ISL6569ACR datasheet(19/22 Pages) RENESAS

19 / 22 page

ISL6569A

FN9092 Rev 2.00

Page 19 of 22

Dec 29, 2004

In the solutions to the compensation equations, there is a

single degree of freedom. For the solutions presented in

Equations 28, RFB is selected arbitrarily. The remaining

compensation components are then selected according to

Equations 28.

In Equations 28, L is the per-channel filter inductance

divided by the number of active channels; C is the sum total

of all output capacitors; ESR is the equivalent-series

resistance of the bulk output-filter capacitance; and VPP is

the peak-to-peak sawtooth signal amplitude as described in

Figure 6 and Electrical Specifications.

Input Supply Voltage Selection

The VCC input of the ISL6569A can be connected to either a

+5V supply directly 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 approximately 20mA.

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 Input Supply Voltage Selection.

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 figure Typical

Application on page 4). Figure 15 and Equation 29 are

provided to assist in the 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 16 to determine the

input-capacitor RMS current requirement given the duty

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

of the combined peak-to-peak inductor current (IC,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.

FIGURE 14. COMPENSATION CIRCUIT FOR ISL6569A BASED

CONVERTER WITHOUT LOAD-LINE

REGULATION

COMP

RFB

IOUT

VDIFF

RFB

CESR



LC C ESR



–

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

LC C ESR



–

RFB

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

0.75VIN



2f

0fHF LCRFBVPP

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

(EQ. 28)

VPP 2



 2f

0fHFLCRFB

0.75 VIN 2fHF LC 1

–





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

0.75VIN 2fHF LC 1

–







2f

0fHF LCRFBVPP

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

FIGURE 15. RT vs SWITCHING FREQUENCY

100

1000

10000

SWITCHING FREQUENCY (kHz)

100

1000

11.09 1.13



log

–



(EQ. 29)

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