LT3585-0/LT3585-1
LT3585-2/LT3585-3
U
OPERATIO
The LT3585 series of parts operate on the edge of dis-
continuous conduction mode. When CHRG/IADJ is driven
higher than 1.1V, the master latch is set. This enables the
part to deliver power to the photoflash capacitor. When
the power switch, Q1, is turned on, current builds up in
the primary of the transformer. When the desired current
level is reached, the output of comparator A1 goes high,
resetting the switch latch that controls the state of Q1, and
the output of the DCM comparator goes low. Q1 now turns
off and the flyback waveform on the SW node quickly rises
added between when A3 goes high and the switch latch
is set, see Figure 2. If the part is enabled, and the CHRG/
IADJ pin is floated, internal circuitry drives the voltage on
the pin to 1.28V. This allows a single I/O port pin, which
can be three-stated, to enable or disable the part as well
as place the part into the input current reduction mode.
This feature effectively reduces the average input current
into the flyback transformer. The magnitude of the delay
decreases with increasing V . This causes the reduced
BAT
averageinputcurrenttoremainrelativelyflatwithchanges
toalevelproportionaltoV .Thesecondarycurrentflows
in V . When CHRG/IADJ is brought higher than 1.6V,
OUT
BAT
through high voltage diode(s), D1, and into the photoflash
capacitor. When the secondary current decays to zero,
the voltage on the SW node collapses. When this voltage
no delay is added. The CHRG/IADJ pin functionality is
shown in Figure 3.
BothV andV haveundervoltagelockout(UVLO).When
BAT
IN
reaches 130mV higher than V , the output of A3 goes
BAT
one of these pins goes below its UVLO voltage, the DONE
high. This sets the switch latch and the power switch, Q1,
pingoeslow.WithaninsufficientbypasscapacitoronV
BAT
turnsbackon. ThiscyclerepeatsuntilthetargetV
level
OUT
or V , the ripple on the pin is likely to activate UVLO and
IN
is reached. When the target V
is reached, the master
OUT
terminate the charge. The applications circuits in the data
sheet suggest values adequate for most applications.
latch resets and the DONE pin goes low.
The input current of an LT3585 series circuit can be
reduced by changing the voltage of the CHRG/IADJ pin.
When this pin is between 1.1V and 1.4V, a time delay is
The LT3585 series also includes an integrated IGBT driver.
There are two output pins, IGBTPU and IGBTPD. The
IGBTPU pin is used to pull the gate of the IGBT up. This
should be done quickly to guarantee proper Xenon lamp
ignition. Tie this pin directly to the gate of the IGBT. The
IGBTPD pin is pinned out separately to allow for greater
flexibilityinchoosingaseriesresistorbetweenthepinand
the gate of the IGBT. This resistor can be used to slow
down the turn off of the IGBT.
Normal Operation
CHRG/IADJ ≥ 1.6V
I
PRI
TIME
V
SW
V
OUT
100V/DIV
TIME
Reduced Input Current
DONE
2V/DIV
CHRG/IADJ Three Stated
I
PRI
CHRG/IADJ
2V/DIV
TIME
LT3585-1
1sec/DIV
Extra Delay Added
V
C
= 3.6V
BAT
OUT
(~5.2µs at V
= 4.2V)
BAT
= 50µF
V
CHRG/IADJ PIN STATE
SW
THREE
STATE*
<0.3V
3V
<0.3V
3V
<0.3V
3585 F02
TIME
3585 F03
*MUST TAKE CHRG/IADJ PIN ABOVE 1.1V, THEN FLOAT
Figure 3. Basic Operation
Figure 2. Normal and Reduced Input Current Waveforms
3585f
10
Linear [ Linear ]
