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PDF ZXSC100 Data sheet ( Hoja de datos )
| Número de pieza | ZXSC100 | |
| Descripción | SINGLE CELL DC-DC CONVERTER SOLUTION | |
| Fabricantes | Zetex Semiconductors | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de ZXSC100 (archivo pdf) en la parte inferior de esta página. Total 19 Páginas | ||
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SINGLE CELL DC-DC CONVERTER SOLUTION
ZXSC100
DESCRIPTION
The ZXSC100 series is designed for DC-DC
applications where step-up voltage conversion from
very low input voltages is required. These applications
mainly operate from single nickel cadmium or nickel
metal hydride battery cells.
The ZXSC100 devices are non-synchronous PFM,
DC-DC controller ICs which drive an external
transistor. Zetex SuperSOT4™ switching transistors,
with saturation resistance as low as 13mΩ, are
recommended as the external switching element.
These bipolar transistors are the best switching
devices available for this type of DC-DC conversion,
enabling high efficiency conversion with input
voltages down to below 1 volt.
The circuit can start up under full load with regulation
maintained down to an input voltage of only 0.926
volts. The solution configuration ensures optimum
efficiency over a wider range of load currents, several
circuit configurations are possible with power
dissipation up to 2W. The step up output voltage is
easily programmed with external resistors, the
non-synchronous architecture and SuperSOT4™
device enabling an output voltage down to the input
voltage level. For best performance the ZXSC100
quiescent current is a small 150µA ensuring minimum
battery drain in no load conditions.
For the best in space saving the ZXSC100 is offered in
the MSOP8 package, however the devices are also
available in SO8 packaging for applications where
space saving is not so critical.
The IC and discrete combination offers the ultimate
cost vs performance solution for single cell DC-DC
conversion.
FEATURES
• SuperSOT4™ switching transistor
ZXT14N20DX:VCE(sat) 45mV max @ 1A load
• Efficiency maintained over a wide range of input
voltages and load currents
82% efficiency @ VBATT=1V
• Startup under full load
• Minimum operating input voltage VBATT=0.926V
• Adjustable output voltage down to VBATT
• Quiescent current typically 150µA referred to
input voltage
• MSOP8 Package
• SO8 Package
• Demonstration boards available
APPLICATIONS
• Cordless Telephones
• MP3 Players
• PDA
• Pagers
• Battery Backup Supplies
• Electronic toothbrush
• GPS Receivers
• Digital Camera
• Palmtop Computers
APPLICATIONS(continued)
• Hand Held Instruments
• Portable Medical Equipment
• Solar Powered Equipment
• LED Flashlight
• LED Backlight
TYPICAL APPLICATION CIRCUIT
VBATT
L1
R1
C1
U1
EM
VDRIVE
BAS ISENSE
RE FB
VCC
GND
ZXSC100
D1
ZHCS2000
3.3V/0.1A
Q1 R3
ZXT14N20DX
C3
C2
R2 R4
ISSUE 1 - JANUARY 2001
1
1 page  
ZXSC100
DEVICE DESCRIPTION
The ZXSC100 is non-synchronous PFM, DC-DC
controller IC which, when combined with a high
performance external transistor, enables the
production of a high efficiency boost converter for use
in single cell applications. A block diagram is shown
for the ZXSC100 in Figure 1.
The driver circuit supplies the external switching
transistor with a defined current, which is
programmed by an external resistor connected
between the RE pin and VCC. The internal reference
voltage for the circuit is 25mV below VCC. To maximise
efficiency the external transistor is switched quickly,
typically being forced off within 20ns.
In higher power applications more current can be
supplied to the switching transistor by using a further
external component. The driver transistor in the IC can
be bypassed with the addition of a discrete PNP. More
information on this circuit configuration can be found
in the applications section.
Figure 1
ZXSC100 Block Diagram
A shutdown circuit turns the device on or off at VCC=1V
with a hysteresis of typically 80mV. At start up,
comparator Comp1 turns the driver circuit and
therefore the external switching transistor on. This
circuit will remain active until the feedback voltage at
the pin FB rises above VREF, which is set to 730mV. An
external resistive divider on the FB pin sets the output
voltage level.
Comparator Comp2 forces the driver circuit and the
external switching transistor off, if the voltage at
ISENSE exceeds 25mV. The voltage at ISENSE is taken
from a current sense resistor connected in series with
the emitter of the switching transistor.
A monostable following the output of Comp2 extends
the turn-off time of the output stage by a minimum of
2us. This ensures that there is sufficient time to
discharge the inductor coil before the next on period.
The AND gate between the monostable and Comp1
output ensures that the switching transistor always
remains on until the ISENSE threshold is reached and
that the minimum discharge period is always
asserted. The pulse width is constant, the pulse
frequency varies with the output load.
ISSUE 1 - JANUARY 2001
5
5 Page 
ZXSC100
Inductor selection
The inductor value must be chosen to satisfy
performance, cost and size requirements of the overall
solution. For the reference designs we recommend an
inductor value of 22µH with a core saturation current
rating greater than the converter peak current value.
Inductor selection has a significant impact on the
converter efficiency. For applications where efficiency
is critical, a 5% improvement can be achieved with a
high performance inductor. This should be selected
with a core saturation current rating much higher than
the peak current of the converter, say 3 times greater.
The resultant reduction in core losses brings about the
efficiency improvement.
Peak current definition
The peak current rating is a design parameter whose
value is dependent upon the overall application. For
the reference designs, a peak current of 1.2A was
chosen to ensure that the converter could provide the
required output power.
In general, the IPK 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 IPK is decided the value of RSENSE can be
determined by:
RSENSE =
VISENSE
IPK
Output power definition
By making the above assumptions for the inductor and
IPK the output power can be determined by:
Output
Power
=
(VOUT
2
−
x
VIN) x
(TOn +
IPK x TDIS
TOFF)
where
TON
=
IPK xL
VIN
and
TDIS
=
IPK xL
(VOUT − VIN)
Note: VOUT = output voltage + rectifier diode VF
Figure 3 shows the discontinuous inductor current and
the relationship between output power, TON, TDIS and
TOFF.
Figure 3
Discontinuous inductor current
Output capacitors
Output capacitors are a critical choice in the overall
performance of the solution. They are required to filter
the output and supply load transient currents. There
are three parameters which are paramount in the
selection of the output capacitors; their capacitance
value, IRIPPLE and ESR. The capacitance value is
selected to meet the load transient requirements. The
capacitors IRIPPLE rating must meet or exceed the
current ripple of the solution.
The ESR of the output capacitor can also affect loop
stability and transient performance. The capacitors
selected for the solution, and indicated in the
reference designs, are optimised to provide the best
overall performance.
ISSUE 1 - JANUARY 2001
11
11 Page | ||
| Páginas | Total 19 Páginas | |
| PDF Descargar | [ Datasheet ZXSC100.PDF ] | |
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