LT3922
operaTion
The LT3922 is a step-up LED driver that utilizes a fixed-
frequency peak current control to accurately regulate the
current through a string of LEDs. It includes two power
switches, their drivers, and a diode for providing power
to the top switch driver. The switches connect an external
inductor at the SW pin alternately to the ground and then
amplifier. The other input to this amplifier comes from
the ISP and ISN pin voltages. An external current sense
resistorbetweenthesepinsshouldbeplacedinserieswith
the string of LEDs such that the voltage across it provides
the feedback to regulate the LED current. The current
regulation amplifier then compares the actual LED current
to the output (V ). The inductor current rises and falls
to the desired LED current and adjusts V as necessary.
OUT
C
accordingly and the peak current can be regulated by
adjusting the duty ratio of the power switches through
the combined effect of the other circuit blocks.
The voltage regulation amplifier overrides the current
regulation amplifier when the FB pin voltage is higher than
an internal 1.2V reference. An external resistor network
from the LED string to the FB pin provides an indication
of the LED string voltage and allows the voltage amplifier
to prevent overvoltage of the LED string.
The synchronous controller ensures the power switches
do not conduct at the same time, and a programmable
oscillator turns on the bottom switch at the beginning of
eachswitchingcycle. Thefrequencyofthisoscillatorisset
by an external resistor at the RT pin and can be overrid-
den by external pulses at the SYNC/SPRD pin. The SYNC/
SPRD pin can also be used to command spread spectrum
frequencymodulation(SSFM),whichreducesradiatedand
conducted electromagnetic interference (EMI).
The ISP, ISN, and FB pin voltages are also monitored to
detect fault conditions like open and short circuits, which
are then reported by pulling FAULT pin low. The response
to a fault can be selected either to try hiccup restarts or to
latchoff by the choice of an external resistor connected to
the SS pin. Refer to the Applications Information section
for a detailed explanation of fault responses.
The bottom switch is turned off by the peak current com-
parator which waits during the on-time for the increasing
inductor current to exceed the target set by the voltage
Finally, pulse-width modulation (PWM) of the LED current
is achieved by turning on and off an external PMOS switch
at the V pin. This target is modified by a signal from the
C
betweentheV
andthestringofLEDs.Anexternalpulse
OUT
oscillatorwhichstabilizestheinductorcurrent.Anetworkof
at the PWM pin controls the state of the PWM driver or
a DC voltage at the PWM pin dictates the duty ratio of an
internal PWM pulse, whose frequency is programmed
with an external resistor at the RP pin. After each pulse,
when the PMOS switch opens, the LT3922 preserves the
passive components at the V pin is necessary to stabilize
C
this regulation loop.
The target for the inductor current is derived from the
desired LED current programmed by the voltage at the
CTRL pin. The analog-to-digital detector and the control
buffer convert either a DC voltage or duty cycle of pulses
at the CTRL pin into the input for the current regulation
voltages of the capacitors at V and V
to ensure a rapid
C
OUT
recovery for the next pulse.
applicaTions inForMaTion
The following is a guide to selecting the external com-
ponents and configuring the LT3922 according to the
requirements of an application.
resistor is sensed by the current regulation amplifier
through the ISP and ISN pins and regulated to a level pro-
grammed by the CTRL pin. The maximum resistor voltage
that can be programmed is 100mV which corresponds to
1A through the LED string when a 100mΩ current sense
resistor is used.
Programming LED Cu rrent with the CTRL Pin
The primary function of the LT3922 is to regulate the cur-
rent in a string of LEDs. This current should pass through
a series current sense resistor. The voltage across this
To allow for this maximum current, the CTRL pin may
be connected directly to the V
pin which provides
REF
3922f
13
For more information www.linear.com/LT3922
Linear Systems [ Linear Systems ]
