3
Transient Load Generator
Probably one of the most interesting tests for a regulator
system is the transient load. From this single test one can
access voltage droop, loop stability and the regulator’s
response to load changes going from no load to full load and
the recovery after rapid load removal. To quickly determine
these characteristics, a pulse load generator is incorporated
on the evaluation board. A current load pulse at about 20A
per position at 1.5V output is activated with two slide
switches. A scope probe connector is provided to monitor the
current pulse and is calibrated to read 10mV/A. Figures 5, 6,
and 7 show the transient response of the Evaluation Board
with 12V input, operating with the internal load generator
which provides slightly over a 40A load step. For all scope
shots: Top trace is PWM 1 output, next is V
COMP
at 1V/div.
Center trace is V
CORE
at 50mV/div and the lower trace is the
load current at 20A/div. DAC set to 1.500V.
Efficiency
Figures 8 and 9 show the efficiency of the converter with
CORE voltage at the two extremes of the DAC voltage and at
1.500V, near the middle of the range. The curves show 12V
input and 5V input. Note the improvement in efficiency as the
output voltage approaches the input voltage, with increasing
duty cycle.
Snubber Networks
Snubbers are not used in this design, but pad locations and
connections to PHASE and ground are provided by R2 - C7
for PHASE 1 and R4 - C9 for PHASE 2.
PC Board Schematic
Figure 11 shows the main schematic. The Power Good
indicator circuit is shown in figure 13. Figure 12 shows the
schematic of the transient load generator.
The layout is shown in Figures 14 and 15, starting with the
silk screen in Figure 14. The Bill of Material is shown in Table 1.
Following the Bill of Materials is quick design guide.
PC Board Layout Considerations
Like all high current supplies where low voltage control
signals in the millivolt range must live with high voltage, high
current switching signals, PC board layout becomes crucial
in obtaining a satisfactory supply.
FIGURE 7. EXPANDED BACK EDGE OF CURRENT PULSE
FIGURE 5. 44A TRANSIENT CURRENT PULSE
FIGURE 6. EXPANDED FRONT EDGE OF CURRENT PULSE
0
5
10
15
20
25
30
35
40
LOAD CURRENT (A)
100
90
80
70
60
50
EFFICIENCY (%)
V
OUT
= 1.10V
V
OUT
= 1.50V
V
OUT
= 1.85V
V
IN
= 12V
FIGURE 8. 12V INPUT EFFICIENCY AT DAC EXTREMES
LOAD CURRENT (A)
100
90
80
70
60
50
0
5
10
15
20
25
30
35
40
EFFICIENCY (%)
V
OUT
= 1.85V
V
OUT
= 1.10V
V
OUT
= 1.50V
V
IN
= 5V
FIGURE 9. 5V INPUT EFFICIENCY AT DAC EXTREMES
ISL6560/62 Evaluation Board
