SC2447 查看數據表（PDF） - Semtech Corporation
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SC2447 Datasheet PDF : 26 Pages
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SC2447
POWER MANAGEMENT
Application Information
The SC2447 consists of two current-mode synchronous
buck controllers with many integrated functions. The
SC2447 can be used to generate:
1) two independent outputs from a common input or
two different inputs or,
2) dual-phase output with current sharing,
3) current sourcing/sinking from common or separate
inputs as in DDR (I and II) memory application.
Step-Down Converter
Starting from the following step-down converter
specifications,
Input voltage range: Vin ∈ [Vin,min , Vin,max ]
Input voltage ripple (peak-to-peak): ∆Vin
Output voltage: V
o
Output voltage accuracy: ε
Output voltage ripple (peak-to-peak): ∆Vo
Nominal output (load) current: Io
Maximum output current limit: I
o,max
Output (load) current transient slew rate: dI (A/s)
o
Circuit efficiency: η
Selection criteria and design procedures for the following
are described.
1) output inductor (L) type and value
2) output capacitor (C ) type and value
o
3) input capacitor (C ) type and value
in
4) power MOSFETs
5) current sensing and limiting circuit
6) voltage sensing circuit
7) loop compensation network
Operating Frequency (fs)
The switching frequency in the SC2447 is user-
programmable. The advantages of using constant
frequency operation are simple passive component
selection and ease of feedback compensation. Before
setting the operating frequency, the following trade-offs
should be considered:
1) Passive component size
2) Efficiency
3) EMI condition
4) Minimum switch on time
5) Maximum duty ratio
For a given output power, the size of the passive
components are inversely proportional to the switching
frequency, whereas MOSFET/Diode switching losses are
proportional to the operating frequency. Other issues such
as heat dissipation, packaging and the cost issues are
also considered. The frequency bands for signal
transmission should be avoided because of EM
interference.
Minimum Switch On Time Consideration
In the SC2447, the falling edge of the clock turns on the
top MOSFET. The inductor current and the sensed voltage
ramp up. After the sensed voltage crosses a threshold
determined by the error amplifier output, the top MOSFET
is turned off. The propagation delay time from the turn-
on of the controlling FET to its turn-off is the minimum
switch on time. This propagation delay time consists of
the propagation delay time (T ) from the current sense
DLPWM
inputs to the PWM output and the propagation delay time
(TDLTG) from the trailing edge of the PWM input to the
trailing edge of the phase voltage.
The SC2447 has a typical propagation delay time from
the current sense inputs to the PWM output of about
85ns at room temperature. The shortest on interval
(TMINON) of the controlling FET is then 85ns+TDLTG. Assuming
that TDLTG is 45ns, the controller either does not turn on
the top MOSFET at all or turns it on for at least 130ns.
TDLTG can be found in the MOSFET driver datasheet.
For a synchronous step-down converter, the operating duty
cycle is VO/VIN. The required on time for the top MOSFET
is VO/(VIN*fs). If the frequency is set such that the required
pulse width is less than 130ns, assuming TDLTG is 45ns,
then the converter will start skipping cycles. Due to
minimum on-time limitation, simultaneously operating at
very high switching frequency and very short duty cycle is
not practical. If the voltage conversion ratio VO/VIN and
hence the required duty cycle is higher, the switching
frequency can be increased to reduce the size of passive
components.
There will not be enough modulating headroom if the on
time is made equal to the minimum on time (TMINON). For
ease of control, set the switching frequency so that the
pulse width is at least 1.5 times the minimum on time.
© 2006 Semtech Corp.
15
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