ZXSC440
PHOTOFLASH CHARGER
Description
The ZXSC440 is a dedicated photoflash charger,charging an
80µF photoflash capacitor to 300V in 3.5 seconds from a 3V
supply.
The flyback conversion efficiency is typically 75%,much
higher than the commonly used discrete charging circuits.
DRIVE
V
FB
SENSE
N/C
Pin Assignments
(Top View)
V
CC
GND
READY
CHARGE
ADVANCE INFORMATION
The Charge pin enables the circuit to be initiated fromthe
camera's microprocessor, using negligible current when flash
is not being used.
The Ready pin signals the microprocessor when the flash is
charged and ready to be fired.
A small amount of hysteresis on the voltage feedback shuts
down the device as long as the capacitor remains fully
charged, again using negligible current.
MSOP-8
Features
•
•
•
•
•
•
Charges a 80µF photoflash capacitor to 300V in
3.5 seconds from 3V
Charges various value photoflash capacitors
Over 75% flyback efficiency
Charge and Ready pins
Consumes only 4.5µA when not charging
Small MSOP-8 low profile package
Applications
•
•
Digital camera flash unit
Film camera flash unit
Typical Application Circuit
ZXSC440
Document number: DS33619 Rev. 3 - 2
1 of 11
www.diodes.com
June 2012
© Diodes Incorporated
ZXSC440
PHOTOFLASH CHARGER
Pin Descriptions
Pin Name
Drive
VFB
Pin #
1
2
3
4
5
6
7
8
Description
Drive output for external switching transistor. Connect to base or
gate of external switching transistor
Reference voltage. Internal threshold set to 300mV. Connect
external resistor network to set output voltage
Inductor current sense input. Internal threshold voltage set to 28mV.
Connect external sense resistor
Initiate photoflash capacitor charging
Signal to microprocessor when photoflash capacitor charged
Ground
Supply voltage, 1.8V to 8V
ADVANCE INFORMATION
Sense
N/C
Charge
Ready
GND
V
CC
Functional Block Diagram
ZXSC440
Document number: DS33619 Rev. 3 - 2
2 of 11
www.diodes.com
June 2012
© Diodes Incorporated
ZXSC440
PHOTOFLASH CHARGER
Absolute Maximum Ratings
(T
A
= 25°C)
Parameter
V
CC
Drive
-0.3 to +10
-0.3 to V
CC
+0.3
-0.3 to V
CC
+0.3
-0.3 to The lower of (+5.0) or (V
CC
+0.3)
-0.3 to The lower of (+5.0) or (V
CC
+0.3)
-40 to +85
-55 to +150
450
Rating
Unit
V
V
V
V
V
°C
°C
mW
ADVANCE INFORMATION
Ready
Charge
VFB, Sense
Operating Temperature
Storage Temperature
Power Dissipation @ 25°C
Electrical Characteristics
(T
A
= 25°C, Vdd = 3V; unless otherwise specified)
Symbol
V
CC
I
Q
(Note 1)
I
STDN
E
FF
(Note 2)
ACC
REF
TCO
REF
T
DRV
F
OSC
V
SENSE
I
SENSE
V
FB
I
FB
(Note 2)
VIH (Note 3)
VIL
dV
LN
I
DRIVE
V
DRIVE
C
DRIVE
VOH
READY
VOL
READY
T
READY
dI
LD
Notes:
Parameter
V
CC
range
Quiescent current
Shutdown current
Efficiency
Reference tolerance
Reference temp co.
Discharge pulse width
Operating frequency
Sense voltage
Sense input current
Feedback volatage
Feedback input current
Shutdown threshold
Shutdown threshold
Line voltage regulation
Transistor drive current
Transistor voltage drive
MOSFET gate drive cpbty
Ready flag output high
Ready flag output low
Load current regulation
V
CC
= 8V
Conditions
Min.
1.8
Typ.
Max.
8
220
Unit
V
µA
µA
%
4.5
85
1.8V < V
CC
< 8V
1.8V < V
CC
< 8V
-3.0
0.005
1.7
200
22
V
FB
= 0V; V
SENSE
= 0V
V
FB
= 0V; V
SENSE
= 0V
-1
291
-1.2
1.5
0
0.5
V
DRIVE
= 0.7V
2
0
300
I
EOR
= -300nA, T
A
= 25°C
I
EOR
= 1mA, T
A
= 25°C
T
A
= 25°C
2.5
0
195
0.01
V
CC
1
3.4
5
V
CC
-0.4
28
-7
300
34
-15
309
-4.5
V
CC
0.55
3.0
%
%/°C
µs
kHz
mV
µA
mV
µA
V
V
%/V
mA
V
pF
V
V
µs
%/mA
INPUT PARAMETERS
OUTPUT PARAMETERS
1. Excluding gate/base drive current.
2. I
FB
is typically half of these at 3V.
3. Shutdown pin voltage must not exceed (V
CC
+0.3V) or 5V, whichever is lower.
ZXSC440
Document number: DS33619 Rev. 3 - 2
3 of 11
www.diodes.com
June 2012
© Diodes Incorporated
ZXSC440
PHOTOFLASH CHARGER
Device Description
Bandgap Reference
All threshold voltages and internal currents are derived
from a temperature compensated bandgap reference
circuit with a reference voltage of 1.22V nominal. If the
REF terminal is used as a reference for external devices,
the maximum load should not exceed ±2µA.
Comp2, at input SENSE. Once the current sense voltage
across the sensing resistor exceeds 28mV, comparator,
Comp2, through gate U1, triggers a re-triggerable
monostable and turns off the output drive stage for 1.7µs.
The inductor discharges into the reservoir capacitor. After
1.7µs a new charge cycle begins, thus ramping the output
voltage. When the output voltage reaches the nominal
value and VFB gets an input voltage of more than 300mV,
the monostable is forced "on" from Comp1 through gate
U1, until the feedback voltage falls below 300mV. The
above action continues to maintain regulation, with slight
hysteresis on the feedback threshold.
ADVANCE INFORMATION
Dynamic Drive Output
Depending on the input signal, the output is either "LOW"
or "HIGH". In the high state a 3.4mA current source (max
drive voltage = V
CC
-0.4V) drives the base or gate of the
external transistor. In order to operate the external
switching transistor at optimum efficiency, both output
states are initiated with a short transient current in order to
quickly discharge the base or the gate of the switching
transistor.
READY Detector
The READY circuit is a re-triggerable 195µs monostable,
which is re-triggered by every down regulating action of
comparator Comp1. As long as regulation takes place,
output READY is "HIGH" (high impedance, 100K to V
CC
).
Short dips of the output voltage of less than 195µs are
ignored. If the output voltage falls below the nominal value
for more than 195µs, output READY goes "LOW". This
can be used to signal to the camera controller that the
flash unit has charged fully and is ready to use.
Switching Circuit
The switching circuit consists of two comparators, Comp1
and Comp2, a gate U1, a monostable and the drive
output. Normally the DRIVE output is "HIGH"; the external
switching transistor is turned on. Current ramps up in the
inductor, the switching transistor and external current
sensing resistor. This voltage is sensed by comparator,
Typical Operating Characteristics
(For typical application circuit at V
IN
=3V and T
A
=25°C unless otherwise stated)
ZXSC440
Document number: DS33619 Rev. 3 - 2
4 of 11
www.diodes.com
June 2012
© Diodes Incorporated
ZXSC440
PHOTOFLASH CHARGER
Application Information
Switching Transistor Selection
The choice of switching transistor has a major impact on
the converter efficiency. For optimum performance, a
bipolar transistor with low V
CE(SAT)
and high gain is
required. The V
CEO
of the switching transistor is also an
important parameter as this sees typically three times the
input voltage when the transistor is switched off. Zetex
SuperSOT™ transistors are an ideal choice for this
application. At input voltages above 4V, suitable Zetex
MOSFET transistors will give almost the same
performance with a simpler drive circuit, omitting the
ZXTD6717 pre-drive stage. Using a MOSFET, the
Schottky diode may be omitted, as the body diode of the
MOSFET will perform the same function, with just a small
loss of efficiency.
Therefore, with a 300V output, a supply of 8 volts and a
1:12 step-up transformer, there will be a 396V across the
diode. This occurs during the current ramp-up in the
primary, as it transforms the input voltage up by the turns
ratio and the polarity at the secondary is such as to add to
the output voltage already being held off by the diode.
ADVANCE INFORMATION
Peak Current Definition
In general, the I
PK
value must be chosen to ensure that
the switching transistor, Q1, is in full saturation with
maximum output power conditions, assuming worse-case
input voltage and transistor gain under all operating
temperature extremes. Once I
PK
is decided the value of
R
SENSE
can be determined by:
R
SENSE
=
V
SENSE
I
PK
Output Rectifier Diode Selection
The diode should have a fast recovery, as any time spent
in reverse conduction removes energy from the reservoir
capacitor and dumps it, via the transformer, into the
protection diode across the output transistor.
This seriously reduces efficiency. Two BAS21 diodes in
series have been used, bearing in mind that the reverse
voltage across the diode is the sum of the output voltage
together with the input voltage multiplied by the step-up
ratio of the transformer:
V
R(DIODE)
= V
OUT(MAX)
+ (V
IN
x T
URNS
R
ATIO
)
Sense Resistor
A low value sense resistor is required to set the peak
current. Power in this resistor is negligible due to the low
sense voltage threshold, V
SENSE
. Below is a table of
recommended sense resistors:
Manufacturer
Cyntec
IRC
Series
RL1220
LR1206
R
DC
(Ω) Range
0.022 - 10
0.010 – 1.0
Size
0805
1206
Tolerance
±5%
±5%
Using a 22mΩ sense resistor results in a peak current of just over 1.2A.
Transformer Parameters
Proprietary transformers are available, for example the
Pulse PAO367, Primary inductance: 24µH, Core: Pulse
PAO367, Turns ratio: 1:12, see Bill of Materials below. If
designing a transformer, bear in mind that the primary
current may be over an amp and, if this flows through 10
turns, the primary flux will be 10 Amp. Turns and small
cores will need an air gap to cope with this value without
saturation. Secondary winding capacitance should not be
too high as this is working at 300V and could soon cause
excessive losses.
ZXSC440 Transformer Specifications
Part No.
T-15-089
T-15-083
SBL-5.6-1
PAO367
Size
(WxLxH) mm
6.4x7.7x4
8x8.9x2
5.6x8.5x4
9.1x9.1x5.1
L
PRI
(µH)
12
20
10
24
L
PRI-LEAK
(nH)
400
500
200
N
10:2
10:2
10:2
12:1
R
PRI
(mΩ)
211
675
103
R
SEC
(Ω)
27
35
26
ZXSC440
Document number: DS33619 Rev. 3 - 2
5 of 11
www.diodes.com
June 2012
© Diodes Incorporated
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