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SI9176 Datasheet(Fiches technique) 6 Page - Vishay Siliconix

Numéro de pièce SI9176
Description  High Performance Step-Down DC-DC Converter With Adjustable Output Voltage
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Fabricant  VISHAY [Vishay Siliconix]
Site Internet  http://www.vishay.com
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Si9176
Vishay Siliconix
New Product
www.vishay.com
6
Document Number: 71753
S-21825—Rev. A, 07-Oct-02
The error amplifier and comparator control the duty cycle of the
PMOS MOSFET to continuously force the REF pin and FB pin
voltages to be equal. As the input-to-output voltage difference
drops, the duty cycle of the PMOS MOSFET can reach 100%
to allow system designers to extract the maximum stored
energy from the battery. The dropout voltage is 190 mV at
600 mA.
During each cycle, the PMOS switch current is limited to a
maximum of 1.5 A (typical) thereby protecting the IC while
continuing to force maximum current into the load.
Pulse Skipping Mode (MODE pin = OPEN)
By leaving the MODE pin open-circuit, the converter runs in
pulse skipping mode (PSM). In PSM mode the oscillator
continues to operate, but switching only occurs if the FB pin
voltage is below the REF voltage at the start of each clock
cycle.
Clock cycles are skipped thereby reducing the
switching frequency to well below 100 kHz and minimizing
switching losses for improved efficiency at loads under
150 mA. Although PSM mode switching frequency varies with
line and load conditions, the minimum PSM frequency will be
kept above 20 kHz for load currents of 30 mA or more to
prevent switching noise from reaching the audio frequency
range.
Each time the PMOS switch is turned on, the inductor current
is allowed to reach 300 mA. Once achieved, the PMOS switch
is turned off and the NMOS switch is turned on in the normal
manner. However, unlike PWM mode, the NMOS switch, turns
off as the switch current approaches zero current to maximize
efficiency. The PMOS switch remains on continuously (100%
duty
cycle)
when
the
input-voltage-to-output-voltage
difference is low enabling maximum possible energy
extraction from the battery.
PSM mode is recommend for load currents of 150 mA or less.
Auto Mode
When the MODE pin grounded, the converter is set to Auto
mode. Switching between PWM mode and PSM modes takes
place automatically without an external control signal. For
heavy load operation, the converter will operate in PWM mode
to achieve maximum efficiency. When delivering light load
currents, the converter operates in PSM mode to conserve
power. The switchover threshold between the two modes is
determined by the peak inductor current, which is 300 mA
nominal. There is hysteresis in the switchover threshold to
provide smooth operation. Thus, the mode PSM-to-PWM
mode switchover current for increasing load currents is higher
than that of PWM-to-PSM mode switchover for decreasing
load currents.
Oscillator
The internal oscillator provides for a fixed 2-MHz switching
frequency.
Dynamic Output Voltage Control (REF)
The Si9176 is designed with an adjustable output voltage
which has a change of VFB to VIN -- VDROP.VOUT is defined
according to the following relationship:
VOUT =
1+R1
R2
×V
FB
Converter Shutdown (SD pin)
With logic LOW level on the SD pin, the Si9176 is shutdown.
Shutdown reduces current consumption to less than 2-mAby
shutting off all of the internal circuits. Both the PMOS and
NMOS transistors are turned off. A logic HIGH enables the IC
to start up as described in “Start-up” section.
Thermal Shutdown
The Si9176 includes thermal shutdown circuitry, which turns
off the regulator when the junction temperature exceeds
165_C. Once the junction temperature drops below 145_C,
the regulator is enabled. If the condition causing the over
temperature, the Si9176 begins thermal cycling, turning the
regulator on and off in response to junction temperature.
Restart from a thermal shutdown condition is the same as
described in the “Start-up” section.




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