SC2446AEVB datasheet(10/27 Pages) SEMTECH

10 / 27 page

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2446A

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:

]

[

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:

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

in) type and value,

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

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

O/VIN. So the required on time for the top

MOSFET is V

O/(VINfs). If the frequency is set such that

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

O/VIN 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

SC2446A. For ease of control, we recommend the

required pulse width to be at least 1.5 times the minimum

on time.

Application Information

SC2446AEVB Datasheet(PDF) 10 Page - Semtech Corporation

SC2446AEVB Datasheet(HTML) 10 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Datasheet Download

Link URL