ISL6558

FN9027 Rev 13.00

Page 14 of 17

August 28, 2015

The total output ripple current can be determined from the

curves in Figure 10. They provide the total ripple current as a

function of duty cycle and number of active channels,

where L is the channel inductor value.

Find the intersection of the active channel curve and duty cycle

for your particular application. The resulting ripple current

multiplier from the y-axis is then multiplied by the normalization

the given application.

INPUT CAPACITOR SELECTION

Use a mix of input bypass capacitors to control the voltage

overshoot across the MOSFETs. Use ceramic capacitors for

the high frequency decoupling and bulk capacitors to supply

the RMS current. Small ceramic capacitors can be placed very

close to the upper MOSFET to suppress the voltage induced in

the parasitic circuit impedances.

Two important parameters to consider when selecting the bulk

input capacitor are the voltage rating and the RMS current

rating. For reliable operation, select a bulk capacitor with voltage

and current ratings above the maximum input voltage and

largest RMS current required by the circuit. The capacitor

voltage rating should be at least 1.25 times greater than the

maximum input voltage and a voltage rating of 1.5 times is a

conservative guideline. The RMS current requirement for a

converter design can be approximated with the aid of Figure 11.

Follow the curve for the number of active channels in the

converter design. Next determine the duty cycle for the

converter and find the intersection of this value and the active

channel curve. Find the corresponding y-axis value, which is the

current multiplier. Multiply the total full load output current, not

the channel value, by the current multiplier value found and the

result is the RMS input current which must be supported by the

input capacitors.

MOSFET SELECTION AND CONSIDERATIONS

The ISL6558 requires two N-Channel power MOSFETs per

active channel or more if parallel MOSFETs are employed.

total gate charge, and thermal management requirements.

In high-current PWM applications, the MOSFET power

dissipation, package selection and heatsink are the dominant

design factors. The power dissipation includes two loss

components; conduction loss and switching loss. These losses

are distributed between the upper and lower MOSFETs

according to duty cycle of the converter (see the equations

below). The conduction losses are the main component of

power dissipation for the lower MOSFETs, Q2 and Q4 of

Figure 1. Only the upper MOSFETs, Q1 and Q3 have

significant switching losses, since the lower device turn on and

off into near zero voltage.

The following equations assume linear voltage-current

transitions and do not model power loss due to the reverse-

recovery of the lower MOSFETs body diode. The gate-charge

losses are dissipated in the HIP660x drivers and don’t heat the

MOSFETs. However, large gate-charge increases the

switching losses. Ensure that both MOSFETs are within their

maximum junction temperature at high ambient temperature by

calculating the temperature rise according to package thermal-

resistance specifications. A separate heatsink may be

necessary depending upon MOSFET power, package type,

ambient temperature and air flow.

KNORM

VOUT

LxFSW

---------------------

=

(EQ. 11)

1.0

0.8

0.6

0.4

0.2

0

0

0.1

0.2

0.3

0.4

0.5

SINGLE

CHANNEL

2 CHANNEL

3 CHANNEL

4 CHANNEL

FIGURE 10. RIPPLE CURRENT vs DUTY CYCLE

ITOTAL

DKNORMxKCM

=

(EQ. 12)

0.5

0.4

0.3

0.2

0.1

0

0

0.1

0.2

0.3

0.4

0.5

SINGLE

CHANNEL

3 CHANNEL

4 CHANNEL

2 CHANNEL

FIGURE 11. CURRENT MULTIPLIER vs DUTY CYCLE

2







------------------------------------------------------------







2

----------------------------------------------------------

+

=

(EQ. 13)

2

r



–





---------------------------------------------------------------------------------

=

(EQ. 14)