LT1375/LT1376
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PIN FUNCTIONS
VIN: This is the collector of the on-chip power NPN switch.
Thispinpowerstheinternalcircuitryandinternalregulator
whentheBIASpinisnotpresent.AtNPNswitchonandoff,
high dl/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.
pin is used as a SENSE pin, connected directly to the 5V
output. Two additional functions are performed by the FB
pin. When the pin voltage drops below 1.7V, switch
current limit is reduced. Below 1V, switching frequency is
also reduced. See Feedback Pin Function section in Appli-
cations Information for details.
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 (LT1376 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%.
GND: The GND pin connection needs consideration for
tworeasons. First, itactsasthereferencefortheregulated
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.
SYNC (LT1375 Only): The SYNC pin is used to synchro-
nizetheinternaloscillatortoanexternalsignal.Itisdirectly
logic compatible and can be driven with any signal be-
tween 10% and 90% duty cycle. The synchronizing range
is equal to initial operating frequency, up to 900kHz. See
Synchronizing section in Applications Information for
details.
FB/SENSE: The feedback pin is used to set output voltage,
using an external voltage divider that generates 2.42V at
the pin with the desired output voltage. The fixed voltage
(-5) parts have the divider included on the chip, and the FB
W
BLOCK DIAGRAM
The LT1376 is a constant frequency, current mode buck
converter. This means that there is an internal clock and
twofeedbackloopsthatcontrolthedutycycleofthepower
switch. In addition to the normal error amplifier, there is a
current sense amplifier that monitors switch current on a
cycle-by-cycle basis. A switch cycle starts with an oscilla-
tor pulse which sets the RS flip-flop to turn the switch on.
When switch current reaches a level set by the inverting
input of the comparator, the flip-flop is reset and the
switch turns off. Output voltage control is obtained by
using the output of the error amplifier to set the switch
current trip point. This technique means that the error
amplifier commands current to be delivered to the output
rather than voltage. A voltage fed system will have low
phase shift up to the resonant frequency of the inductor
and output capacitor, then an abrupt 180° shift will occur.
The current fed system will have 90° phase shift at a much
lower frequency, but will not have the additional 90° shift
until well beyond the LC resonant frequency. This makes
itmucheasiertofrequencycompensatethefeedbackloop
and also gives much quicker transient response.
Most of the circuitry of the LT1376 operates from an
internal 2.9V bias line. The bias regulator normally draws
power from the regulator input pin, but if the BIAS pin is
13756fd
7
Linear [ Linear ]
