IC PHOTOFLASH CAP CHARGER 6-DFN

LT3484EDCB-0#TRMPBF

Manufacturer Part NumberLT3484EDCB-0#TRMPBF
DescriptionIC PHOTOFLASH CAP CHARGER 6-DFN
ManufacturerLinear Technology
LT3484EDCB-0#TRMPBF datasheet
 


Specifications of LT3484EDCB-0#TRMPBF

ApplicationsPhotoflash Capacitor ChargerCurrent - Supply5mA
Voltage - Supply1.7 V ~ 16 VOperating Temperature-40°C ~ 85°C
Mounting TypeSurface MountPackage / Case6-DFN
Lead Free Status / RoHS StatusLead free / RoHS CompliantOther namesLT3484EDCB-0#TRMPBFTR
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OPERATIO
The LT3484-0/LT3484-1/LT3484-2 are designed to charge
photoflash capacitors quickly and efficiently. The opera-
tion of the part can be best understood by referring to
Figure 1. When the CHARGE pin is first driven high, a one
shot sets both SR latches in the correct state. The power
NPN device, Q1, turns on and current begins ramping up
in the primary of transformer T1. Comparator A1 monitors
the switch current and when the peak current reaches 1.4A
(LT3484-0), 1A(LT3484-2) or 0.7A (LT3484-1), Q1 is
turned off. Since T1 is utilized as a flyback transformer, the
flyback pulse on the SW pin will cause the output of A3 to
be high. The voltage on the SW pin needs to be at least
40mV higher than V
for this to happen.
BAT
During this phase, current is delivered to the photoflash
capacitor via the secondary and diode D1. As the second-
ary current decreases to zero, the SW pin voltage will begin
to collapse. When the SW pin voltage drops to 40mV
above V
or lower, the output of A3 (DCM Comparator)
BAT
will go low. This fires a one shot which turns Q1 back on.
This cycle will continue to deliver power to the output.
Output voltage detection is accomplished via R2, R1, Q2,
and comparator A2 (V
Comparator). Resistors R1 and
OUT
R2 are sized so that when the SW voltage is 31.5V above
V
, the output of A2 goes high which resets the master
IN
latch. This disables Q1 and halts power delivery. NPN
transistor Q3 is turned on pulling the DONE pin low,
indicating that the part has finished charging. Power
delivery can only be restarted by toggling the CHARGE pin.
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APPLICATIO S I FOR ATIO
Choosing the Right Device
(LT3484-0/LT3484-1/LT3484-2)
The only difference between the three versions of the
LT3484 is the peak current level. For the fastest possible
charge time, use the LT3484-0. The LT3484-1 has the
lowest peak current capability, and is designed for appli-
cations that need a more limited drain on the batteries. Due
to the lower peak current, the LT3484-1 can use a physi-
cally smaller transformer. The LT3484-2 has a current
limit in between that of the LT3484-0 and the LT3484-1.
LT3484-0/LT3484-1/LT3484-2
The CHARGE pin gives full control of the part to the user.
The charging can be halted at any time by bringing the
CHARGE pin low. Only when the final output voltage is
reached will the DONE pin go low. Figure 2 shows these
various modes in action. When CHARGE is first brought
high, charging commences. When CHARGE is brought
low during charging, the part goes into shutdown and
V
no longer rises. When CHARGE is brought high
OUT
again, charging resumes. When the target V
reached, the DONE pin goes low and charging stops.
Finally the CHARGE pin is brought low again so the part
enters shutdown and the DONE pin goes high. Both V
and V
have undervoltage lockout (UVLO). When one of
IN
these pins goes below its UVLO voltage, the DONE pin
goes low. With an insufficient bypass capacitor on V
V
, the ripple on the pin is likely to activate the UVLO and
IN
terminate the charge. The application diagrams suggest
values adequate for most applications.
100V/DIV
Figure 2. Halting the Charging Cycle with the CHARGE Pin
W
U
Transformer Design
The flyback transformer is a key element for any
LT3484-0/LT3484-1/LT3484-2 design. It must be de-
signed carefully and checked that it does not cause exces-
sive current or voltage on any pin of the part. The main
parameters that need to be designed are shown in Table 1.
The first transformer parameter that needs to be set is the
turns ratio N. The LT3484-0/LT3484-1/LT3484-2 accom-
plish output voltage detection by monitoring the flyback
waveform on the SW pin. When the SW voltage reaches
31.5V higher than the V
delivery. Thus, the choice of N sets the target output
OUT
LT3484-2
V
= 3.6V
IN
C
= 50µF
V
OUT
OUT
V
DONE
5V/DIV
V
CHARGE
5V/DIV
1s/DIV
3484 F02
voltage, the part will halt power
BAT
voltage is
BAT
or
BAT
3484012f
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