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SI9122 датащи(PDF) 11 Page - Vishay Siliconix

номер детали SI9122
подробное описание детали  500-kHz Half-Bridge DC/DC Controller with Integrated Secondary Synchronous Rectification Drivers
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производитель  VISHAY [Vishay Siliconix]
домашняя страница  http://www.vishay.com
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SI9122 датащи(HTML) 11 Page - Vishay Siliconix

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Document Number: 71815
S-80038-Rev. J, 14-Jan-08
www.vishay.com
11
Vishay Siliconix
Si9122
conditions the oscillator frequency is reduced by the current
overload protection to enable a constant current to be
maintained into a low impedance circuit.
Current Limit
Current mode control providing constant current operation is
achieved by monitoring the differential voltage VCS between
the CS1 and CS2 pins, which are connected to a current
sense resistor on the primary low-side MOSFET. In the
absence of an overcurrent condition, VCS is less than lower
current limit threshold VTLCL (typical 100 mV); CL_CONT is
pulled up linearly via the 120 µA current source (IPU) and
both DL and DH switch at half the oscillator set frequency.
When a moderate overcurrent condition occurs (VTLCL < VCS
< VTHCL), the CL_CONT capacitor will be discharged at a rate
that is proportional to VCS - 100 mV by the IPD current
source. Both driver outputs are in frequency fold-back mode
and the switching frequency becomes roughly 20 % of
normal switching frequency. When a severe overcurrent
condition occurs (VTHCL < VCS), the NMOS discharges
CL_CONT capacitor immediately at 2 mA rate and the
CL_CONT voltage will be clamped to 1.2 V disabling both DL
and DH outputs.
Before VCS reaches severe overcurrent condition, a lowering
of the CL_CONT voltage results in PWM control of the output
drive being taken over by the current limit control loop
through CL_CONT. Current control initially reduces the
switching duty cycle toward the minimum the chip can reach
(DMIN). If this duty cycle reduction still cannot lower the load
current, then the switching frequency will start to fold back to
minimum 1/5 of the nominal frequency. This prevents the
on-time of the primary drivers from being reduced to below
100 ns and avoids current tails. If VCS > VTHCL, the switching
will then stop.
With constant current mode control and frequency foldback,
protection of the MOSFET switches is increased. The
converter reverts to voltage mode operation immediately
when the primary current falls below the limit level, and
CL_CONT capacitor is charged up and clamped to 6.5 V. The
soft-start function does not apply during current limit period,
as this would constitute hiccup mode operation.
VIN Voltage Monitor - VINDET
The chip provides a means of sensing the voltage of VIN, and
withholding operation of the output drivers until a minimum
voltage of VREF (3.3 V, 300 mV hysteresis), is achieved. This
is achieved by choosing an appropriate resistive tap between
the ground and VIN, and comparing this voltage with the
reference voltage. When the applied voltage is greater than
VREF, the output drivers are activated as normal. VINDET also
provides the input to the voltage feed forward function.
However, if the divided voltage applied to the VINDET pin is
greater than VCC - 0.3 V, the high-side driver, DH, will stop
switching until the voltage drops below VCC - 0.3 V. Thus, the
resistive tap on the VIN divider must be set to accommodate
the normal VCC operating voltage to avoid this condition.
Alternatively, a zener clamp diode from VINDET to GND may
also be used.
Shutdown Mode
If VINDET is forced below the lower threshold, a minimum of
350 mV (VSD), the device will enter SHUTDOWN mode. This
powers down all unnecessary functions of the controller,
ensures that the primary switches are off and results in a low
level current demand from the VIN or VCC supplies.
Figure 5. High-Voltage Pre-Regulator Circuit
CVCC
0.5 µF
CEXT
2 nF
GND
VCC
PNP Ext
REG_COMP
VREF
VIN
VINEXT
HVDMOS
14.5 V
REXT
(Si9122)
Auxillary
VCC
Figure 6 . Current Limit Circuit
-
+
GM
CEXT
+
-
AV
AV
150 mV
-
+
GM
Peak Detect
Blank
CS1
CS2
IPD
0 to 240 µA (nom)
IPU
120 µA (nom)
AV
VOFFSET
OSC
CL_CLAMP
REXT
CL_CONT
AV
100 mV
VCC


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