ISL85415

7. Application Guidelines

FN8373 Rev.5.01

Page 24 of 30

Jun.4.20

7.4

Minimum On/Off-Time Limitation

Minimum on-time (tMIN_ON) is the shortest duration of time that the HS FET can be turned on and minimum off-

time (tMIN_OFF) is the shortest duration of time that the HS FET can be turned off. The typical tMIN_ON is 90ns and

the typical tMIN_OFF is 150ns. For a given tMIN_ON and tMIN_OFF, the higher the switching frequency, the narrower

the range of allowed duty cycle, which translates to a smaller range of allowed VIN.

For a given output voltage (VOUT) and switching frequency (fsw), the maximum allowed input voltage is given by

Equation 5:

The minimum allowed voltage is given by Equation 6:

Table 2 shows the recommended switching frequencies for the various VOUT to operate up to the maximum VIN of

36V.

7.5

Output Inductor Selection

The inductor value determines the converter’s ripple current. Choosing an inductor current requires a somewhat

arbitrary choice of ripple current, ΔΙ. A reasonable starting point is 30% of total load current. The inductor value

can then be calculated using Equation 7:

Increasing the value of inductance reduces the ripple current and threrefore the ripple voltage. However, the

larger inductance value can reduce the converter’s response time to a load transient. The inductor current rating

should be set so that it does not saturate in overcurrent conditions. For typical ISL85415 applications, inductor

values are generally in the 10µH to 47µH range. In general, higher VOUT results in higher inductance.

7.6

Buck Regulator Output Capacitor Selection

An output capacitor is required to filter the inductor current. The current mode control loop allows the use of low

ESR ceramic capacitors, so it supports very small circuit implementations on the PC board. Electrolytic and

polymer capacitors can also be used.

While ceramic capacitors offer excellent overall performance and reliability, the actual in-circuit capacitance must

be considered. Ceramic capacitors are rated using large peak-to-peak voltage swings and with no DC bias. In the

DC/DC converter application, these conditions do not reflect reality. As a result, the actual capacitance can be

considerably lower than the advertised value. Consult the manufacturers data sheet to determine the actual

in-application capacitance. Most manufacturers publish capacitance vs DC bias so that this effect can be easily

accommodated. The effects of AC voltage are not frequently published, but an assumption of ~20% further

reduction generally suffices. The result of these considerations can mean an effective capacitance 50% lower

than nominal and this value should be used in all design calculations. Nonetheless, ceramic capacitors are a very

good choice in many applications due to their reliability and extremely low ESR.

Table 2.

Recommended Switching Frequencies for Various VOUT

VIN (Maximum) (V)

VOUT (V)

fSW (kHz)

36

5

500

36

3.3

500

36

2.5

500

36

1.8

300

(EQ. 5)

VIN max



VOUT

fSW tMIN_ON



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

=

(EQ. 6)

VIN min



VOUT

1f

– SW tMIN_OFF



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

=

(EQ. 7)

L

VIN VOUT

–

fSW I



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

VOUT

VIN

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



=