LT3579 datasheet(28/40 Pages) LINER

28 / 40 page

LT3579/LT3579-1

35791f

APPENDIX

Table 7 shows a list of several discrete PMOS manufa-

cturers. Consult the manufacturers for detailed information

on their entire selection of PMOS devices.

Table 7. Discrete PMOS Manufacturers

Vishay

www.vishay.com

Fairchild Semiconductor

www.fairchildsemi.com

Central Semiconductor

www.centralsemi.com

COMPENSATION – ADJUSTMENT

To compensate the feedback loop of the LT3579, a series

resistor-capacitor network in parallel with an optional single

capacitor must be connected from the VC pin to GND. For

most applications, choose a series capacitor in the range

of 1nF to 10nF with 2.2nF being a good starting value.

The optional parallel capacitor should range in value from

22pF to 180pF with 47pF being a good starting value. The

compensation resistor, RC, is usually in the range of 5k to

50k. A good technique to compensate a new application is

to use a 100kΩ potentiometer in place of the series resistor

RC. With the series and parallel capacitors at 2.2nF and

47pF respectively, adjust the potentiometer while observing

the transient response and the optimum value for RC can

be found. Figures 17a to 17c illustrate this process for the

circuit of Figure 20 with a load current stepped between

0.7A and 1.5A. Figure 17a shows the transient response

with RC equal to 1k. The phase margin is poor as evidenced

by the excessive ringing in the output voltage and inductor

current. In Figure 17b, the value of RC is increased to 3.5k,

which results in a more damped response. Figure 17c

shows the results when RC is increased further to 8k. The

transient response is nicely damped and the compensation

procedure is complete.

COMPENSATION – THEORY

Like all other current mode switching regulators, the

LT3579 needs to be compensated for stable and efﬁcient

operation. Two feedback loops are used in the LT3579: a

fast current loop which does not require compensation,

and a slower voltage loop which does. Standard Bode plot

analysis can be used to understand and adjust the voltage

Figure 17a. Transient Response Shows Excessive Ringing

Figure 17b. Transient Response Is Better

Figure 17c. Transient Response Is Well Damped

100μs/DIV

VOUT

500mV/DIV

AC COUPLED

2A/DIV

ILOAD

1A/DIV

3579 F17a

RC = 1k

100μs/DIV

VOUT

500mV/DIV

AC COUPLED

2A/DIV

ILOAD

1A/DIV

3579 F17b

RC = 3.5k

100μs/DIV

VOUT

500mV/DIV

AC COUPLED

2A/DIV

ILOAD

1A/DIV

3579 F17c

RC = 8k

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