ISL6744ABZ datasheet(13/18 Pages) INTERSIL

13 / 18 page

FN9147.8

September 22, 2005

based on the nature of the intended load, multiple buck

regulators. To minimize the output impedance of the filter, a

SANYO OSCON 16SH150M capacitor in parallel with a

μF ceramic capacitor were selected.

Current Limit Threshold

The current limit threshold is fixed at 0.6V nominal, which is

the reference to the overcurrent protection comparator. The

current level that corresponds to the overcurrent threshold

must be chosen to allow for the dynamic behavior of an open

loop converter. In particular, the low inductor ripple current

under steady state operation increases significantly as the

duty cycle decreases.

Figures 8 and 9 show the behavior of the inductor ripple

under steady state and overcurrent conditions. In this

example, the peak current limit is set at 11A. The peak

current limit causes the duty cycle to decrease resulting in a

reduction of the average current through the inductor. The

implication is that the converter can not supply the same

output current in current limit that it can supply under steady

state conditions. The peak current limit setpoint must take

this behavior into consideration. A 5.11

Ω current sense

resistor was selected for the rectified secondary of current

transformer T2 for the ISL6744Eval 1, corresponding to a

peak current limit setpoint of about 11A.

Performance

The major performance criteria for the converter are

efficiency, and to a lesser extent, load regulation. Efficiency,

load regulation and line regulation performance are

demonstrated in the following Figures.

As expected, the output voltage varies considerably with line

and load when compared to an equivalent converter with a

closed loop feedback. However, for applications where tight

regulation is not required, such as those applications that

use downstream DC/DC converters, this design approach is

viable.

FIGURE 8. STEADY STATE SECONDARY WINDING

VOLTAGE AND INDUCTOR CURRENT

0.9950

0.9960

0.9970

0.9980

0.9990

1.000

TIME (ms)

V (L1:1)

I (L1)

FIGURE 9. SECONDARY WINDING VOLTAGE AND

INDUCTOR CURRENT DURING CURRENT LIMIT

OPERATION

V (L1:1)

I (L1)

0.986

0.988

0.990

0.992

0.994

1.000

TIME (ms)

0.996

0.998

FIGURE 10. EFFICIENCY vs LOAD VIN = 48V

100

3456

789

LOAD CURRENT (A)

FIGURE 11. LOAD REGULATION AT VIN = 48V

12.5

12.25

11.75

11.5

345

678

LOAD CURRENT (A)

11.25

ISL6744

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