Allegro A8731 The allegro a8731 xenon photoflash charger ic is designed to meet the needs of ultra-low power, small form factor cameras, particularly camera-phones. Datasheet

A8731
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
Features and Benefits
Description
▪ Low quiescent current draw (0.01 μA in shutdown mode)
▪ Primary-side output voltage sensing; no resistor divider
required
▪ User-adjustable current limit from 0.4 to 1.2 A
▪ 1.1 V logic (VHI(min)) compatibility
▪ Integrated IGBT driver with internal gate resistors
▪ Flexible dual trigger inputs for IGBT driver
▪ Optimized for mobile phone, 1-cell Li+ battery applications
▪ No primary-side Schottky diode needed
▪ Zero-voltage switching for lower loss
▪ >75% efficiency
▪ Charge complete indication
▪ Integrated 40 V DMOS switch
The Allegro® A8731 Xenon photoflash charger IC is designed
to meet the needs of ultra-low power, small form factor cameras,
particularly camera-phones.
The charge time is adjustable by setting the charge current
limit from 0.4 to 1.2 A maximum. By using primary-side
voltage sensing, the need for a secondary-side resistive voltage
divider is eliminated. This has the additional benefit of reducing
leakage currents on the secondary side of the transformer. To
extend battery life, the A8731 features very low supply current
draw—typically 0.01 μA in shutdown mode and 10 μA in
standby mode.
The A8731 has a flash dual trigger IGBT driver. The IGBT driver
also has internal gate resistors for minimum external component
count. The charge and trigger voltage logic thresholds are set
at 1.1 VHI (min) to support applications implementing low
voltage control logic.
The A8731 is available in a 10-contact 3 mm × 3 mm DFN
package with a 0.75 nominal overall package height, and an
exposed pad for enhanced thermal performance.
Applications
▪ Mobile phone flash
▪ Digital and film camera flash
Package: 10-contact DFN with exposed
thermal pad (package EJ)
Approximate Scale 1:1
Typical Applications
1 : 10
1 : 10
C2
Battery Input +
2.3 to 5.5 V
C1
C2
COUT
100 μF
315 V
VIN
Battery Input +
2.3 to 5.5 V
C1
VOUT Detect
VOUT Detect
SW
ISET
RSET
SW
ISET
Control
Block
COUT
100 μF
315 V
VIN
ISW sense
RSET
Control
Block
ISW sense
VPULLUP
VPULLUP
100 kΩ
DONE
CHARGE
DONE
DONE
VIN
VIN
TRIGGER1
IGBT Driver
IGBT Gate
TRIGGER1
IGBT Driver
IGBT Gate
GATE
GATE
TRIGGER2
TRIGGER2
GND
Figure 1. Typical application with separate trigger inputs.
A8731-DS, Rev. 1
100 kΩ
DONE
CHARGE
GND
Figure 2. Typical application with single trigger input.
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
Selection Guide
Part Number
A8731EEJTR-T
Package
Packing
10-contact DFN
Tape and reel, 1500 pieces per reel
*Contact Allegro for additional ordering information.
Absolute Maximum Ratings
Characteristic
Symbol
Notes
Rating
Units
–0.3 to 40
V
–0.3 to 6.0
V
–0.6 to VIN + 0.3 V
V
–0.3 to VIN + 0.3 V
V
DC voltage.
SW Pin
VSW
VIN Pin
VIN
(VSW is self-clamped by internal active clamp
and is allowed to exceed 40 V during flyback
spike durations. Maximum repetitive energy
during flyback spike: 0.5 μJ at frequency
≤ 400 kHz.)
Care should be taken to limit the current when
–0.6 V is applied to these pins.
¯N̄¯Ē¯ Pins
CHARGE, TRIGGERx, D̄¯Ō
Remaining Pins
Operating Ambient Temperature
Maximum Junction
Storage Temperature
TA
–40 to 85
ºC
TJ(max)
Range E
150
ºC
Tstg
–55 to 150
ºC
Thermal Characteristics
Characteristic
Package Thermal Resistance
Symbol
RθJA
Value
Units
On 2-layer PCB with 0.88 in.2 area of 2 oz. copper each side,
based on JEDEC standard
Test Conditions*
65
ºC/W
On 4-layer PCB based on JEDEC standard
45
ºC/W
*Additional thermal information available on Allegro website.
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
2
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
Functional Block Diagram
VIN
SW
VSW – VBAT
DCM
Detector
ISET
toff(max)
ISET Buffer
VDSref
Control Logic
DMOS
18 μs
HmL
Triggered Timer
OCP
S
Q
R
Q
ton(max)
18 μs
Enable
S
Q
R
Q
DONE
One Shot
CHARGE
VIN
IGBT Driver
GATE
TRIGGER1
TRIGGER2
GND
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
3
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
Pin-out Diagram
ISET
1
GATE
2
VIN
3
GND
CHARGE
10 NC
9
DONE
8
TRIGGER1
4
7
SW
5
6
TRIGGER2
PAD
(Contacts Down View)
Terminal List Table
Number
Name
1
ISET
2
GATE
3
VIN
Function
Sets the maximum switch current; connect an external resistor to GND to
set the desired peak current
IGBT gate drive – sink/source
Input voltage; connect to a 2.3 to 5.5 V battery supply
4
GND
5
CHARGE
Ground connection
6
TRIGGER2
7
SW
8
TRIGGER1
9
¯N̄¯Ē¯
D̄¯Ō
10
NC
No connection , electrically floating pin
–
PAD
Exposed pad for enhanced thermal dissipation; connect to ground plane
Pull high to initiate charging; pull low to enter low-power standby mode
IGBT input trigger 2
Drain connection of internal power MOSFET switch; connect to
transformer primary winding
IGBT input trigger 1
Pulls low when output reaches target value and CHARGE pin is high;
goes high during charging or whenever CHARGE pin is low
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
4
A8731
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
ELECTRICAL CHARACTERISTICS typical values valid at VIN = 3.6 V, RSET = 33 kΩ, ISWlim = 1.0 A, and TA=25°C, unless otherwise noted
Characteristics
VIN Voltage Range
UVLO Enable Threshold
UVLO Hysteresis
VIN Supply Current
Symbol
VIN
VINUV
VINUVhys
IIN
Test Conditions
VIN rising
Shutdown (CHARGE = 0 V,
TRIGGER1 and TRIGGER2 = 0 V)
Charging complete
Charging (CHARGE = VIN,
TRIGGER1 and TRIGGER2 = 0 V)
Min.
2.3
–
–
Typ.
–
2.05
150
Max.
5.5
2.2
–
Unit
V
V
mV
–
0.01
0.5
μA
–
10
50
μA
–
2
–
mA
1.08
–
–
–
–
–
–
1.2
0.4
28
1.2
1000
0.25
–
1.32
–
–
–
–
–
2
A
A
kA/A
V
Ω
Ω
μA
–
–
0.5
μA
–
1.1
–
–
–
–
–
36
–
–
100
20
18
18
–
–
0.4
–
–
–
–
μA
V
V
kΩ
us
μs
μs
Current Limits
Switch Current Limit1
SW / ISET Current Ratio
ISET Pin Voltage While Charging
ISET Pin Internal Resistance
Switch On-Resistance
Switch Leakage Current2
ISWlimMAX
ISWlimMIN
ISW/ISET
VSET
RSET(INT)
RSWDS(on)
ISWlk
CHARGE Input Current
ICHARGE
CHARGE Input Voltage2
VCHARGE
CHARGE Pull-Down Resistor Value
CHARGE ON/OFF Delay
Maximum Switch-Off Timeout
Maximum Switch-On Timeout
RCHPD
tCH
toffMAX
tonMAX
¯N̄¯Ē¯ Output Leakage Current2
D̄¯Ō
IDONElk
¯N̄¯Ē¯ Output Low Voltage2
D̄¯Ō
VDONEL
RSET = 26.7 kΩ
RSET = 85 kΩ
CHARGE = high
CHARGE = high
VIN = 3.6 V, ID = 800 mA, TA = 25°C
VSW = VIN(max), over temperature range
Combined VIN and SW leakage current at TA=25°C
VIN= 5.5 V in Shutdown
VCHARGE = VIN
High, over input supply range
Low, over input supply range
Time between CHARGE = 1 and charging enabled
–
–
1
μA
–
31
–
–
–
31.5
200
20
100
32
400
–
mV
V
mV
V/μs
VTRIG(H) Input = logic high, over input supply range
1.1
–
VTRIG(L) Input = logic low, over input supply range
–
–
TRIGGER, TRIGGER2 Pull-Down Resistor RTRIGPD
–
100
GATE Resistance to VIN
RSrcDS(on) VIN = 3.6 V, VGATE =1.8 V
–
23
GATE Resistance to GND
RSnkDS(on) VIN = 3.6 V, VGATE = 1.8 V
–
30
Propagation Delay (Rising)
tDr
–
110
Propagation Delay (Falling)
tDf
–
140
Measurement taken at pin, CL= 6500 pF, VIN = 3.6 V
Output Rise Time
tr
–
290
Output Fall Time
tf
–
360
1Current limit guaranteed by design and correlation to static test. Refer to application section for peak current in actual circuits.
2Specifications over the range T = –40°C to 85°C; guaranteed by design and characterization.
A
–
0.4
–
–
–
–
–
–
–
V
V
kΩ
Ω
Ω
ns
ns
ns
ns
Output Comparator Trip Voltage2
Output Comparator Overdrive
dV/dt Threshold of ZVS Comparator
IGBT Driver
TRIGGER, TRIGGER2 Input Voltage2
VOUTTRIP
VOUTOV
dV/dt
¯N̄¯Ē¯ pin
32 μA into D̄¯Ō
Measured as VSW – VIN
Pulse width = 200 ns (90% to 90%)
Measured at SW pin
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
5
A8731
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
IGBT Drive Timing Definition
TRIGGER
50%
tDr
50%
tr
tDf
90%
GATE
10%
tf
90%
10%
IGBT Drive Timing Characteristic Performance
CGATE = 6200 pF. VIN = 3.6 V, Time = 200 ns/div; CH1 = TRIGGER input, 2 V/div,
CH2 = Gate driver output, 1 V/div
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
6
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
A8731 Operation Timing Diagram
VIN
UVLO
CHARGE
SW
Target VOUT
VOUT
DONE
T2
T1
T3
TRIGGER1 and
TRIGGER2
IGBTDRV
A
B
C
D
E
F
Explanation of Events
A: Start charging by pulling CHARGE to high, provided that VIN is above UVLO level.
B: Charging stops when VOUT reaches the target voltage.
C: Start a new charging process with a low-to-high transition at the CHARGE pin.
D: Pull CHARGE to low to put the controller in low-power standby mode.
E: Charging does not start, because VIN is below UVLO level when CHARGE goes high.
F: After VIN goes above UVLO , another low-to-high transition at the CHARGE pin is required to
start the charging.
T1, T2, T3 (Trigger instances): IGBT driver output pulled high whenever both TRIGGER pins are
logic high. It is recommended to avoid applying any trigger pulses during charging.
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
7
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
Performance Characteristics
Charging Time at Various Peak Current Levels
Common Parameters
Symbol
Parameter Units/Division
C1
VOUT
50 V
C2
VBAT
1V
C3
IIN
100 mA
t
time
200 ms
Conditions Parameter
Value
VBATT
3.6 V
COUT
20 μF
Conditions
Parameter
RSET
ISWlim
Value
26.7 kΩ
≈1.2 A
VOUT
VBAT
C1
C2
IIN
C1
C2
C3
C3
t
VOUT
C1
VBAT
Conditions
Parameter
RSET
ISWlim
Value
33.2 kΩ
≈1.0 A
C2
IIN
C1
C2
C3
C3
t
VOUT
C1
VBAT
Conditions
Parameter
RSET
ISWlim
Value
39 kΩ
≈0.9 A
C2
IIN
C1
C2
C3
C3
t
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
8
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
Efficiency versus Battery Voltage
Charge Time versus Battery Voltage
Transformer Lp= 8 μH, N = 10.2; COUT= 20 μF / 330 V UCC; TA=25°
Transformer Lp= 8 μH, N = 10.2; COUT= 20 μF / 330 V UCC; TA=25°
71
3.5
2.0
45
≈ 0.8
67
39
≈ 0.9
66
33.2
≈ 1.0
65
26.7
≈ 1.2
69
68
1.5
1.0
0.5
64
62
RSET
(kΩ)
55
IP
(A)
≈ 0.65
61
45
≈ 0.8
60
39
≈ 0.9
59
33.2
≈ 1.0
58
26.7
≈ 1.2
63
57
56
55
0
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
2.0
6.0
2.5
3.0
3.5
VBAT (V)
COUT= 20 μF. For larger or smaller capacitances, charging time
scales proportionally.
4.0
VBAT (V)
4.5
5.0
5.5
6.0
Special low-profile transformer with relatively low inductance
(Lp= 8 μH) and high winding resistance (Rp = 0.37 Ω). Higher efficiency can be achieved by using transformers with higher Lp, which reduces
switching frequency and therefore switching loses, and lower resistance,
which reduces conduction losses.
Average Input Current versus Battery Voltage
XFM Lp= 8 μH, N = 10.2, COUT= 20 μF 330 V UCC, TA=25°
0.55
0.50
0.45
0.40
IIN (A)
Time (s)
2.5
IP
(A)
≈ 0.65
Efficiency (%)
3.0
70
RSET
(kΩ)
55
0.35
0.30
RSET
(kΩ)
26.7
IP
(A)
≈ 1.2
33.2
≈ 1.0
39
≈ 0.9
45
≈ 0.8
55
≈ 0.65
0.25
0.20
0.15
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
VBAT (V)
An increase in ISWlim with respect to VBAT actually keeps the average input current
roughly constant throughout the battery voltage range. Normally, if ISWlim is kept
constant, the average current will drop as VBAT goes higher.
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
9
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
Application Information
transformer primary inductance, Lp. If necessary, the
The CHARGE pin enables the part and starts charging. following expressions can be used to determine ISWlim
¯ open-drain indicator is pulled low when
The ¯¯¯
D¯ Ō¯¯N̄¯Ē
more accurately:
CHARGE is high and target output voltage is reached.
Pulling the CHARGE pin low stops charging and
ISET = VSET / (RSET + RSET(INT) – K × RGND(INT) ), (2)
forces the chip into low-power standby mode.
where:
Selection of Switching Current Limit
RSET(INT) is the internal resistance of the ISET pin
The A8731 features continuously adjustable peak
(1 kΩ typical),
switching current between 0.4 and 1.2A. This is done
by selecting the value of an external resistor RSET,
RGND(INT) is the internal resistance of the bonding
connected from the ISET pin to GND, which deterwire for the GND pin (27 mΩ typical), and
mines the ISET bias current, and therefore the switching current limit, ISWlim.
K = (K′ + VIN × K″), with K′ = 24350 and
To the first order approximation, ISWlim is related to
K″ ≈ 1040 at TA = 25°C. Then,
ISET and RSET according to the following equations:
ISWlim = ISET × K + VBAT / LP × tD ,
(3)
(1)
ISWlim = ISET × K = VSET / RSET × K ,
where tD is the delay in SW turn-off (0.1 μs typical).
where K = 28000 when battery voltage is 3.6 V.
Figure 3 can be used to determine the relationship
In real applications, the actual switching current
limit is affected by input battery voltage, and also the
between RSET and ISWlim at various battery voltages.
General Operation Overview
1.3
1.2
1.1
VIN = 5.5 V
VIN = 4.5 V
ISWlim (A)
1.0
VIN = 3.6 V
VIN = 3.0 V
0.9
VIN = 2.3 V
0.8
0.7
0.6
0.5
0.4
25
30
35
40
45
50
55
60
65
70
75
80
85
90
RSET (kΩ)
Figure 3. Peak Current Limit versus ISET Resistance. VIN = VBAT, transformer LP = 8 μH, TA = 25°C.
10
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
Smart Current Limit (Optional)
With the help of some simple external logic, the user
can change the charging current according to the battery voltage. For example, assume that ISET is normally 36 μA (for ISWlim = 1.0 A). Referring to figure 4,
when the battery voltage drops below 2.5 V, the signal
at BL (battery-low) goes high. The resistor RBL, connecting BL to the ISET pin, then injects 10 μA into
RSET. This effectively reduces ISET current to 26 μA
(for ISWLIM = 0.73 A).
Timer Mode and Fast Charging Mode
The A8731 achieves fast charging times and high efficiency by operating in discontinuous conduction mode
(DCM) through most of the charging process The
BL
RBL
ISET
RSET
relationship of Timer Mode and Fast Charging Mode
is shown in figure 5.
The IC operates in Timer Mode when beginning to
charge a completely discharged photoflash capacitor, usually when the output voltage, VOUT, is less
than approximately 15 to 20 V. Timer Mode is a fixed
period, 18 μs, off-time control. One advantage of
having Timer Mode is that it limits the initial battery
current surge and thus acts as a “soft-start.” A timeexpanded view of a Timer Mode interval is shown in
figure 6.
As soon as a sufficient voltage has built up at the
output capacitor, the IC enters Fast-Charging Mode.
In this mode, the next switching cycle starts after the
secondary side current has stopped flowing, and the
switch voltage has dropped to a minimum value. A
proprietary circuit is used to allow minimum-voltage
switching, even if the SW pin voltage does not drop to
0 V. This enables Fast-Charging Mode to start earlier
Figure 4. Smart Current Limit reference circuit
VOUT
Timer Mode
Fast Charging Mode
VBAT
VSW
VBAT
IIN
VOUT
ISW
Figure 5. Timer Mode and Fast Charging Mode. t =200 ms/div,
VOUT =50 V/div, VBAT =1 V/div., IIN =100 mA/div., VBAT =3.6 V,
COUT =20 μF/330 V, RSET=46 kΩ (ISWlim≈0.75 A).
Figure 6. Timer Mode expanded view. VOUT ≤ 14 V, t = 2 μs / div.,
VBAT = 3.6 V, RSET = 33.2 kΩ.
11
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
A8731
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
than previously possible, thereby reducing the overall
charging time. Minimum-voltage switching is shown
in figure 7.
During Fast-Charging Mode, when VOUT is high
enough (over 50 V), true zero-voltage switching
(ZVS) is achieved. This further improves efficiency
as well as reduces switching noise. A ZVS interval is
shown in figure 8.
IGBT Gate Driver Inputs
The TRIGGER1 and TRIGGER2 pins are ANDed
together inside the IC to control the IGBT gate driver.
If only one trigger signal is needed, tie both trigger
pins together and use as a single input.
Ambient Light Sensing
Ambient Light Sensing (ALS) can be easily implemented for the A8731 using the TRIGGER2 pin plus
three external components. This configuration is
shown in figure 9.
The phototransistor current is proportional to the
intensity of the light that it receives. When there is
sufficient ambient light (for example, during daylight
outdoor photographing), a current of about 30 μA
can flow through the phototransistor. This forces the
voltage at TRIGGER2 pin to fall to 0.8 V or lower,
so it prohibits TRIGGER1 from firing the flash. The
exact threshold of ambient light required to prohibit
flash firing can be adjusted by RTGR1. The smaller this
resistance, the brighter the ambient light must be to
prohibit flash firing.
When ambient conditions are dark, the current flowing through the phototransistor is in less than 1 μA.
Because the TRIGGER2 pin is biased at 1.4 V or
higher, TRIGGER1 is allowed to activate the IGBT
gate driver (and thereby fire the flash).
The capacitor CTGR1 and resistor RTGR1 form an
integrator for light exposure. When the flash fires,
bright light bounces back from subject and enters the
phototransistor. In example A in figure 10, the flash
terminates after just 30 μs, without fully discharging
the photoflash capacitor.
VOUT
Minimum Voltage
Switching
VSW
VSW
VBAT
VOUT
ISW
Figure 7. Minimum voltage switching. VOUT ≥ 15 V; t =1 μs/div.,
VBAT = 3.6 V, RSET = 33.2 kΩ.
Zero Voltage
Switching
VSW
VBAT
VBAT
ISW
VOUT
ISW
Figure 8. Zero voltage switching. VOUT = 120 V. t = 0.2 μs/div.,
VBAT = 3.6 V, RSET = 33.2 kΩ.
12
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
Battery Input
2.5 to 5.5 V
If the subject is far away, the reflected
light intensity is lower, so the phototransistor current is also lower. In example B, the
flash stays on for longer time (60 μs) and
discharges more energy from the photoflash capacitor.
Using a larger CTGR1 causes the time
constant of the integrator to increase, so a
longer pulse is required before the flash is
terminated.
1 : 10
+
C1
C2
COUT
100 μF
315 V
VIN
VOUT Detect
ISET
SW
Control
Block
RSET
ISW sense
VPULLUP
DONE
CHARGE
DONE
VIN
IGBT Driver
TRIGGER1
A
IGBT Gate
GATE
RTGR1
100 kΩ
PNZ121S
Phototransistor
TRIGGER2
CTGR1
1 μF
GND
A It is recommend to use a regulated system voltage for the bias. If battery
voltage is used, the ALS sensitivity will vary with battery voltage, and there
would be a small leakage current even when the camera is turned off.
Figure 9. ALS typical application
VOUT
C1
VOUT
VTRIGGER2
VTRIGGER2
C2
C2
C3
VTRIGGER1
C3
VGATE
C1
C4
t
(A)
VTRIGGER1
C2
C1
C2
C3
C3
C4
VGATE
C1
C4
Common Parameters
Symbol
Parameter Units/Division
C1
VOUT
50 V
C2
VTRIGGER2
1V
5V
C3
VTRIGGER1
C4
VGATE
5V
t
time
20 μs
C4
t
(B)
Figure 10. Adaptive timing of photoflash. (A) Subject near to camera, and (B) subject far from camera.
13
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
Transformer Selection
1. The transformer turns ratio, N, determines the output voltage:
N = NS / NP ,
VOUT = 31.5 × N – Vd ,
(4)
(5)
where 31.5 is the typical value of VOUTTRIP , and Vd is
the forward drop of the output diode.
2. The primary inductance, LP , determines the on-time
of the switch:
ton = (–LP / R ) × ln (1 – ISWlim × R /VIN) ,
(6)
where R is the total resistance in the primary current
path (including RSWDS(on) and the DC resistance of the
transformer).
If VIN is much larger than ISWlim × R, then ton can be
approximated by:
ton = ISWlim × LP /VIN .
(7)
The minimum pulse width for toff determines what
is the minimum LP required for the transformer. For
example, if ISWlim = 0.7 A, N = 10, and VOUT = 315 V,
then LP must be at least 9 μH in order to keep toff at
200 ns or longer. These relationships are illustrated in
figure 11.
In general, choosing a transformer with a larger LP
results in higher efficiency (because a larger LP means
lower switch frequency and hence lower switching
loss). But transformers with a larger LP also require
more windings and larger magnetic cores. Therefore, a
trade-off must be made between transformer size and
efficiency.
Component Selection
Selection of the flyback transformer should be based
on the peak current, according to the following table:
IPeak Range
3. The secondary inductance, LS, determines the offtime of the switch. Given:
LS / LP = N × N , then
toff = (ISWlim / N) × LS /VOUT
(8)
= (ISWlim × LP × N) /VOUT .
(9)
ton
LP
(A)
Supplier
Part Number
0.4 to 1.0
TDK
LDT565630T-002
14.5
0.5 to 1.2
TDK
LDT565630T-003
10.5
LDT565620ST-203
8.2
0.7 to 1.0
TDK
0.7 to 1.2
Mitsumi
0.8 to 1.2
Tokyo Coil
(μH)
C5-KT2.2L
8.0
T-19-243
6.5
toff
VSW
ISW
Vr
tf
VIN
VIN
ISW
VSW
tneg
Figure 11. Pulse width relationship definitions.
14
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
Package EJ, 3 mm x 3 mm 10-Contact DFN
with Exposed Thermal Pad
0.30
3.00 ±0.15
0.85
0.50
10
10
3.00 ±0.15
1.64
3.10
A
1
2
1
11X
D
SEATING
PLANE
0.08 C
+0.05
0.25 –0.07
C
C
2.38
PCB Layout Reference View
0.75 ±0.05
0.50
1
For Reference Only
(reference JEDEC MO-229WEED)
Dimensions in millimeters
Exact case and lead configuration at supplier discretion within limits shown
2
0.40 ±0.10
1.64
B
10
2.38
A Terminal #1 mark area
B Exposed thermal pad (reference only, terminal #1
identifier appearance at supplier discretion)
C Reference land pattern layout (reference
IPC7351 SON50P300X300X80-11WEED3M);
All pads a minimum of 0.20 mm from all adjacent pads; adjust as
necessary to meet application process requirements and PCB layout
tolerances; when mounting on a multilayer PCB, thermal vias at the
exposed thermal pad land can improve thermal dissipation (reference
EIA/JEDEC Standard JESD51-5)
15
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Mobile Phone Xenon Photoflash Capacitor Charger
With IGBT Driver
A8731
Revision History
Revision
Revision Date
Rev. 1
April 19, 2012
Description of Revision
Miscellaneous format changes
Copyright ©2008-2012, Allegro MicroSystems, Inc.
Allegro MicroSystems, Inc. reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the
information being relied upon is current.
Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the
failure of that life support device or system, or to affect the safety or effectiveness of that device or system.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use;
nor for any infringement of patents or other rights of third parties which may result from its use.
For the latest version of this document, visit our website:
www.allegromicro.com
16
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com