9

FN9011.3

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

approximately 1.63m

Ω. For this example, six 1500µF, 4V

Sanyo OS-CON capacitors provide a maximum ESR of

1.66m

Ω, 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

1.66m

Ω x 8A = 13.3mV.

Sixteen 22

µF 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

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 470

µF, 16V capacitors have a maximum

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.

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

resistor value is then 142mV / 28A = 5.07m

Ω. 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 / 5m

Ω = 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.

3

1

0.01

0.1

100nH

1

µH10µH

Inductance

10

0k

Hz

20

0k

Hz

50

0k

Hz

1M

Hz

2M

Hz

Frequency = Channel fsw

FIGURE 6. INDUCTOR SELECTION CURVES

0.3

0.2

0.1

0

0

0.1

0.2

0.3

0.4

0.5

FIGURE 7. CURRENT MULTIPLIER vs. DUTY CYCLE

ISL6560