SC2441ITSTRT datasheet(13/35 Pages) SEMTECH

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 2005 Semtech Corp.

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POWER MANAGEMENT

SC2441

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:

]

[

max

min

in ∈

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

o,max

Output (load) current transient slew rate: dI

o (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 (C

o) type and value,

3) input capacitor (C

in) 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:

in=3.3V, Vo=1.2V, Io=4A, fs=500kHz.

Operating Frequency (f

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

duty cycle is V

. So the required on time for the top

MOSFET is V

/(V

S). If the frequency is set such that

the required pulse width is less than 180ns, 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 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

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