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CD43-100 Schematic ( PDF Datasheet ) - Linear Technology

Teilenummer CD43-100
Beschreibung Dual Micropower DC/DC Converter with Positive and Negative Outputs
Hersteller Linear Technology
Logo Linear Technology Logo 




Gesamt 8 Seiten
CD43-100 Datasheet, Funktion
FEATURES
s Generates Well-Regulated Positive and
Negative Outputs
s Low Quiescent Current:
20µA in Active Mode (per Converter)
<1µA in Shutdown Mode
s Operates with VIN as Low as 1.2V
s Low VCESAT Switch: 250mV at 300mA
s Uses Small Surface Mount Components
s High Output Voltage: Up to ±34V
s Tiny 10-Pin MSOP Package
U
APPLICATIO S
s Small TFT LCD Panels
s Handheld Computers
s Battery Backup
s Digital Cameras
LT1945
Dual Micropower DC/DC
Converter with Positive and
Negative Outputs
DESCRIPTIO
The LT®1945 is a dual micropower DC/DC converter in a
10-pin MSOP package. Each converter is designed with a
350mA current limit and an input voltage range of 1.2V to
15V, making the LT1945 ideal for a wide variety of appli-
cations. Both converters feature a quiescent current of
only 20µA at no load, which further reduces to 0.5µA in
shutdown. A current limited, fixed off-time control scheme
conserves operating current, resulting in high efficiency
over a broad range of load current. The 36V switch allows
high voltage outputs up to ±34V to be easily generated
without the use of costly transformers. The LT1945’s low
off-time of 400ns permits the use of tiny, low profile
inductors and capacitors to minimize footprint and cost in
space-conscious portable applications.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
VIN
2.7V
TO 5V
C1
4.7µF
Dual Output (+12V, –20V) Converter
L1
10µH
C4
0.1µF
D1
8 10
VIN
2
SHDN1
SW1
1
NFB1
LT1945
4
SHDN2
5
FB2
GND PGND PGND SW2
37 96
D2
100pF 365k
24.9k
–20V
10mA
C2
1µF
115k
L2
10µH
C1: TAIYO YUDEN JMK212BJ475
C2, C3: TAIYO YUDEN TMK316BJ105
C4: TAIYO YUDEN EMK107BJ104
D1, D2, D3: ZETEX ZHCS400
L1, L2: MURATA LQH3C100
4.7pF
D3
C3
1M 1µF
12V
20mA
1945 TA01
Efficiency at VIN = 3.6V
90
85
+12V OUTPUT
80
–20V OUTPUT
75
70
65
60
55
50
0.1
1 10
LOAD CURRENT (mA)
100
1945 TA01a
1945f
1






CD43-100 Datasheet, Funktion
LT1945
APPLICATIO S I FOR ATIO
For higher output voltages, the formula above will give
large inductance values. For a 2V to 20V converter (typical
LCD bias application), a 47µH inductor is called for with
the above equation, but a 10µH or 22µH inductor could be
used without excessive reduction in maximum output
current.
Inductor Selection—Inverting Charge Pump Regulator
For the inverting regulator, the voltage seen by the internal
power switch is equal to the sum of the absolute value of
the input and output voltages, so that generating high
output voltages from a high input voltage source will often
exceed the 36V maximum switch rating. For instance, a
12V to – 30V converter using the inverting topology would
generate 42V on the SW pin, exceeding its maximum
rating. For this application, an inverting charge pump is
the best topology.
The formula below calculates the approximate inductor
value to be used for an inverting charge pump regulator
using the LT1945. As for the boost inductor selection, a
larger or smaller value can be used. For designs with
varying VIN such as battery powered applications, use the
minimum VIN value in the equation below.
( )VOUT VIN MIN + VD
L = ILIM tOFF
Current Limit Overshoot
For the constant off-time control scheme of the LT1945,
the power switch is turned off only after the 350mA current
limit is reached. There is a 100ns delay between the time
when the current limit is reached and when the switch
actually turns off. During this delay, the inductor current
exceeds the current limit by a small amount. The peak
inductor current can be calculated by:
IPEAK
=
ILIM
+

VIN(MAX)
L
VSAT
100ns
Where VSAT = 0.25V (switch saturation voltage). The
current overshoot will be most evident for regulators with
high input voltages and smaller inductor values. This
overshoot can be beneficial as it helps increase the amount
of available output current for smaller inductor values.
This will be the peak current seen by the inductor (and the
diode) during normal operation. For designs using small
inductance values (especially at input voltages greater
than 5V), the current limit overshoot can be quite high.
Although it is internally current limited to 350mA, the
power switch of the LT1945 can handle larger currents
without problem, but the overall efficiency will suffer. Best
results will be obtained when IPEAK is kept below 700mA
for the LT1945.
Capacitor Selection
Low ESR (Equivalent Series Resistance) capacitors should
be used at the output to minimize the output ripple voltage.
X5R or X7R multilayer ceramic capacitors are the best
choice, as they have a very low ESR and are available in
very small packages. Y5V ceramics are not recommended.
Their small size makes them a good companion to the
LT1945’s MS10 package. Solid tantalum capacitors (like
the AVX TPS, Sprague 593D families) or OS-CON capaci-
tors can be used, but they will occupy more board area
than a ceramic and will have a higher ESR. Always use a
capacitor with a sufficient voltage rating.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as close as
possible to the LT1945. A 4.7µF input capacitor is suffi-
cient for most applications. Table 2 shows a list of several
capacitor manufacturers. Consult the manufacturers for
more detailed information and for their entire selection of
related parts.
Table 2. Recommended Capacitors
CAPACITOR TYPE
VENDOR
Ceramic
Taiyo Yuden
(408) 573-4150
www.t-yuden.com
Ceramic
AVX
(803) 448-9411
www.avxcorp.com
Ceramic
Murata
(714) 852-2001
www.murata.com
1945f
6

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