SC2441AEVB(2006) View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2441AEVB
(Rev.:2006)

1.8V to 20V Input 2-Phase Synchronous Step-down Controllers with Step-up Converter

Semtech Corporation 

SC2441A

POWER MANAGEMENT

Applications Information

Operating Frequency (fs)

The switching frequency in the SC2441A 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.

ROSC vs. SRotescpv-sdSoinwgnle CChhanannelnSewlitSchwinigtcFhreiqnugenFcyrequency

150

130

110

90

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

should be considered. The frequency bands for signal

transmission should be avoided because of EM

interference.

The switching frequency of both step-down controllers is

set with an external resistor from Pin 10 to the signal

ground. The set frequency is inversely proportional to the

resistor value (Figure 7) and can be approximated as:

ROSC = 101618 × FSW -1.22

ROSC is in KΩ and FSW is in KHz.

70

50

30

10

100

300

500

700

900

Switching Frequency (KHz)

1100

Figure 7. ROSC vs. Step-down Channel Free-running

Frequency

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

SC2441A 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.

The internal oscillator starts to operate once VIN exceeds

its UVLO threshold. The oscillator output, CLK, (see Figure

2) clocks the step-up converter. The frequency divider

generates two out-of-phase clocks, CLK1 and CLK2, at

a half of CLK frequency. CLK1 and CLK2 clock the step-

down channels. The switching frequency of the step-up

converter is twice those of the step-down controllers. If

both step-down channels are running at 250KHz, then

the boost section will be running at 500KHz.

For a synchronous step-down converter, the operating

duty cycle is VO /VIN. So the required on time for the top

MOSFET is VO/(VINfS). 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

 2006 Semtech Corp.

20

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