SC2442 Просмотр технического описания (PDF) - Semtech Corporation
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SC2442 Datasheet PDF : 10 Pages
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SC2442/H
POWER MANAGEMENT
Application Information
Undervoltage Lockout
Sync and Enable
The undervoltage lockout circuitry monitors the AVCC pin.
During undervoltage lockout all output drives are turned
off and both SS pins are discharged to ground. Typically,
for AVCC increasing, normal operation will not occur until
AVCC reaches 4.6V. For AVCC falling, undervoltage lock-
out will not occur until AVCC falls below 4.5V.
Voltage Regulator
When the SYNC/EN pin is pulled below 1V, all output
drives are turned off and both SS pins are discharged to
ground. When the SYNC/EN pin is pulled high above
1.8V, normal operation occurs. When an external clock
signal is applied that is marginally higher in frequency to
that set by the ROSC resistor, the internal oscillator will
synchronize to this signal. The external signal should have
TTL compatible transition.
Using an external PNP transistor as shown in the Typical
Applications Circuit, the SC2442/H provide a regulated
AVCC supply. The same AVCC with adequate filtering can
be connected to PVCC to provide power for the output
drives. The AVCC is regulated at 7V typical which provides
optimum drive for most low Voltage power MOSFETs. The
BDI pin will provide at least 3 mA to regulate the external
PNP.
For VIN voltages below 8V, the PNP pass transistor will
always be operating in saturation and it is recommended
to connect VIN directly to AVCC and PVCC so long as
maximum input voltage is below 8V. The BDI pin may be
left unconnected. Alternately if an additional supply <8V is
available in addition to VIN, it can be used for separately
powering both the VCCs.
Soft Start and Hiccup Mode
The SC2442/H controllers utilize asynchronous start up
to provide glitch free output rise times. During start up, the
SS1 and SS2 pins are held low and the gate drive signals
are also pulled low. Once AVCC reaches 4.6V and above,
the SS capacitors are charged by a 10 μA internal current
source. The error amplifier outputs are clamped by the re-
spective SSx voltages. As the SSx pin voltage goes above
the oscillator valley voltage of 1.5V, the high-side driver
will begin switching. The low-side driver will not begin
switching until the SSx voltage has reached 3.3V.
Operating Frequency
The operating frequency is set by a resistor from ROSC
pin to AGND. ROSC sets the clock frequency Fc that is
twice the operating frequency of each converter. The
clock frequency is given by
Fc = 33,000 / ROSC
Fc is in kHz and ROSC is in kOhm.
Current Limiting
The SC2442/H provide cycle-by-cycle current limiting by
sensing the current in the input line. A non inductive resistor
should be used for precise current sensing and limiting.
When the voltage drop across a sense resistor in the input
line exceeds 105 mV, the PWM pulse is latched off and is
not reset until the next clock cycle. Overcurrent condition
affects only the high side driver. An RC filter should be
placed across the sense resistor as shown in the Typical
Application Circuit to reduce noise due to turn-on spikes.
The filter capacitor should be connected between VIN and
respective OCx pins for proper filtering. Typically a ceramic
or similar low esr capacitor is placed between the current
sense resistor and the switching circuit. If this capacitor
value is large, it can significantly distort the current
feedback waveform aross the sense resistors. This should
be taken into account while designing the overcurrent
protection circuit.
During normal operation, the SC2442/H will enter hiccup
mode if the SS pin voltage is above 3.3V and the FB pin
voltage is below 0.525V (70% of 0.750V). If this occurs
the GDxH and GDxL signals will go low and the SS pin will
begin to sink 2 μA. The 2 μA of sink current will slowly
discharge the SS capacitor until its voltage reaches 1V,
which will trigger the SS pin to begin sourcing 10 μA. The
convertor will operate in the asynchronous mode during
hiccup.
Gate Drive Considerations
The SC2442/H provide high side gate drive with
bootstrapping as shown in the Typical Application Circuit.
A ceramic capacitor is recommended between each BSTx
pin to the corresponding Phase Node. Each gate drive can
source and sink a minimum of 0.5A current with 100 nS
dead time between transistions to prevent shoot throughs.
© 2007 Semtech Corp.
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