SC2447TSTRT datasheet(15/26 Pages) SEMTECH

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POWER MANAGEMENT

SC2447

The SC2447 consists of two current-mode synchronous

buck controllers with many integrated functions. The

SC2447 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.

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 MOSFETs

5) current sensing and limiting circuit

6) voltage sensing circuit

7) loop compensation network

Operating Frequency (f

The switching frequency in the SC2447 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) Efficiency

3) EMI condition

4) Minimum switch on time

5) Maximum duty ratio

For a given output power, the size of the passive

components are inversely proportional to the switching

frequency, whereas MOSFET/Diode switching losses are

proportional to the operating frequency. Other issues such

as heat dissipation, packaging and the cost issues are

also considered. The frequency bands for signal

transmission should be avoided because of EM

interference.

Minimum Switch On Time Consideration

In the SC2447, 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. This propagation delay time consists of

the propagation delay time (T

DLPWM) from the current sense

inputs to the PWM output and the propagation delay time

DLTG) from the trailing edge of the PWM input to the

trailing edge of the phase voltage.

The SC2447 has a typical propagation delay time from

the current sense inputs to the PWM output of about

85ns at room temperature. The shortest on interval

MINON) of the controlling FET is then 85ns+TDLTG. Assuming

that T

DLTG is 45ns, the controller either does not turn on

the top MOSFET at all or turns it on for at least 130ns.

DLTG can be found in the MOSFET driver datasheet.

For a synchronous step-down converter, the operating duty

cycle is V

O/VIN. The required on time for the top MOSFET

is V

O/(VIN*fs). If the frequency is set such that the required

pulse width is less than 130ns, assuming T

DLTG is 45ns,

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 size of passive

components.

There will not be enough modulating headroom if the on

time is made equal to the minimum on time (T

MINON). For

ease of control, set the switching frequency so that the

pulse width is at least 1.5 times the minimum on time.

Application Information

SC2447TSTRT Datasheet(PDF) 15 Page - Semtech Corporation

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