SC4530 datasheet(14/23 Pages) SEMTECH

14 / 23 page

SC4530

Applications Information (Continued)

Feed-Forward Compensation

A feed-forward capacitor C

4 (connected across the upper

feedback resistor R

) is needed for stability. An initial

estimate of C

4 can be found using Equation (0) below:

(0)

The value of C

4 can be optimized empirically by observing

the inductor current and the output voltage during load

transient. Starting with the initial estimate, C

4 is tuned until

there is no excessive ringing or overshoot in the inductor

current or the output voltage during load transient.

Mode Transition and the FB Pin

If the upper feedback resistor R

(Figure 3, page 9) is large

and is about the same magnitude as R

2, then fast switching

transients may couple into the FB pin, disturbing or

delaying the transition from light-load operating mode

to continuous-conduction mode (CCM). As described

previously, the output ripple voltage is very low in

continuous-conduction mode. Delayed CCM transition

extends the load range in which the converter produces

larger output voltage ripples. This disturbance becomes

more pronounced when V

IN is increased above 2V and

SW_OUT_IL_28V to 5V@174mA_C4=10pF_onset CCM=176mA

OUT

50mV/div

AC Coupled

200mA/div

20V/div

4ms/div

4 = 10pF, C5 not Placed

OUT = 5V, IOUT = 174mA

CCM Onset I

OUT = 176mA

(a)

SW_OUT_IL_28V to 5V@110mA_C4=10pF_C5=33pF_onset CCM=126mA

OUT

50mV/div

AC Coupled

200mA/div

20V/div

10ms/div

4 = 10pF, C5 = 33pF

OUT = 5V, IOUT = 110mA

CCM Onset I

OUT = 126mA

(b)

Figure 7. Switching Waveforms of a 28V to 5V

Converter Just Before It Enters Continuous-

Conduction Mode

Figure 8. Switching Waveforms of a 28V to 3.3V

Converter Just Before It Enters Continuous-

Conduction Mode

SW_OUT_IL_28V to 3.3V@145mA_C4=22pF_onset CCM=150mA

OUT

50mV/div

AC Coupled

200mA/div

20V/div

4ms/div

4 = 22pF, C5 not Placed

OUT = 3.3V, IOUT = 145mA

CCM Onset I

OUT = 150mA

(a)

SW_OUT_IL_28V to 3.3V@118mA_C4=22pF_C5=47pF_onset CCM=121mA

10ms/div

OUT

50mV/div

AC Coupled

200mA/div

20V/div

4 = 22pF, C5 = 47pF

OUT = 3.3V, IOUT = 118mA

CCM Onset I

OUT = 121mA

(b)

SC4530 датащи(PDF) 14 Page - Semtech Corporation

SC4530 датащи(HTML) 14 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

датащи скачать

ссылки URL