LT1374-5 Ver la hoja de datos (PDF) - Linear Technology
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LT1374-5 Datasheet PDF : 28 Pages
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LT1374
PIN FUNCTIONS
FB/SENSE: The feedback pin is the input to the error
amplifier which is referenced to an internal 2.42V source.
An external resistive divider is used to set the output
voltage. The fixed voltage (-5) parts have the divider
included on-chip and the FB pin is used as a SENSE pin,
connected directly to the 5V output. Three additional
functions are performed by the FB pin. When the pin
voltage drops below 1.7V, switch current limit is reduced.
Below 1.5V the external sync function is disabled. Below
1V, switching frequency is also reduced. See Feedback Pin
Function section in Applications Information for details.
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 0.07Ω FET structure. Effi-
ciency improves from 75% for conventional bipolar de-
signs to > 89% for these new parts.
VIN: This is the collector of the on-chip power NPN switch.
This pin powers the internal circuitry and internal regulator
when the BIAS pin is not present. At NPN switch on and off,
high dI/dt edges occur on this pin. Keep the external
bypass and catch diode close to this pin. All trace induc-
tance on this path will create a voltage spike at switch off,
adding to the VCE voltage across the internal NPN.
GND: The GND pin connection needs consideration for
two reasons. First, it 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. The second consideration is EMI caused
by GND pin current spikes. Internal capacitance between
the VSW pin and the GND pin creates very narrow (<10ns)
current spikes in the GND pin. If the GND pin is connected
to system ground with a long metal trace, this trace may
radiate excess EMI. Keep the path between the input
bypass and the GND pin short.
VSW: 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 is
clamped with the external catch diode. Maximum negative
switch voltage allowed is – 0.8V.
SYNC: (R and SO-8 packages only) 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 synchroniz-
ing range is equal to initial operating frequency, up to
1MHz. This pin replaces SHDN on -SYNC option parts. See
Synchronizing section in Applications Information for
details.
SHDN: The shutdown pin is used to turn off the regulator
and to reduce input drain current to a few microamperes.
Actually, this pin has two separate thresholds, one at
2.38V to disable switching, and a second at 0.4V to force
complete micropower shutdown. The 2.38V threshold
functions as an accurate undervoltage lockout (UVLO).
This can be used to prevent the regulator from operating
until the input voltage has reached a predetermined level.
VC: The VC 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 1V for very light loads and 2V at maximum load.
It can be driven to ground to shut off the regulator, but if
driven high, current must be limited to 4mA.
BIAS: (SO package only) 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 voltage forces most of the internal circuitry to draw
its operating current from the output voltage rather than
the input supply. This is a much more efficient way of
doing business if the input voltage is much higher than the
output. Minimum output voltage setting for this mode of
operation is 3.3V. Efficiency improvement at VIN = 20V,
VOUT = 5V, and IOUT = 25mA is over 10%.
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