SW
C
pins of the TSSOP package must be shorted
pin is the output of the error amplifier and the
sn1765 1765fas
BR
<0.8V.