ISL6560EVAL1 Intersil, ISL6560EVAL1 Datasheet - Page 9

• Manufacturer Part Number: ISL6560EVAL1
• Description: EVALUATION BOARD ISL6560
• Manufacturer: Intersil

Specifications of ISL6560EVAL1

• Main Purpose: Special Purpose DC/DC, VRM Supply
• Outputs And Type: 1, Non-Isolated
• Power - Output: 60W
• Voltage - Output: 1.5V
• Current - Output: 40A
• Voltage - Input: 5V, 12V
• Regulator Topology: Buck
• Frequency - Switching: 400kHz
• Board Type: Fully Populated
• Utilized Ic / Part: ISL6560
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

These curves help to visualize that in some cases, major
changes in some parameters only result in subtle changes in
other parameters. For example, going from 175kHz to the
200kHz channel frequency.
Output Capacitors
The combined series resistance and inductance of the
output capacitors is one of the limiting factors in the supply’s
response to transient loads. Most DC/DC converters do not
have the bandwidth or operating frequency to respond to
rapid load changes. Therefore, attention must be paid to the
filter network, for it must be the major source of energy
during step load changes. The output capacitors must
respond by supplying the initial load current, until the
regulator loop responds and the inductor current slews.
Bulk capacitors store energy, but are limited by the effective
series resistance and inductance path to their reservoir of
energy. Considering only the series resistance, the total
effective series resistance of the parallel connected
capacitors should be equal to or less than the effective DC
R
approximately 1.63mΩ. For this example, six 1500µF, 4V
Sanyo OS-CON capacitors provide a maximum ESR of
1.66mΩ, roughly meeting the design target.
To a first order, output ripple voltage is the product of the
capacitor’s ESR and the ripple current. In this design it is
1.66mΩ x 8A = 13.3mV.
Sixteen 22µF ceramic capacitors help provide high-
frequency bypassing by providing lower inductance and low
high-frequency impedance. It is essential that additional
ceramic capacitors also be place at the load to help stabilize
the load voltage and minimize additional droop at the load.
Input Capacitors
The input capacitors are also critical to supply operation.
They must provide enough energy to prevent the input
voltage from dropping due to load transients. In addition, the
OUT
0.01
0.1
3
1
100nH
of the supply. As mentioned earlier R
Frequency = Channel fsw
FIGURE 6. INDUCTOR SELECTION CURVES
Inductance
9
1µH
OUT
is
10µH
ISL6560
high peak currents can cause heating of these capacitors
and can result in premature failure if not properly designed.
The value of the RMS current that these capacitors must
share can be approximated with the aid of the curve of
Figure 7. The dotted lines show determination of the current
multiplier.
For the 40A design with the 1.8V/12V = 0.15 duty cycle, the
RMS current is 0.24 x 40A = 9.6A. From this curve, it is
evident that the maximum current is only 10A. If the duty
cycle was 50%, each channel would be ON for its full cycle
and the ripple would go to zero.
Rubycon ZA series capacitors were selected for the input
capacitors. Their 470µF, 16V capacitors have a maximum
RMS current rating of 1.6A at 105
capacitors are required.
On any switching supply, high frequency decoupling may be
necessary on the supply input to keep the high peak current,
fast rise current pulses contained within the supply. Often a
small inductor is placed in series with the input line to help
reduce this potential source of EMI. Ceramic capacitors to
ground also help lower the high frequency impedance to
shunt the high frequency components to reduce and contain
the high speed current pulses.
R
Each channel supplies a current of 20A. Add the 4A ripple
(half of the ripple current) component and the minimum
voltage across the current sense resistor that will trip the
comparator is the minimum limit of 142mV. The R
resistor value is then 142mV / 28A = 5.07mΩ. A 5mΩ
resistor was used for this function to insure the minimum
current.
The maximum current is also important. The maximum
threshold voltage for the current comparator is 172mV. The
maximum current would be: 172mV / 5mΩ = 34.4A per
channel. The 4A of ripple current per channel must be
subtracted to yield 30.4A per channel. The maximum output
current would be two times the channel current, or 60.8A.
SENSE
0.3
0.2
0.1
0
FIGURE 7. CURRENT MULTIPLIER vs. DUTY CYCLE
0
Selection
0.1
DUTY CYCLE (V
0.2
o
C ambient. For 9.6A, six
0.3
O
/V
IN
)
0.4
SENSE
FN9011.3
0.5