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PDF VA7920 Data sheet ( Hoja de datos )

Número de pieza VA7920
Descripción HIGH EFFICIENCY DC-DC STEP UP CONVERT
Fabricantes Vimicro 
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No Preview Available ! VA7920 Hoja de datos, Descripción, Manual

VA7910/VA7920
REV 1.1/Feb 2006
HIGH EFFICIENCY DC-DC STEP UP CONVERT
FEATURES
„ 0.9V start up voltage;
„ 0.9V minimum operating voltage;
„ Above 95% efficiency;
„ Low battery voltage detector;
„ Fixed 3.3V and 5V output or adjustable
output from 2V to 5V;
„ Internal synchronous rectifier;
„ Zero shut down current;
„ Little external elements
„ Ultra small TSSOP8, MSOP8 package;
APPLICATION
„ One to three-cell battery powered devices;
„ PDA and handheld instruments;
„ Cell phones;
„ Pagers;
„ GPS
„ Digital cameras;
INTRODUCTION
VA7910/VA7920 series circuits are high
efficiency DC-DC step up converts, with a few
external components to realize the conversion
from the battery voltage to the expect output
voltage. It can be widely used in PDA cell
phoneshand-held devices and so on.
The start up voltage is guaranteed at above
0.9V input and the device keeps working.
With an internal synchronous P-MOS rectifier,
it doesn’t require external diode to rectify.
VA7910FF is completely compatible with
L6920 of ST Corporation. And VA7910DF is
completely compatible with MAX1674/MAX1675
of MAXIM Corporaton.
FUNCTIONAL DIAGRAM
FB
VREF
Zero crossing
comparator
-
Multi-way
selector
+ -+
Voltage
com parator
-
Logic
controller
+
Reference
Voltage
comparator
LBI
Current
limit
circuit
LBO
SHDN
Fig1 Function Diagram
OUT
LX
GND
www.vimicro.com
© 2003 Vimicro Corporation
-1-
DSC-VA7910/VA7920 -1

1 page




VA7920 pdf
VA7910/VA7920
Fig4 VA7920 typical application diagram
1OPERATION PRINCIPLE
Through controlling the internal N-MOS active
switch and P-MOS synchronous rectifier on/off
with the external inductor (L1) and output
capacitor (C3), VA7910/VA7920 can realize the
step-up conversion function.
After the circuit comes into steady state, the
internal active N-MOS switch will conduct when
the output voltage drops below the expected
value and the P-MOS synchronous rectifier turns
off, then the external capacitor supply to the load
and at the same time, the energy transfers from
the battery to the inductor. When the current in
the inductor increases above the active switch
current limit or the transfer time exceed the
active switch maximum On-Time, the N-MOS will
turn off.
The internal P-MOS conducts simultaneously
with the N-MOS turning off, so the current of the
inductor will drop to induce faradism, then the
energy of the faradism and the battery transfer to
the capacitor and supply the load together.
Because the added faradism, the circuit realize
the voltage step-up function. In fact, the
synchronous rectifier minimum On-Time is equal
to the active switch minimum Off-Time (tOFFMIN).
after this time, the P-MOS synchronous rectifier
turns off as soon as the output voltage goes
lower than the expected value or the current
flowing of the inductor goes down to zero.
Notes that, the N-MOS active switch may not
conduct as soon as the internal P-MOS
synchronous rectifier turns off unless the output
voltage also drops below the expected value. So,
the active switch and synchronous rectifier turns
off simultaneously is possible.
When the load of the device is lighterthe
current flowing in the inductor L1 rises and falls
smoothly, the device actually works in PFM
mode. With the increasing of the current of the
load, the current in the inductor are overlapped in
the course of rising and smooth in the course of
falling. The current flowing in the inductor all
goes down to zero under these two conditions,
and the internal active switch and synchronous
rectifier turns off simultaneously possibly, the
device works in discontinuous mode. When the
current increases to induce that the inductor
current can’t drop to zero, the stored energy of
the inductor during the period of the active switch
maximum On-Time is equal to the transferred
energy during the period of the synchronous
rectifier minimum On-Time. At this time, the
active switch and synchronous rectifier becomes
on and off by turns, the device works in
continuous mode. Works in this mode, the larger
the current of the load is, the nearer the ripple
current of the inductor closes to the active switch
current limit. The load current limit is defined by
the following relationship:
ILOAD-LIM=
VIN
VOUT
×(ILIM-TOFFMIN×
VOUT-VIN )×η........................ 1
2L
Whereη is the efficiency.
If the load current rises above the limit
ILOAD-LIM, the device output voltage will be
variable to induce a failing step-up conversion.
2START-UP
One of the key features of VA7910/VA7920 is
the startup at the supply voltage down to 1V
(The star-up voltage relates to the load)
-5-

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