SC2446 Datasheet, PDF (11/38 Pages) Semtech Corporation – Dual-Phase Single or Two Output Synchronous Step-Down Controllers

SC2446
POWER MANAGEMENT
Application Information
SC2446 consists of two current-mode synchronous buck
controllers with many integrated functions. By proper
application circuitry configuration, SC2446 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 SC2446 is described in the following.
Step-down Converter
Starting from the following step-down converter
specifications,
Input voltage range: V_in ∈ [V_in,min, V_in,max ]
Input voltage ripple (peak-to-peak): ∆V_in
Output voltage: V_o
Output voltage accuracy: ε
Output voltage ripple (peak-to-peak): ∆V_o
Nominal output (load) current: I
o
Maximum output current limit: I_o,max
Output (load) current transient slew rate: dI_o (A/s)
Circuit efficiency: η
Selection criteria and design procedures for the following
are described.
1) output inductor (L) type and value,
2) output capacitor (C_o) type and value,
3) input capacitor (C ) type and value,
in
4) power MOSFET’s,
5) current sensing and limiting circuit,
6) voltage sensing circuit,
7) loop compensation network.
Operating Frequency (f_s)
The switching frequency in the SC2446 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
For a given output power, the sizes of the passive
components are inversely proportional to the switching
frequency, whereas MOSFET’s/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 SC2446 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. The SC2446 has a minimum on time of
about 150ns 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
150ns.
For a synchronous step-down converter, the operating duty
cycle is V_O/V_IN. So the required on time for the top MOSFET
is V_O/(V_INfs). If the frequency is set such that the required
pulse width is less than 150ns, 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_O/V_IN and hence 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
SC2446. For ease of control, we recommend the required
pulse width to be at least 1.5 times the minimum on time.
 2004 Semtech Corp.
11
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