5
LT1961
1961fa
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. If required, the current limit
can be reduced during start up 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 at the FB pin is needed for this feature to
operate.
V
IN
: This pin powers the internal circuitry and internal
regulator. Keep the external bypass capacitor close to this
pin.
GND: Short GND pins 3 and 4 and the exposed pad on the
PCB. The GND is 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 of the IC. This
condition occurs when the load current flows through the
metal path between the GND pins and the load ground
point. Keep the ground path short between the GND pins
and the load and use a ground plane when possible. Keep
the path between the input bypass and the GND pins short.
The exposed pad should be attached to a large copper area
to improve thermal resistance.
V
SW
: The switch pin is the collector of the on-chip power
NPN switch and has large currents flowing through it.
Keep the traces to the switching components as short as
possible to minimize radiation and voltage spikes.
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
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 1.35V 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
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 0.3V for very light loads and 0.9V at maximum
load.
PIN FUNCTIONS
UUU
