LT3922
pin FuncTions
SW: Switch Pins. These pins are internally connected to
the power devices and drivers. They should always be
tied together. In normal operation, the voltage of these
pins will switch between the output voltage and zero at
the programmed frequency. Do not force any voltage on
these pins.
ISPandISN(andtherefore, theregulatedcurrentsupplied
to the load). Alternatively, a digital pulse at this pin with
duty cycle from 12.5% to 62.5% can be used to program
the regulated voltage. Below 200mV or 10% duty cycle,
the CTRL pin voltage disables switching. For more detail,
see Typical Performance Characteristics and Applications
Information sections.
BST: Boost Pin. This pin supplies the top power switch
GATE driver. Connect a 100nF capacitor between this
pin and SW close to the package. An internal diode from
ISP:PositiveCurrentSensePin.Thispinisoneoftheinputs
to the internal current sense error amplifier. It should be
connected to the positive side of the external sense resis-
tor. Use Kelvin connection for accurate current sensing.
INTV to BST will charge the capacitor when the SW pin
CC
switches low.
INTV : Internally Regulated, Low-Voltage Supply Pin.
ISN: Negative Current Sense Pin. This pin is one of the
inputs to the internal current sense error amplifier. It
should be connected to the negative side of the external
sense resistor. Use Kelvin connection for accurate cur-
rent sensing.
CC
This pin provides the power for the converter switch GATE
drivers. Do not force any voltage on this pin. Place a 2.2µF
bypass capacitor to GND close to the package.
V : Input Voltage Pin. This pin supplies power to the
IN
internal, high-performance analog circuitry. Connect a
V :CompensationPin. Aresistorandcapacitorconnected
C
bypass capacitor between this pin and GND.
in series from this pin to GND stabilize the current and
voltage regulation. Typical resistor and capacitor values
are from 0k to 100k and from 0.1nF to 10nF, respectively.
EN/UVLO: Enable and Undervoltage Lockout Pin. A volt-
age at this pin greater than 1.33V will enable switching,
and a voltage less than 0.1V is guaranteed to shut down
the internal current bias and sub-regulators. A resistor
network between this pin and ground can be used to set
the pin voltage and automatically lockout the part when
FB: Feedback Pin. When the voltage at this pin is near 1.2V
the regulated current is automatically reduced from the
programmed value. A resistor network between this pin
and V
can be used to set a limit for the output voltage.
OUT
V isbelowacertainlevel.Nointernalcomponentspullup
If the voltage at the FB pin reaches 1.266V, a FB overvolt-
IN
or down on this pin, so it requires an external voltage bias
age lockout comparator disables switching.
for normal operation. This pin may be tied directly to V .
IN
SS: Soft-Start Pin. At startup and recovery from fault
conditions, a 20μA current charges the capacitor and the
FBvoltagetrackstherisingvoltageatthispinuntiltheload
current reaches its programmed level. Typical values for
the capacitor are 10nF to 100nF. Using a single resistor
OVLO:InputOvervoltageLockoutPin.Whenthevoltageat
this pin rises above 1.205V, the system disables switch-
ing and resets the soft-start capacitor. Do not leave this
pin open. Tie this pin to GND when the OVLO function is
not used.
from SS to INTV , the LT3922 can be set in two differ-
CC
ent fault modes for the shorted LED conditions: hiccup
(no resistor) and latchoff (100k). Refer to the Application
Information section for a detailed explanation.
V
: Reference Voltage Pin. This pin provides a buffered
REF
2Vreferencecapableof3mAdrive.Itcanbeusedtosupply
resistor networks for setting the voltages at the CTRL and
PWM pins. Bypass with a 1μF capacitor to GND.
ISMON: Output Current Monitoring Pin. This pin provides
a buffered voltage output equal to 10mV for every 1mV
between ISP and ISN.
CTRL: Control Pin. An analog voltage from 250mV to
1.25V at this pin programs the regulated voltage between
3922f
10
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