ISL6551EVAL1 Intersil, ISL6551EVAL1 Datasheet - Page 40

• Manufacturer Part Number: ISL6551EVAL1
• Description: EVALUATION BOARD ISL6551
• Manufacturer: Intersil

Specifications of ISL6551EVAL1

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Isolated
• Voltage - Output: 3.3V
• Current - Output: 60A
• Voltage - Input: 36 ~ 75V
• Regulator Topology: Buck
• Frequency - Switching: 470kHz
• Board Type: Fully Populated
• Utilized Ic / Part: ISL6551
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

Output Start Up
The start up characteristic of the output voltage heavily
depends on the load. For a “pure” resistive load or an
electronic load with low slew rate (0.01A/us), the output
voltage comes up smoothly, as shown in Figures 96 and 97.
In the case of high slew rate electronic load, the
monotonicity of the output voltage is lost, as shown in
Figures 98 and 99. During the startup, the load demands
more current than what the converter can deliver, which
causes the output dipping. The higher the load is, the higher
the current ramp is needed, and the higher the error voltage
is required to push the duty cycle up further. The error
voltage is limited by the soft start voltage (Vclamp) that
comes up with a slower speed, therefore, the duty cycle is
limited causing repetitive up/downs at the output voltage, as
shown in Figure 100. For applications with similar behavior
of the electronic load, this problem can be resolved by
speeding up the startup of the soft start with two possible
options: 1) increase the soft start speed above the start-up
speed of the error voltage by reducing the capacitive load at
the CSS pin of ISL6551; or 2) set the soft start at the output
reference pin (EANI) and completely remove all capacitive
load at the CSS pin. In general, the second option is the
practical one. Note that the delay at the electronic load is
caused by its turn-on threshold.
FIGURE 96. OUTPUT VOLTAGE (CHANNEL 1) AT V
V
OUT
=3.3V, AND 0.083Ω RESISTIVE LOAD
40
Application Note 1002
IN
=48V,
FIGURE 98. OUTPUT VOLTAGE (CHANNEL 1) AT V
FIGURE 99. OUTPUT VOLTAGE (CHANNEL 1) AT V
FIGURE 97. OUTPUT VOLTAGE (CHANNEL 1) AT V
V
RESISTIVE MODE (CHANNEL 4, 1/4 OF THE LOAD)
V
CONSTANT CURRENT MODE. CHANNEL 2:
ERROR VOLTAGE; CHANNEL 3: VCLAMP
VOLTAGE; CHANNEL 4: CURRENT RAMP
(ISENSE). EACH CHANNEL IS 1V/DIV AND
2MS/DIV.
V
ELECTRONIC CONSTANT CURRENT MODE
(CHANNEL 4, 1/4 OF THE LOAD)
OUT
OUT
4
OUT
=2.64V, AND I
=2.64V, AND I
=2.64V, AND I
OUT
OUT
OUT
=60A, 1A/US ELECTRONIC
=60A, 1A/US ELECTRONIC
=60A, 0.01A/US
3
IN
IN
IN
=75V,
2
=48V,
=75V,
1