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VA7910 Schematic ( PDF Datasheet ) - Vimicro

Teilenummer VA7910
Beschreibung HIGH EFFICIENCY DC-DC STEP UP CONVERT
Hersteller Vimicro
Logo Vimicro Logo 




Gesamt 9 Seiten
VA7910 Datasheet, Funktion
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
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© 2003 Vimicro Corporation
-1-
DSC-VA7910/VA7920 -1






VA7910 Datasheet, Funktion
VA7910/7920 will change the startup mode
into operation mode as soon as the output
voltage goes over 1.8V. During the startup, the
internal P-MOS synchronous rectifier is off and
the energy transferred to the capacitor (C3) and
the load from the battery and inductor (L1)
through its intrinsic body diode.
During startup, the internal N-MOS active
switch turns on/off at a given frequency and duty.
The active switch with a very low RDSON thanks to
the internal charge pump used to power the
MOS gate. The energy transferred to inductor L1
from the battery when the active switch is on,
and when it is off, the energy transferred to the
capacitor C3 and load from the battery and L1.
Current limit and zero crossing detection are
still available during the startup. If the current in
inductor is detected over the active switch
current limit, the active switch turns off at once;
otherwise, if the current in the inductor drops to
zero, the active switch turns on at once.
VA7910/VA7920
3SHUT DOWN
VA7910/VA7920 includes a shutdown control
pinSHDN pin.In shutdown mode (the pin
pulled low), all the internal circuits are turned off,
ultra low leakage current delivered to the battery
(The typical value is 100nA).
Notes that, in shutdown mode, the P-MOS
synchronous rectifier body diode causes a
parasitic path between the power supply and
output pin. And at that moment, the battery
current can’t be avoided if the output connected
with a load.
4LOW BATTERY DETECTION
VA7910 also includes low battery detector
comparator, comparing the LBI voltage and the
internal reference voltage. There is a 3% internal
hysteresis to avoid the oscillation in LBO.
LBO is an open drain output, so a pull up
resistor is required for a proper use.
APPLICATION ISSUES
1OUTPUT VOLATGE SELECTION
Output voltage is selected depending on FB
pin. There are three connecting choices
available for FB.
aFB connected to GND The output voltage
is an invariable value of 5V. Please refer to the
typical application as shown in fig 3 and fig 4.
Connect 2 and 3 of J with a shunt. R4 and R5 is
not used.
bFB connected to OUT pin The output
voltage is an invariable value of 3.3V. Please
refer to the typical application as shown in fig 3
and fig 4. Connect 1 and 2 of J with a shunt. R4
and R5 also is not used.
cFB connected to an external resistor divider
The output voltage is adjustable from 2V to 5V.
Please refer to the typical application as shown
in fig 3 and fig 4. Let R4 be connected between 1
and 2 and R5 be connected between 2 and 3 of J.
And the output voltage is defined by the following
relationship:
VOUT=1.15V1+R4/R5.................... 2
R4 and R5 should be selected as larger as
possible to minimize power consumption to
improve the efficiency. But due to current sunk by
FB pin to affect the calculation precision, R4 and
R5 should selected between 150 kΩ~4MΩ, thus
the current in R4 and R5 is between 1uA and
5uA.
2LOW BATTERY DETECTOR
Low battery detection is realized by
comparing the voltage between LBI pin and the
reference (VREF=1.15V). We can use external
resistor divider to realize the battery detection
which above the 1.15V reference voltage (as
shown in Fig 3 and Fig 4)
Supposing VTRIP as the to be detected voltage,
we can defined the relationship with R1 and R2
as following formula:
VTRIP=1.15V1+R1/R2.................... 3
Based on the same consideration of R4 and
R5 selection, R1 and R2 should be selected
between 150 kΩ~4MΩ.
3OUTPUT CAPACITOR SELECTION
The output capacitor affects the efficiency and
output ripple directly, so its choice has to be
considered particularly carefully.
Output capacitor should be selected between
10uF~100uF. In order to get higher efficiency and
smaller output ripple, a very low ESR capacitor
should be chosen. Ceramic capacitors are the
lowest ESR capacitor but they are expensive.
Another possible choice is low ESR tantalum
capacitor.
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