SC2446A Semtech Corporation, SC2446A Datasheet - Page 10

• Manufacturer Part Number: SC2446A
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

SC2446A consists of two current-mode synchronous
buck controllers with many integrated functions. By
proper application circuitry configuration, SC2446A 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.
The application information related to the converter
design using SC2446A is described in the following.
Step-down Converter
Starting from the following step-down converter
specifications,
Input voltage range:
Input voltage ripple (peak-to-peak): V
Output voltage: V
Output voltage accuracy:
Output voltage ripple (peak-to-peak): V
Nominal output (load) current: I
Maximum output current limit: I
Output (load) current transient slew rate: dI
Circuit efficiency:
Selection criteria and design procedures for the following
are described.
1) output inductor (L) type and value,
2) output capacitor (C
3) input capacitor (C
4) power MOSFET’s,
5) current sensing and limiting circuit,
6) voltage sensing circuit,
7) loop compensation network.
Operating Frequency (f
The switching frequency in the SC2446A 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) Circuitry efficiency
3) EMI condition
4) Minimum switch on time and
5) Maximum duty ratio
POWER MANAGEMENT
Application Information
2005 Semtech Corp.
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For a given output power, the sizes of the passive
components are inversely proportional to the switching
frequency, whereas MOSFETs/Diodes switching losses
are proportional to the operating frequency. Other issues
such as heat dissipation, packaging and the cost issues
are also to be considered. The frequency bands for signal
transmission should be avoided because of EM
interference.
Minimum Switch On Time Consideration
In the SC2446A the falling edge of the clock turns on
the top MOSFET gate. 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 gate 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. The SC2446A has a
minimum on time of about 120ns at room temperature.
This is the shortest on interval of the controlling FET. The
controller either does not turn on the top MOSFET at all
or turns it on for at least 120ns.
For a synchronous step-down converter, the operating
duty cycle is V
MOSFET is V
the required pulse width is less than 120ns, 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 V
the required duty cycle is higher, the switching frequency
can be increased to reduce the sizes of passive
components.
There will not be enough modulation headroom if the on
time is simply made equal to the minimum on time of the
SC2446A. For ease of control, we recommend the
required pulse width to be at least 1.5 times the minimum
on time.
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fs). If the frequency is set such that
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SC2446A
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