ISL6556B datasheet(21/24 Pages) RENESAS

21 / 24 page

ISL6556B

FN9097 Rev 4.00

Page 21 of 24

December 28, 2004

Input Supply Voltage Selection

The VCC input of the ISL6556B can be connected either

directly to a +5V supply or through a current limiting resistor to

a +12V supply. An integrated 5.8V shunt regulator maintains

the voltage on the VCC pin when a +12V supply is used. A

300

 resistor is suggested for limiting the current into the

VCC pin to a worst-case maximum of approximately 25mA.

Switching Frequency

There are a number of variables to consider when choosing

the switching frequency, as there are considerable effects on

the upper-MOSFET loss calculation. These effects are

outlined in MOSFETs, and they establish the upper limit for

the switching frequency. The lower limit is established by the

requirement for fast transient response and small output-

voltage ripple as outlined in Output Filter Design. Choose

the lowest switching frequency that allows the regulator to

meet the transient-response requirements.

Switching frequency is determined by the selection of the

frequency-setting resistor, RT (see the figures labeled

Typical Application on pages 4 and 5). Figure 13 and

Equation 27 are provided to assist in selecting the correct

value for RT.

Input Capacitor Selection

The input capacitors are responsible for sourcing the ac

component of the input current flowing into the upper

MOSFETs. Their RMS current capacity must be sufficient to

handle the ac component of the current drawn by the upper

MOSFETs which is related to duty cycle and the number of

active phases.

For a two phase design, use Figure 14 to determine the

input-capacitor RMS current requirement given the duty

cycle, maximum sustained output current (IO), and the ratio

of the per-phase peak-to-peak inductor current (IL,PP) to IO.

Select a bulk capacitor with a ripple current rating which will

minimize the total number of input capacitors required to

support the RMS current calculated. The voltage rating of

the capacitors should also be at least 1.25 times greater

than the maximum input voltage.

Figures 15 and 16 provide the same input RMS current

information for three and four phase designs respectively.

Use the same approach to selecting the bulk capacitor type

and number as described above.

Low capacitance, high-frequency ceramic capacitors are

needed in addition to the bulk capacitors to suppress leading

and falling edge voltage spikes. The result from the high

current slew rates produced by the upper MOSFETs turn on

and off. Select low ESL ceramic capacitors and place one as

close as possible to each upper MOSFET drain to minimize

board parasitic impedances and maximize suppression.

1.0203 10



10.6258- 1.03167



log



1200

–

(EQ. 27)

FIGURE 13. RT vs SWITCHING FREQUENCY

100

1000

10000

SWITCHING FREQUENCY (kHz)

100

1000

0.3

0.1

0.2

FIGURE 14. NORMALIZED INPUT-CAPACITOR RMS CURRENT

vs DUTY CYCLE FOR 2-PHASE CONVERTER

00.4

1.0

0.2

0.6

0.8

DUTY CYCLE (VO / VIN)

IL,PP = 0

IL,PP = 0.5 IO

IL,PP = 0.75 IO

DUTY CYCLE (VO / VIN)

FIGURE 15. NORMALIZED INPUT-CAPACITOR RMS CURRENT

vs DUTY CYCLE FOR 3-PHASE CONVERTER

00.4

1.0

0.2

0.6

0.8

0.3

0.1

0.2

IL,PP = 0

IL,PP = 0.25 IO

IL,PP = 0.5 IO

IL,PP = 0.75 IO

ISL6556B Datasheet(PDF) 21 Page - Renesas Technology Corp

ISL6556B Datasheet(HTML) 21 Page - Renesas Technology Corp

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL