ISL6432CB Просмотр технического описания (PDF) - Renesas Electronics

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ISL6432CB

PWM and Triple Linear Power Controller for Home Gateway Applications

Renesas Electronics 

ISL6432

functionality. Read the ‘ACPI Implementation’ section under

‘Application Guidelines’ to see if this additional constraint

concerns your application. To ensure the parallel combination

of the feedback resistors equals a certain chosen value, RFB,

use the following equations:

RS

=

V-----O----U----T--

VFB



RFB

RP = V----R-O----SU----T-----–-V----VF----BF----B- , where

VOUT - the desired output voltage,

VFB - feedback (reference) voltage, 0.8V.

Application Guidelines

Soft-Start Interval

The soft-start function controls the output voltages rate of rise to

limit the current surge at start-up. The soft-start function is

integrated on the chip and the soft-start interval is thus fixed.

Layout Considerations

MOSFETs switch very fast and efficiently. The speed with

which the current transitions from one device to another

causes voltage spikes across the interconnecting

impedances and parasitic circuit elements. The voltage

spikes can degrade efficiency, radiate noise into the circuit,

and lead to device overvoltage stress. Careful component

layout and printed circuit design minimizes the voltage spikes

in the converter. Consider, as an example, the turn-off

transition of the upper PWM MOSFET. Prior to turn-off, the

upper MOSFET was carrying the full load current. During the

turn-off, current stops flowing in the upper MOSFET and is

picked up by the lower MOSFET or Schottky diode. Any

inductance in the switched current path generates a large

voltage spike during the switching interval. Careful

component selection, tight layout of the critical components,

and short, wide circuit traces minimize the magnitude of

voltage spikes.

There are two sets of critical components in a DC-DC

converter using a ISL6432 controller. The switching power

components are the most critical because they switch large

amounts of energy, and as such, they tend to generate equally

large amounts of noise. The critical small signal components

are those connected to sensitive nodes or those supplying

critical bypass current.

The power components and the controller IC should be placed

first. Locate the input capacitors, especially the high-frequency

ceramic decoupling capacitors, close to the power switches.

Locate the output inductor and output capacitors between the

MOSFETs and the load. Locate the PWM controller close to

the MOSFETs.

+5VIN

LIN

+

CIN

VOUT2

+

COUT2

+12V

CVCC

VCC GND

OCSET

COCSET

ROCSET

UGATE

PHASE

Q1

LOUT

VOUT1

DRIVE2

Q3

LGATE

Q2

COUT1+

CR1

VOUT3

+

COUT3

Q4

ISL6432

VOUT4

DRIVE3 DRIVE4

+

COUT4

PGND

Q5

+3.3VIN

KEY

ISLAND ON POWER PLANE LAYER

ISLAND ON CIRCUIT OR POWER PLANE LAYER

VIA CONNECTION TO GROUND PLANE

FIGURE 4. PRINTED CIRCUIT BOARD POWER PLANES AND

ISLANDS

The critical small signal components include the bypass

capacitor for VCC and the feedback resistors. Locate these

components close to their connecting pins on the control IC.

A multi-layer printed circuit board is recommended. Figure 4

shows the connections of the critical components in the

converter. Note that the capacitors CIN and COUT each

represent numerous physical capacitors. Dedicate one solid

layer for a ground plane and make all critical component ground

connections with vias to this layer. Dedicate another solid layer

as a power plane and break this plane into smaller islands of

common voltage levels. The power plane should support the

input power and output power nodes. Use copper filled polygons

on the top and bottom circuit layers for the PHASE nodes, but

do not unnecessarily oversize these particular islands. Since the

PHASE nodes are subjected to very high dV/dt voltages, the

stray capacitor formed between these islands and the

surrounding circuitry will tend to couple switching noise. Use the

remaining printed circuit layers for small signal wiring. The wiring

traces from the control IC to the MOSFET gate and source

should be sized to carry 2A peak currents.

PWM Controller Feedback Compensation

The PWM controller uses voltage-mode control for output

regulation. This section highlights the design consideration for

a PWM voltage-mode controller. Apply the methods and

considerations only to the PWM controller.

Figure 5 highlights the voltage-mode control loop for a

synchronous-rectified buck converter. The output voltage

(VOUT) is regulated to the Reference voltage level, 0.8V. The

error amplifier (Error Amp) output (VE/A) is compared with the

FN9019 Rev 0.00

Jun 1, 2001

Page 7 of 12

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