VA7920.pdf 데이터시트 (총 9 페이지) - 파일 다운로드 VA7920 데이타시트 다운로드

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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
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DSC-VA7910/VA7920 -1

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ORDERING INFORMATION
Type
VA7910FF
VA7910DF
Package
TSSOP8
MSOP8
VA7910/VA7920
Pin Number
8
8
Print Mark
TBA
TBA
PIN-OUT
FB 1 VA7910FF 8 OUT
LBI 2 VA7910DF 7 LX
LBO 3 Top View 6 GND
REF 4 (Not to scale) 5 SHDN
TSSOP8
MSOP8
Fig2 Pin-Out Diagram
PIN DESCRIPITION
Pin Name
FB
LBI
LBO
REF
SHDN
GND
LX
OUT
Pin Num
VA7910 VA7920
Pin Function
Output voltage selected pin. Connect to Ground for 5V output;
1 1 Connect to OUT for 3.3V output; Connect to a resistor divider for
adjustable output from 2V to 5V.
2
Low battery detector input. Internal detection threshold voltage is
1.15VVREF.
Low battery detector output. The output goes low when VLBI is lower
3
than the internal threshold voltage, otherwise, the output shown
high impedance. This pin is open drain configuration, it must add an
external pull-up resistor in application.
Reference voltage. Bypass this output to GND with a 0.1μF
4 3 capacitor for filtering high frequency noise.
The maximum current delivered to this pin is 10μA.
Shutdown Pin. The device will shut down when the voltage of this
5 4 pin is below 0.3V; The device is operating when the voltage is
above 0.6V.
6 6 Ground.
7 5 Connect with external inductor.
Power output pin. This pin supplies the power for the internal circuit
8 2 and it must connect to an external about 10uF to 100uF capacitor
for filtering and an about 1uF capacitor for coupling.
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VA7910/VA7920
ABSOLUTE MAXIMUM RATINGS
Supply voltage ( VOUT ) ...........................................0.3V~+6V
FBLBILBOLXSHDN
REF input voltage ..................... 0.3VVOUT0.3V
Operation temperature ( TA ) ..............40℃~+85
Storage temperature............................ 65℃~150
Junction temperature.......................................... 150
Power dissipation PDTA25℃)
TSSOP8Thermal resistanceθJA=TBD/W.... TBD
MSOP8Thermal resistanceθJA=TBD/W...... TBD
Leading temperatureSoldering10s.............. 300
ESDHuman body mode............................................2kV
CautionStresses above those listed parameter may cause permanent damage to the devices. These
are stress ratings only and operation of the device at these or any other conditions above those indicated
in the operational section of this specification is not implied. Exposure to the listed ratings for extended
periods may affect device reliability.
ELECTRICAL CHARACTERIZATION (1)
Inductor input voltage: VIN=2VVFBVOUT, unless otherwise noted. Operation temperature with mark
is:-40℃≤TA85℃;without mark isTA25℃;The typical values are at TA25℃)
PARAMETER
SYMBOL TEST CONDITION
MIN TYP MAX UNIT
Minimum input voltage
Start up voltage
VIN
0.9
0.9 1.3
V
V
Start up voltage tempco
-2 mV/
Output voltage
FB voltage
FB input current
VOUT
VFB
VFB=VOUT
VFB=0
VOUT=2V~5V
3.2 3.3 3.4
4.85 5 5.15
1.13 1.15 1.16
50
V
V
nA
Output voltage range
Connect to external
resistor divider
2
5V
Static output current
IQ VFB=1.4VVOUT=3.3V
Shutdown current into OUT
pin
ISD
VSHDN=0
14.8 16.9 17.4 μA
0.1 1 μA
Reference voltage
Active switch maximum
On-Time
VREF
tONMAX
IREF=0
IREF=10μA
1.13 1.15 1.16
1.125 1.145 1.155
3.3 5 6.9
V
μs
Active switch minimum
Off-Time
tOFFMIN
0.65 1
1.5 μs
Active switch ON
resistance
Synchronous rectifier
switch ON resistance
RDSN
RDSP
VA7910ILX=100mA
VA7920ILX=100mA
VA7910ILX=100mA
VA7920ILX=100mA
0.1 0.2
0.1 0.2
0.15 0.3
0.3 0.4
Ω
Ω
Ω
Ω
Active switch current limit
VA7910
ILIM
VA7920
0.8 1.0 1.2
0.8 1.0 1.2
A
A
LX leakage current
ILXOFF
VOUT5.5V
VLX5.5VVSHDN =0
1 μA
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VA7910/VA7920
ELECTRICAL CHARACTERIZATION (2)
Inductor input voltage: VIN=2VVFBVOUT, unless otherwise noted. Operation temperature with mark
is:-40℃≤TA85℃;without mark is”: TA25℃;The typical values are at TA25℃)
PARAMETER
SYMBOL TEST CONDITION
MIN TYP MAX UNIT
LBI internal detection
threshold
1.13 1.15 1.16 V
LBI input current
50 nA
LBO low output voltage
LBO OFF leakage current
VLBI=0ISINK=1mA
0.2 0.4 V
1 μA
SHDN Input current
1 μA
SHDN LOW input voltage
SHDN High input voltage
VILSHDN
VIHSHDN
0.3 V
0.6 V
DETAILED DESCRIPTION
VA7910/VA7920 is a high efficiency DC-DC
step up convert circuit widely used for the
devices powered by one to three-cells Li-ion/
Li-polymer battery or hydro-nickel battery and
need a steady high voltage.
The static operating current of
VA7910/VA7920 is extra low. With an internal
synchronous rectifier to improve the efficiency,
the device needn’t external diode for rectify that
can reduce the board space and save the cost.
VA7910/VA7920 realizes the high efficiency
step up conversion through controlling the
internal active switch maximum On-Time and
minimum Off-Time and tailoring the inductor
maximum current limit. Fig3 and fig 4 shows the
typical application circuit. Following is the
detailed description.
Fig3 VA7910 typical application diagram
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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)
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