ISL6744 Datasheet, PDF (13/18 Pages) Intersil Corporation – Intermediate Bus PWM Controller

ISL6744
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
22µF 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.
14
13
12
11
10
9
8
0.9950
0.9960
V (L1:1)
I (L1)
0.9970 0.9980
TIME (ms)
0.9990
1.000
FIGURE 8. STEADY STATE SECONDARY WINDING
VOLTAGE AND INDUCTOR CURRENT
15
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 5.11Ω 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.
100
95
90
85
85
75
70
0 1 2 3 4 5 6 7 8 9 10
LOAD CURRENT (A)
FIGURE 10. EFFICIENCY vs LOAD V_IN = 48V
10
5
0.986 0.988
V (L1:1)
I (L1)
0.990
0.992 0.994
TIME (ms)
0.996
0.998
1.000
FIGURE 9. SECONDARY WINDING VOLTAGE AND
INDUCTOR CURRENT DURING CURRENT LIMIT
OPERATION
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
12.5
12.25
12
11.75
11.5
11.25
11
0 1 2 3 4 5 6 7 8 9 10
LOAD CURRENT (A)
FIGURE 11. LOAD REGULATION AT V_IN = 48V
13
FN9147.8
September 22, 2005