ISL62391IRTZ datasheet(16/20 Pages) INTERSIL

16 / 20 page

FN6666.4

December 22, 2008

The LC output filter has a double pole at its resonant frequency

that causes rapid phase change. The R3 modulator used in the

ISL62391, ISL62392 makes the LC output filter resemble a first

order system in which the closed loop stability can be achieved

with the recommended Type-II compensation network. Intersil

provides a PC-based tool (example page is shown later) that

can be used to calculate compensation network component

values and help simulate the loop frequency response.

3.3V Linear Regulator

In addition to the two SMPS outputs, the ISL62391, ISL62392

also provides a fixed 3.3V LDO output (LDO3) capable of

sourcing 100mA continuous current. LDO3 draws its power

from PVCC and can be independently enabled from both

SMPS channels.

LDO3 also has a current limit feature with a nominal level of

180mA. Currents in excess of the limit will cause the LDO3

voltage to drop dramatically, limiting the power dissipation.

Thermal Monitor and Protection

LDO3 and PVCC LDOs can dissipate non-trivial power inside

the ISL62391, ISL62392 at high input-to-output voltage ratios

and full load conditions. To protect the silicon, ISL62391,

ISL62392 continually monitors the die temperature. If the

temperature exceeds +150°C, all outputs will be turned off to

sharply curtail power dissipation. The outputs will remain off

until the junction temperature has fallen below +135°C.

General Application Design Guide

This design guide is intended to provide a high-level

explanation of the steps necessary to design a single-phase

power converter. It is assumed that the reader is familiar with

many of the basic skills and techniques referenced in the

following section. In addition to this guide, Intersil provides

complete reference designs that include schematics, bills of

materials, and example board layouts.

Selecting the LC Output Filter

The duty cycle of an ideal buck converter is a function of the

input and the output voltage. This relationship is written as

Equation 16:

The output inductor peak-to-peak ripple current is written as

Equation 17:

A typical step-down DC/DC converter will have an IP-P of

20% to 40% of the maximum DC output load current. The

value of IP-P is selected based upon several criteria, such as

MOSFET switching loss, inductor core loss, and the resistive

loss of the inductor winding. The DC copper loss of the

inductor can be estimated by Equation 18:

Where ILOAD is the converter output DC current.

The copper loss can be significant so attention has to be

given to the DCR selection. Another factor to consider when

choosing the inductor is its saturation characteristics at

elevated temperature. A saturated inductor could cause

destruction of circuit components, as well as nuisance OCP

faults.

A DC/DC buck regulator must have output capacitance CO

into which ripple current IP-P can flow. Current IP-P develops a

corresponding ripple voltage VP-P across CO, which is the

sum of the voltage drop across the capacitor ESR and of the

voltage change stemming from charge moved in and out of

the capacitor. These two voltages are written as Equation 19:

and Equation 20:

If the output of the converter has to support a load with high

pulsating current, several capacitors will need to be paralleled

to reduce the total ESR until the required VP-P is achieved.

The inductance of the capacitor can cause a brief voltage dip

if the 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 sure that IP-P is shared by a sufficient quantity

of paralleled capacitors so that they operate below the

maximum rated RMS current at fSW. Take into account that

the rated value of a capacitor can fade as much as 50% as

the DC voltage across it increases.

ISL62391, ISL62392

RBOTTOM

CINT = 100pF

REF

FIGURE 27. COMPENSATION REFERENCE CIRCUIT

RTOP

RFB

CFB

COMP

---------

(EQ. 16)

(EQ. 17)

–

()

•

SW L

•

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

(EQ. 18)

COPPER

LOAD

DCR

•

ΔV

ESR

P-P E

•

(EQ. 19)

ΔV

P-P

O f

•

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

(EQ. 20)

ISL62391, ISL62392

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