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PDF UM1660DA Data sheet ( Hoja de datos )
| Número de pieza | UM1660DA | |
| Descripción | Low Power DC/DC Boost Converter | |
| Fabricantes | Union Semiconductor | |
| Logotipo |  |
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UM1660
Low Power DC/DC Boost Converter
UM1660S SOT23-5
UM1660DA DFN6 2.0×2.0
General Description
The UM1660 is a PFM controlled step-up DC-DC converter with a switching frequency up to
1MHz. The device is ideal to generate output voltage for small to medium LCD bias supplies and
white LED backlight supplies from a single cell Li-Ion battery. The part can also be used to
generate standard 3.3V/5V to 12V power conversions.
With a high switching frequency of 1MHz, a low profile and small board area solution can be
achieved using a chip coil and an ultra small ceramic output capacitor. The UM1660 has an
internal 400mA switch current limit, offering lower output voltage ripple. The low quiescent
current (typically 36µA) together with an optimized control scheme, allows device operation at
very high efficiencies over the entire load current range.
Applications
Features
LCD Bias Supply
White LED Supply for LCD Backlights
Digital Still Camera
PDAs, Organizers and Handheld PCs
Cellular Phones
Standard 3.3V/5V to 12V Conversion
2.0V to 6.0V Input Voltage Range
Adjustable Output Voltage up to 28V
400mA Internal Switch Current
Up to 1MHz Switching Frequency
36µA Typical No Load Quiescent Current
1µA Maximum Shutdown Current
Internal Soft-Start
Available in Tiny SOT23-5 and DFN6
2.0×2.0 Packages
Pin Configurations
Top View
(Top View)
54
SW 1
5 VIN
PHO
GND 2
FB 3
4 EN
(Top View)
VIN 1
GND 2
EN 3
6 SW
5 NC
4 FB
123
M: Month Code
UM1660S
SOT23-5
AAG
Marking Pin1
M: Month Code
UM1660DA
DFN6 2.0×2.0
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http://www.union-ic.com Rev.05 Feb.2014
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1 page  
UM1660
Operation
The UM1660 features a constant off-time control scheme. Operation can be best understood by
referring to the function block diagram. The converter monitors the output voltage, and as soon as
the feedback voltage falls below the reference voltage of typically 1.233V, the internal switch
turns on and the current ramps up. The switch turns off as soon as the inductor current reaches the
internally set peak current of typically 400mA. The second criteria that turns off the switch is the
maximum on-time of 6µs (typical). This is just to limit the maximum on-time of the converter to
cover for extreme conditions. As the switch is turned off the external Schottky diode is forward
biased delivering the current to the output. The switch remains off for a minimum of 400ns
(typical), or until the feedback voltage drops below the reference voltage again. Using this PFM
peak current control scheme the converter operates in discontinuous conduction mode (DCM)
where the switching frequency depends on the output current, which results in very high
efficiency over the entire load current range.
Peak Current Control
The internal switch turns on until the inductor current reaches the typical dc current limit (ILIM) of
400mA. There is approximately a 100ns delay from the time the current limit is reached and when
the internal logic actually turns off the switch. During this 100ns delay, the peak inductor current
will increase. This increase demands a larger saturation current rating for the inductor. This
saturation current can be approximated by the following equation:
I peak(typ)
I LIM
Vin
L
100 ns
The higher the input voltage and the lower the inductor value, the greater the peak current.
Soft-Start
All inductive step-up converters exhibit high inrush current during start-up if no special
precaution is made. This can cause voltage drops at the input rail during start up and may result in
an unwanted or early system shut down. The UM1660 limits this inrush current by increasing the
current limit in two steps from ILIM/4 for 256 cycles to ILIM/2 for the next 256 cycles, and then full
current limit.
Enable
Pulling the enable pin (EN) to ground shuts down the device reducing the shutdown current to
1µA (typical). Since there is a conductive path from the input to the output through the inductor
and Schottky diode, the output voltage is equal to the input voltage during shutdown. The enable
pin needs to be terminated and should not be left floating. Using a small external transistor
disconnects the input from the output during shutdown as shown in the figure below.
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http://www.union-ic.com Rev.05 Feb.2014
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5 Page 
UM1660
Last, the selected inductor should have a saturation current that meets the maximum peak current
of the converter (as calculated in the peak current control section). Use the maximum value for
ILIM for this calculation.
Another important inductor parameter is the dc resistance. The lower the dc resistance, the higher
the efficiency of the converter.
Setting the Output Voltage
The output voltage is calculated as:
Vout 1.233V (1 R1)
R2
For battery powered applications a high impedance voltage divider should be used with a typical
value for R2 of 200kΩ and a maximum value for R1 of 2.2MΩ. Smaller values might be used to
reduce the noise sensitivity of the feedback pin.
A feedforward capacitor across the upper feedback resistor R1 is required to provide sufficient
overdrive for the error comparator.
The lower the switching frequency of the converter, the larger the feedforward capacitor value
required. A good starting point is to use a 10pF feedforward capacitor. As a first estimation, the
required value for the feedforward capacitor at the operation point can also be calculated using the
following formula:
CFF
2
1
fs
R1
20
Where:
R1 = Upper resistor of voltage divider
fS = Switching frequency of the converter at the nominal load current (See previous section for
calculating the switching frequency)
CFF = Choose a value that comes closest to the result of the calculation
The larger the feedforward capacitor the worse the line regulation of the device. Therefore, when
concern for line regulation is paramount, the selected feedforward capacitor should be as small as
possible.
Output Capacitor Selection
The output capacitor limits the output ripple and maintains the output voltage during large load
transitions. Ceramic capacitors with X5R or X7R temperature characteristics are highly
recommended due to their small size, low ESR, and small temperature coefficients. For most
applications, a 1μF ceramic capacitor is sufficient. For some applications a reduction in output
voltage ripple can be achieved by increasing the output capacitor.
Input Capacitor Selection
For good input voltage filtering, low ESR ceramic capacitors are recommended. A 4.7µF ceramic
input capacitor is sufficient for most of the applications. For better input voltage filtering this
value can be increased.
Diode Selection
Schottky diode is a good choice for UM1660 because of its low forward voltage drop and fast
reverse recovery. Using Schottky diode can get better efficiency. The current rating of the diode
should meet the peak current rating of the converter as it is calculated in the peak current control
section. Use the maximum value for ILIM for this calculation.
________________________________________________________________________
http://www.union-ic.com Rev.05 Feb.2014
11/15
11 Page | ||
| Páginas | Total 15 Páginas | |
| PDF Descargar | [ Datasheet UM1660DA.PDF ] | |
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