SC2453_05 datasheet(10/22 Pages) SEMTECH

10 / 22 page

 2005 Semtech Corp.

www.semtech.com

SC2453

POWER MANAGEMENT

Applications Information (Cont.)

Vdiode drop.

Shoot through control circuitry provides a 30ns dead time

to ensure both the upper and lower MOSFET will not turn

on simultaneously and cause a shoot through condition.

Error Amplifier and PWM Controller

In closed loop operation, the internal oscillator ramp

ranges from 0.75V to 3.8V. The error amplifier output

ranges determines duty-ratio of a converter. The synchro-

nous continuous-conduction mode of operation allows

the SC2453 to regulate the output irrespective of the

direction of the load current.

The SC2453 uses voltage-mode control for good noise

immunity and ease of compensation. The low-side

MOSFET of each channel is turned off at the falling-edge

of the phase timing clock. After a brief non-overlapping

time interval of 30ns, the high-side MOSFET is turned

on. The phase inductor current ramps up. When the in-

ternal ramp reaches the threshold determined by the

error amplifier output, the high-side MOSFET is turned

off. As long as phase voltage collapses below 1.5V, the

low-side MOSFET is turned on.

Buck Converter

Buck converter design includes the following specifica-

tions:

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

sssss)))))

The switching frequency in the SC2453 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 SC2453, the falling edge of the clock turns on the

top MOSFET. The inductor current and the sensed voltage

ramp up. After the internal ramp 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 SC2453 has a minimum

on time of about 50ns 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 50ns.

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 50ns, 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.

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