SC2440AEVB datasheet(12/28 Pages) SEMTECH

12 / 28 page

Applications Information

the MHz fundamental.

Switching frequency is also limited by the minimum con-

trollable on time when stepping down from high V

IN to

low V

OUT. This will be described in next section.

Table 1. Programming the Channel Frequency

Channel f

FREE-RUN (kHz)

ROSC Resistor (kW)

200

300

78.7

400

57.6

500

45.3

600

35.7

700

30.9

800

24.9

900

22.

000

9.6

7.4

200

5.4

300

3.3

400

500

600

0.0

700

9.09

800

8.25

900

7.50

2000

6.65

200

6.9

2200

5.62

2300

4.99

2400

4.53

2500

4.02

Minimum On Time Considerations

The switching duty cycle of a DC-DC converter is the ra-

tio of the switch on time to the switching period. For a

non-synchronous step-down regulator, the duty cycle D

in continuous-conduction mode (CCM) is given by:

SC2440A

Setting the Output Voltage

The regulator output voltage is set with an external resis-

tor divider (Figure 4) with its center tap tied to the FB pin.

()

The percentage error due the input bias current of the er-

ror amplifier is

Using a smaller R

2 (preferably < 20kW) makes the effect of

the amplifier input bias current insignificant compared to

the V

OUT tolerance resulting from the use of % resistors.

Setting the Operating Frequency

The master oscillator in the SC2440A feeds a toggle flip-

flop which in turn generates the individual channel clocks

CLK and CLK2 (Figure 2). The phase clocks run at half the

master oscillator frequency and are shifted in phase by

80o. The external resistor from the ROSC pin to ground

programs the channel free-running frequency f

FREE-RUN. Ta-

ble lists the suggested programming resistors for vari-

ous channel frequencies.

Before choosing the operating frequency, tradeoffs

among efficiency, operating duty cycle, component size

and EMI must be considered. High frequency operation

reduces the size of passive components but switching

losses are higher. Lowering the switching frequency im-

proves efficiency, however the required inductor and ca-

pacitor are larger. It is also worth noting that a dual DC-DC

converter with each channel switching at MHz will pro-

duce a switching noise spectrum at integer multiples of

< 200nA

SC2440A

VOUT

Figure 4. R

1 and R2 Set the Output Voltage

< 200nA

SC2440A

VOUT

Figure 4. R

1 and R2 Set the Output Voltage

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