SC2441EVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2441EVB
Semtech Corporation
'SC2441EVB' PDF : 35 Pages View PDF
SC2441
POWER MANAGEMENT
Applications Information
The SC2441 consists of two current-mode synchronous
buck controllers and an auxiliary boost converter. The
SC2441 can be used to generate
1) two independent step-down outputs or
2) dual phase single output with current sharing and
3) a step-up output
The application information using SC2441 for the control
of step-down and step-up converters are described below.
Step-down Converter
Specifications of a step-down converter are given by the
followings
Input voltage range: Vin ∈ [Vin,min , Vin,max ]
Input voltage ripple (peak-to-peak): ∆Vin
Output voltage: Vo
Output voltage accuracy: ε
Output voltage ripple (peak-to-peak): ∆Vo
Nominal output (load) current: Io
Maximum output current limit: Io,max
Output (load) current transient slew rate: dIo (A/s)
Circuit efficiency: η.
Based on these converter specifications, selection criteria
and design procedures for the following components are
described.
1) output inductor (L) type and value,
2) output capacitor (Co) type and value,
3) input capacitor (Cin) type and value,
4) power switch MOSFET’s,
5) current sensing and limiting circuitry,
6) voltage sensing circuitry,
7) loop compensation circuitry.
To illustrate the design process, the following example is
used:
Vin=3.3V, Vo=1.2V, Io=4A, fs=500kHz.
Operating Frequency (fs)
The switching frequency in the SC2441 is user-
programmable. The advantages of constant frequency
operation are simple passive component selection and fast
transient response with simple frequency compensation.
Before setting the operating frequency, the following trade-
offs should be considered.
1) passive component sizes
2) converter efficiency
3) EMI
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 Limitation
In both step-down controllers, the falling edge of the
clock turns on the top MOSFET. The inductor current
ramps up so does the sensed voltage. 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
SC2441 has a minimum on time of about 180ns 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 180ns.
For a synchronous step-down converter, the operating
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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 input voltage is 3.3V and the operating
frequency is 1MHz, the lowest output voltage will be 0.6V.
There will not be enough modulation headroom if the on
time is simply made equal to the minimum on time of the
SC2441. For ease of control, we recommend the required
pulse width to be at least 1.5 times the minimum on time.
Maximum Duty-cycle Consideration
When operating at 500KHz, the maximum top MOSFET
on duty-cycle is 90%. The top MOSFET therefore turns off
for at least 200ns every cycle regardless of the switching
frequency. This places an upper bound on the voltage
2005 Semtech Corp.
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