ISL62391, ISL62392, ISL62391C, ISL62392C

FN6666 Rev 8.00

Page 18 of 22

August 25, 2015

load transient has an extremely high slew rate. Low inductance

capacitors should be considered in this scenario. A capacitor

dissipates heat as a function of RMS current and frequency. Be

capacitors so that they operate below the maximum rated RMS

capacitor can fade as much as 50% as the DC voltage across it

increases.

Selection of the Input Capacitor

The important parameters for the bulk input capacitance are

the voltage rating and the RMS current rating. For reliable

operation, select bulk capacitors with voltage and current

ratings above the maximum input voltage and capable of

supplying the RMS current required by the switching circuit.

Their voltage rating should be at least 1.25x greater than the

maximum input voltage, while a voltage rating of 1.5x is a

preferred rating. Figure 28 is a graph of the input RMS ripple

current (normalized relative to output load current) as a function

of duty cycle and is adjusted for a converter efficiency of 80%.

The ripple current calculation is written as Equation 21:

Where:

- x is a multiplier (0 to 1) corresponding to the inductor

peak-to-peak ripple amplitude expressed as a percentage

- D is the duty cycle that is adjusted to take into account the

efficiency of the converter which is written as Equation 22.

In addition to the bulk capacitance, some low ESL ceramic

capacitance is recommended to decouple between the drain of

the high-side MOSFET and the source of the low-side

MOSFET.

MOSFET Selection and Considerations

Typically, a MOSFET cannot tolerate even brief excursions

beyond their maximum drain to source voltage rating. The

MOSFETs used in the power stage of the converter should

voltage tolerance of the input power source and the voltage

spike that occurs when the MOSFET switches off.

There are several power MOSFETs readily available that are

optimized for DC/DC converter applications. The preferred

high-side MOSFET emphasizes low gate charge so that the

device spends the least amount of time dissipating power in

the linear region. Unlike the low-side MOSFET, which has the

drain-source voltage clamped by its body diode during turn off,

minimize conduction loss. It should be noted that this is an

optimal configuration of MOSFET selection for low duty cycle

applications (D < 50%). For higher output, low input voltage

solutions, a more balanced MOSFET selection for high- and

low-side devices may be warranted.

For the low-side (LS) MOSFET, the power loss can be assumed

to be conductive only and is written as Equation 23:

For the high-side (HS) MOSFET, the conduction loss is written

as Equation 24:

For the high-side MOSFET, the switching loss is written as

Equation 25:

Where:

inductor current minus 1/2 of the inductor ripple current

current plus 1/2 of the inductor ripple current

Selecting The Bootstrap Capacitor

The selection of the bootstrap capacitor is written as Equation

26:

(EQ. 21)



DD

2

–



D

x

2

12

------







+

=

D



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

=

(EQ. 22)

FIGURE 28. NORMALIZED RMS INPUT CURRENT

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

DUTY CYCLE

x = 1

x = 0.75

x = 0.50

x = 0.25

x = 0

(EQ. 23)



DS ON

–







(EQ. 24)

DS ON



=

(EQ. 25)





2

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





2

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

+

=

V

BOOT

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

=

(EQ. 26)