Dec 2003 Photoflash Capacitor Chargers Fit into Tight Spots

LINEAR TECHNOLOGY
DECEMBER 2003
IN THIS ISSUE…
COVER ARTICLE
Photoflash Capacitor Chargers
Fit into Tight Spots ........................1
Albert Wu
Issue Highlights .............................2
LTC® in the News ............................2
DESIGN FEATURES
Digital Programmable Oscillator Is
Smaller, Sturdier and More Versatile
than Crystal Oscillators .................7
Albert Huntington
Hot Swap™ Controller with PowerUp Timeout Function Simplifies Hot
Swapping Boards with Multiple
Power Supplies .............................10
Anthony Ng and YK Sim
Low Voltage Wizardry Provides the
Ultimate Power-On Reset Circuit ...14
Bob Jurgilewicz
New Power for Ethernet—
Disconnect and Clean Up
(Epilogue to a 3-part series) ..........17
Jacob Herbold
Save Board Space with a High
Efficiency Dual Synchronous,
600mA, 1.5MHz Step-Down
DC/DC Regulator...........................19
Damon Lee
White LED Driver Minimizes Space,
Maximizes Efficiency and Flexibility
.....................................................22
Steven Martin
High Input Voltage Monolithic
Switcher Steps Up and Down
Using Single Inductor ...................24
VOLUME XIII NUMBER 4
Photoflash Capacitor
Chargers Fit into
by Albert Wu
Tight Spots
Introduction
Take a walk through any electronics
retailer and you will notice an obvious trend: Cameras are being added
to PDAs, cell phones and other portable devices. This is due, of course,
to shrinking electronics required for
digital imaging. Even as imaging electronics shrink, the imaging pixel count
grows. The corresponding increase
in image quality demands a corresponding improvement in photoflash
technology. LED-based photoflash
units are certainly compact enough
to fit in the smallest devices, but LED
units cannot meet the light output and
spectral quality required of one megapixel or higher sensors. A xenon-bulb
based flash unit offers better performance, but traditionally takes more
space. Now there is a way to fit a
xenon-bulb photoflash unit into the
tightest spaces. The solution is to use
one of Linear Technology’s LT®3468
photoflash capacitor chargers.
The LT3468 series is available in a
5-Lead ThinSOT™ package. All output
voltage detection is implemented inside the part, substantially reducing
external parts count to a mere four
components. A new patented control
technique allows the use of ultra-small
transformers while maintaining high
efficiency. Imaging devices using these
parts can save significant space while
still achieving well controlled battery
current, fast charge times and high
efficiency.
Overview
A typical application for the LT3468 is
shown in Figure 1a. The high level of
continued on page 3
DANGER HIGH VOLTAGE—OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
Jay Celani
Hot Swap Controller Enforces
Tracking in Split Supply Systems
.....................................................28
T1
1:10.4
VIN
2.5V TO 8V
C1
4.7µF
Ted Henderson
Jeff Heath
DESIGN IDEAS
............................................... 33–37
(complete list on page 33)
New Device Cameos.......................38
Design Tools .................................39
Sales Offices.................................40
5
CHARGE
7, 8
1
1
SW
VIN
R1
100k
DONE
4
320V
+
VDD
Voltage Margining Made Easy.......31
D1
5, 6
3
4
LT3468
DONE
GND
D2
COUT
PHOTOFLASH
CAPACITOR
2
CHARGE
C1: 4.7µF, X5R OR X7R, 10V
T1: TDK PART# LDT565630T-001, LPRI = 6µH, N = 10.4
D1: VISHAY GSD2004S DUAL DIODE CONNECTED IN SERIES
D2: ZETEX ZHCS400 OR EQUIVALENT
R1: PULL UP RESISTOR NEEDED IF DONE PIN USED
Figure 1a. Compact, 320V photoflash capacitor charging circuit needs no zener
, LTC, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, FilterCAD, Hot Swap, LinearView, Micropower SwitcherCAD, Multimode
Dimming, No Latency ΔΣ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelProtect, PowerPath, PowerSOT,
SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, UltraFast and VLDO are trademarks of Linear Technology Corporation.
Other product names may be trademarks of the companies that manufacture the products.
DESIGN FEATURES
LT3468, continued from page 1
The LT3468-1 is a lower current version of the LT3468. Figure 2a shows a
typical application circuit while Figure
2b shows the charge time. The input
current for the LT3468-1 is typically
250mA, while that of the LT3468 is
about 550mA.
10
9
CHARGE TIME (s)
8
7
6
5
4
COUT = 100µF
3
2
1
0
Operation
COUT = 50µF
2
3
4
5
6
VIN (V)
7
8
9
Figure 1c. The LT3468 makes it
possible to fit an entire photoflash
charging circuit into 80mm2
Figure 1b. Charge times
integration inside the part results in
a very simple circuit that takes little
valuable board space. Figure 1c shows
an entire charging circuit fitting into
80mm2. The tallest component on
the board is the transformer, which
is only 3mm in height. Despite the tiny
components, charge time is excellent
due to the high power, integrated low
resistance NPN power switch.
To better understand the operation
of the part, refer to Figure 3 for the
following overview. Note that the only
difference between the LT3468 and
the LT3468-1 is the switch current
limit (1.4A for the LT3468, 0.7A for
the LT3468-1). A low-to-high transition on the CHARGE pin initiates the
part. An edge triggered one-shot triggered by the CHARGE pin puts the
various latches inside the part into
the proper state.
DANGER HIGH VOLTAGE—OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
T1
1:10.2
C1
4.7µF
R1
100k
CHARGE
4
8
1
320V
3
4
5
1
VIN
SW
10
9
+
VDD
DONE
D1
5
COUT
PHOTOFLASH
CAPACITOR
D2
LT3468-1
2
GND
DONE
CHARGE
8
CHARGE TIME (s)
VIN
2.5V TO 8V
7
6
5
4
COUT = 50µF
3
2
C1: 4.7µF, X5R OR X7R, 10V
T1: TDK PART# LDT565630T-002, LPRI = 14.5µH, N = 10.2
D1: VISHAY GSD2004S DUAL DIODE CONNECTED IN SERIES
D2: ZETEX ZHCS400 OR EQUIVALENT
R1: PULL UP RESISTOR NEEDED IF DONE PIN USED
1
0
3468 TA04
COUT = 20µF
2
3
Figure 2a. LT3468-1 photoflash circuit uses tiny 3mm tall transformer
4
5
6
VIN (V)
7
8
9
3468 TA06
Figure 2b. Charge time
Table 1. Comparison chart for Linear Technology’s photoflash charger parts
Peak SW Current (A)
LT3468
LT3468-1
LT3420
LT3420-1
1.4
0.7
1.4
1.0
40
20
840
500
Secondary Current
at SW Turn On (mA)
Average Input Current (mA)
(VIN = 3.3V, VOUT = 300V)
0
550
250
Minimum Battery Voltage (V)
2.5
1.8
Integrated Output Detection?
Yes
No
Automatic Refresh?
No
Yes
Common Battery
Combinations
1–2 Li-ion cell
4 AA cells
4 NIMH cells
1–2 Li-ion cell
2–4 AA cells
2–4 NIMH cells
Package
TSOT-5L
MSOP-10L
Linear Technology Magazine • December 2003
3
DESIGN FEATURES
D1
T1
TO BATTERY
VOUT
PRIMARY
C1
SECONDARY
D2
3
DONE
5
VIN
R2
60k
Q3
+
SW
1
COUT
PHOTOFLASH
CAPACITOR
DCM COMPARATOR
+
ONESHOT
A3
–
+
–
36mV
Q2
Q1
ENABLE
MASTER
LATCH
Q
S
R1
2.5k
Q
R
DRIVER
R
+
S
A2
+
1.25V
REFERENCE
–
A1
VOUT COMPARATOR
CHARGE
4
Q1
Q
RSENSE
20mV
–
+–
ONESHOT
2
GND
3486 BD
LT3468: RSENSE = 0.015Ω
LT3468-1: RSENSE = 0.03Ω
Figure 3. Block diagram of the LT3468
The part begins charging by turning on the power NPN transistor Q1.
With Q1 on, the current in the primary
of the flyback transformer increases.
When it reaches the current limit,
Q1 is turned off and the secondary
of the transformer delivers current to
the photoflash capacitor via diode D1.
During this time, the voltage on the SW
pin is proportional to the output voltage. Since the SW pin is higher than VIN
by an amount roughly equal to (VOUT
+ 2 • VD)/N, the output of the DCM
Comparator is high. In this equation,
VOUT is the photoflash capacitor voltage, VD is the rectifying diode forward
drop, and N is the turns ratio of the
transformer.
Once the current in the secondary
of the transformer decays to zero,
the voltage on the SW pin collapses
to VIN, or lower. As a result, the output of the DCM comparator goes low,
which triggers the one-shot. This leads
to Q1 turning on again and the cycle
repeats.
Output voltage detection is accom4
VIN = 3.6V
VOUT = 100V
VIN = 3.6V
VOUT = 300V
VSW
10V/DIV
VSW
10V/DIV
IPRI
1A/DIV
IPRI
1A/DIV
1µs/DIV
1µs/DIV
Figure 4a. LT3468 switching
waveform at 100V output
Figure 4b. LT3468 switching
waveform at 300V output
VIN = 3.6V
VOUT = 100V
VIN = 3.6V
VOUT = 300V
VSW
10V/DIV
VSW
10V/DIV
IPRI
1A/DIV
IPRI
1A/DIV
1µs/DIV
Figure 4c. LT3468-1 switching
waveform at 100V output
1µs/DIV
Figure 4d. LT3468-1 switching
waveform at 300V output
Linear Technology Magazine • December 2003
DESIGN FEATURES
plished via comparator A2. When the
SW pin is 31.5V higher than VIN on
any cycle, the output of A2 goes high.
This resets the master latch and the
part stops delivering power to the photoflash capacitor. Power delivery can
only restart by taking the CHARGE
pin low and then high.
Note that the flux in the flyback
transformer is brought to zero on
each switching cycle. This is generally referred to as boundary mode
since the transformer is operated
in between continuous conduction
mode and discontinuous conduction
mode (CCM and DCM respectively).
When the CHARGE pin is forced low
at anytime, the LT3468 ceases power
delivery and goes into shutdown mode,
thus reducing quiescent current to less
than 1µA. Figure 4 shows some typical
switching waveforms for the LT3468
and LT3468-1.
For a given photoflash capacitor size,
the device which results in the highest average input current offers the
fastest charge time. The limit on how
much current the photoflash charger
can draw is usually set by the batteries, and how much load they can
handle. The LT3420 offers the fastest
charge times of the chargers discussed
here.
The following equations predict
the charge times (T) in seconds for
the four parts:
Which Part to Use?
LT3420:
The LT3468 and LT3468-1 round out
Linear Technology’s photoflash capacitor charger line to four chargers that
can suit just about any photoflash
need: the LT3468, LT3468-1, LT3420,
and the LT3420-1. Table 1 shows the
major functional differences between
these four parts.
Choosing a device is a matter of balancing the inherent trade-off between
average input current and charge time.
LT3468:
2
2
COUT •  VOUT(FINAL) – VOUT(INIT) 


,
T=
0.65 • VIN
LT3468-1:
2
2
COUT •  VOUT(FINAL) – VOUT(INIT) 


,
T=
0.32 • VIN
2
2
COUT •  VOUT(FINAL) – VOUT(INIT) 


,
T=
1.2 • VIN
the initial output voltage, and VIN is the
battery or input voltage to which the
flyback transformer is connected.
These equations are developed for
specific transformers, namely the TDK
LDT565630T-001 for the LT3468,
the TDK LDT565630T-002 for the
LT3468-1, the TDK SRW10EPCU01H003 for the LT3420 and the
Kijima Musen SBL-5.6S-2 for the
LT3420-1. If other transformers are
used, the constant in the denominator
of each the above equations changes
slightly because of differing transformer efficiencies.
Generally speaking, the LT3468 is
used for photoflash capacitors in the
80µF to 160µF range commonly found
in mid- to high-end digital cameras.
The LT3468-1 is used for photoflash
capacitors in the 10µF–80µF range,
which are likely to be required in ultra
small digital cameras and cell phonebased cameras. For designs required
to operate from 2AA cells, the LT3420
and LT3420-1 are the right choice,
as they are designed to operate on a
battery voltage down to 1.8V.
LT3420-1:
Output Voltage Detection
where COUT is the value of the photoflash capacitor in Farads, VOUT-FINAL is
the target output voltage,VOUT-INIT is
A major benefit of the LT3468 and
LT3468-1 is the complete integration
of output voltage detection inside the
part. The output voltage is sensed via
the flyback transformer as described
in the operation section above. The
2
2
COUT •  VOUT(FINAL) – VOUT(INIT) 


,
T=
0.55 • VIN
Table 2. Pre-designed transformers and typical specifications (unless otherwise noted)
For Use With
Transformer
Name
Size (mm)
(W × L × H)
LT3468
LT3468-1
SBL-5.6-1
SBL-5.6S-1
5.6 × 8.5 × 4.0
5.6 × 8.5 × 3.0
LT3468
LT3468-1
LT3468/LT3468-1
LT3468-1
LDT565630T-001 5.8 × 5.8 × 3.0
LDT565630T-002 5.8 × 5.8 × 3.0
T-15-089
T-15-083
6.4 × 7.7 × 4.0
8.0 × 8.9 × 2.0
Linear Technology Magazine • December 2003
LPRILPRI Leakage
(µH)
(nH)
N
RPRI
(mΩ)
RSEC
(Ω)
103
305
26
55
10
24
200 Max
400 Max
10.2
10.2
6
14.5
200 Max
500 Max
10.4
10.2
12
20
100 Max 10 Max
240 Max 16.5 Max
400 Max 10.2 211 Max
500 Max 10.26 75 Max
27 Max
35 Max
Vendor
Kijima Musen
Hong Kong Office
852-2489-8266 (ph)
[email protected]
(email)
TDK
Chicago Sales Office
(847) 803-6100 (ph)
www.components.tdk.com
Tokyo Coil Engineering
Japan Office
0426-56-6336(ph)
www.tokyo-coil.co.jp
5
DESIGN FEATURES
Table 3a. Performance comparison of LT3468 and two
microprocessor-controlled photoflash charging units from actual digital cameras
LT3468
µP-Controlled
Flyback #1
µP-Controlled
Flyback #2
Charge Time (seconds)
(VIN = 3V, VOUT charged
from 50V to 320V, 120µF
photoflash capacitor)
6.3
13.6
7.5
Average Input Current (mA)
500
430
750
Comparison of the LT3468
and LT3468-1 to Discrete
Photoflash Chargers
Table 3b. Normalized performance comparison of LT3468 and two
microprocessor-controlled photoflash charging units from actual digital cameras
Normalized Charge Time
(seconds)
(VIN = 3V, VOUT charged
from 50V to 320V, 120µF
photoflash capacitor)
LT3468
µP-Controlled
Flyback #1
µP-Controlled
Flyback #2
6.3
11.7
11.2
Average Input Current (mA)
Normalized to 500mA
500
output voltage is thus set by the turns
ratio, N, of the transformer. Choose N
with the following equation:
N = (VOUT + 2)/31.5, where VOUT is
the desired output voltage.
Because most of the output detection circuitry, other than the
transformer, is integrated inside the
IC, the accuracy of the output detection
can be very good. The 31.5V comparator voltage is precision trimmed and
is specified at ±1.6% over the full
operating temperature range. To find
the worst case deviation on the output
voltage, simply add this deviation to
the worst case deviation in the turns
manufacturers to produce transformer
designs that are optimized for the
LT3468 and LT3468-1. In most applications, these transformers, shown
in Table 2, will suffice. Of particular
interest are the ultra small transformers now available—as small as 5.8mm
× 5.8mm × 3.0mm—which still achieve
excellent efficiency and charge time.
ratio N of the transformer. Typical
guaranteed deviations of N are in the
2%–3% range, although there is likely
much room for improvement here.
Consult your transformer vendor for
more information. Figure 5 shows a
histogram of the VOUT distribution
for a sample (~100 units) of LT3468
prototype boards. As you can see,
the distribution is tight in a range of
±5V, which is equivalent to a tolerance
under ±1.5%
Pre-Designed Transformers
Linear Technology Corporation has
worked with several transformer
There are numerous benefits to using
the LT3468 series of parts—best seen
when the LT3468 series is compared to
the current method used by many digital camera manufacturers. Figure 6
shows a typical microprocessor-controlled flyback photoflash capacitor
charger. Due to cost and microprocessor limitations, no sensing of primary
current is done. In this case, only the
output voltage is sensed in order to
halt charging at the appropriate time.
The microprocessor must control the
gate of the NFET with appropriate ON
and OFF times. The OFF times must
be large enough so that the current in
the primary of the transformer always
stays in control. Since no direct sensing of the current is used, the OFF
time must be conservative so that the
flux in the transformer is always reset
to zero each cycle. Thus, the flyback
converter is operated heavily in the
discontinuous mode region. This has
several unwelcome consequences,
including high peak currents in the
primary of the transformer and the
discrete NFET. The high peak currents
are difficult to filter out and cause voltcontinued on page 16
30
25
5V
VBAT
UNITS
20
15
µP
COMPLEX
PHOTOFLASH
CONTROL
CODE
10
5
0
310
313
316
319 322
VOUT (V)
325
GATE
DRIVER
COUT
A–D INPUT PORT
328
Figure 5. Output voltage histogram
of ~100 LT3468 prototype boards.
6
PWM
OUTPUT
PORT
Figure 6. Typical microprocessor-controlled flyback photoflash capacitor charger. Due to cost
and microprocessor limitations, no sensing of primary current is done—only the output voltage is
sensed in order to halt charging at the appropriate time.
Linear Technology Magazine • December 2003
DESIGN FEATURES
Noise Sensitivity
In any supervisory application, supply noise riding on the monitored DC
voltage can cause spurious resets, particularly when the monitored voltage
approaches the reset threshold. One
common mitigation technique is to add
hysteresis to the input comparator,
but this has drawbacks. The amount
of added hysteresis, usually specified
as a percentage of the trip threshold,
effectively degrades the advertised accuracy of the part. The LTC2903 does
not use hysteresis.
To minimize spurious resets while
maintaining threshold accuracy, the
LTC2903 employs two forms of noise
filtering. The first line of defense
incorporates proprietary tailoring of
the comparator transient response.
Transient events receive electronic
integration in the comparator and
must exceed a certain magnitude
and duration to cause the comparator to switch.
LT3468, continued from page 6
age dips on the supply powering the
converter. In the end, the efficiency of
the converter suffers which leads to
longer charge times.
To illustrate this, two mid-range
digital cameras from an industryleading company are analyzed.
Both camera photoflash units use
a microprocessor controlled flyback
converter. The first microprocessor
controlled circuit is simple while the
second uses numerous external components to implement a more complex
control scheme. Table 3a shows a comparison of the performance parameters
between the LT3468 circuit and the
microprocessor-based circuits. More
telling, though, is Table 3b, which
16
400
TYPICAL TRANSIENT DURATION (µs)
tion before the reset line falls. In our
5V example, using the 1.5% accurate
supervisor, the system ICs must work
down to 4.35V. System ICs working
with a sloppier ±2.5% accurate supervisor must operate down to 4.25V,
increasing the required system voltage
margin, and the likelihood of system
malfunction.
350
300
250
200
RESET OCCURS
ABOVE CURVE
150
100
50
0
1
10
0.1
100
RESET COMPARATOR OVERDRIVE VOLTAGE (% OF VRT)
Figure 8. Typical transient duration vs
overdrive required to trip comparator
Figure 8 illustrates the typical
transient duration versus comparator overdrive (as a percentage of the
trip threshold) required to trip the
comparators. Once any comparator
is switched, the reset line pulls low.
The reset time-out counter starts once
all inputs return above threshold,
and the nominal reset delay time is
200 milliseconds. The counter clears
whenever any input drops back below
its threshold. This reset delay time effectively provides further filtering of
the voltage inputs and is the second
line of defense against noise. A noisy
input with frequency components of
sufficient magnitude above f = 1/tRST
= 5Hz holds the reset line low, preventing oscillatory behavior on the
reset line.
makes the same comparison, but
normalizes the input current.
The performance benefits of the
LT3468 are obvious as shown in the
nearly 44% reduction in charge time
when compared to the microprocessor-based solutions. In addition to the
charge time reduction, the LT3468
solution requires fewer, and smaller,
components thus significantly reducing the overall size of the circuit.
Conclusion
The LT3468 and LT3468-1 provide a
simple and efficient means to charge
photoflash capacitors. The high levels
of integration inside the parts result
in tight output voltage distributions,
A reset line holding low provides a
remarkably good indication of power
supply problems. Common supply
problems include improperly set
output voltage and/or poor supply
regulation.
Although all four comparators have
built-in glitch filtering, use a bypass
capacitor on the V1 and V2 inputs because the greater of V1 or V2 provides
the VCC for the part (a 0.1µF ceramic
capacitor satisfies most applications).
Apply filter capacitors on the V3 and
V4 inputs if supply noise overcomes
the built in filtering.
Conclusion
The LTC2903 quad supply monitor
greatly improves system reliability by
eliminating false resets and maintaining very high accuracy. Its proprietary
reset pull-down circuit solves the long
standing low voltage POR problem.
The reset output can now maintain
a logic-low at power-supply voltages
down to zero volts. The reset output
is guaranteed to sink at least 5µA
(VOL = 0.15V) for V1, V2 or V3 down
to 0.5V. The LTC2903 monitors four
voltages with 1.5% accuracy (over
the entire temperature range) using comparators with built-in noise
rejection. Non-standard voltages can
be monitored with the 0.5V threshold
adjustable input.
small solution size, lower total solution cost and minimal microprocessor
software overhead. When compared
to traditional methods, charge times
can be lowered by more than 44%.
The LT3468 family offers a range of
input currents for flexibility in the
trade-off between input current and
charge time.
for
the latest information
on LTC products,
visit
www.linear.com
Linear Technology Magazine • December 2003
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