ZXRD1033NQ16 데이터 시트보기 (PDF) - Zetex => Diodes

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ZXRD1033NQ16

HIGH EFFICIENCY SIMPLESYNC PWM DC-DC CONTROLLERS

Zetex => Diodes 

ZXRD1000 SERIES

DETAILED DESCRIPTION

The ZXRD1000 series can be configured to use either

N or P channel MOSFETs to suit most applications.

The most popular format, an all N channel

synchronous solution gives the optimum efficiency. A

feature of the ZXRD1000 series solution is the unique

method of generating the synchronous drive, called

SimpleSync . Most solutions use an additional

output from the controller, inverted and delayed from

the main switch drive. The ZXRD1000 series solution

uses a simple overwinding on the main choke (wound

on the same core at no real cost penalty) plus a small

ferrite bead . This means that the synchronous FET is

only enhanced when the main FET is turned off. This

reduces the ‘blanking period’ required for shoot-

through protection, increasing efficiency and allowing

smaller catch diodes to be used, making the controller

simpler and less costly by avoiding complex timing

circuitry. Included on chip are numerous functions that

allow flexibility to suit most applications. The nominal

switching frequency (200kHz) can be adjusted by a

simple timing capacitor, C3. A low battery detect circuit

is also provided. Off the shelf components are available

from major manufacturers such as Sumida to provide

either a single winding inductor for non-synchronous

applications or a coil with an over-winding for

synchronous applications. The combination of these

switching characteristics, innovative circuit design and

excellent user flexibility, make the ZXRD1000 series

DC-DC solutions some of the smallest and most cost

effective and electrically efficient currently available.

Using Zetex’s HDMOS low RDS(on) devices, ZXM64N02X

for the main and synchronous switch, efficiency can

peak at upto 95% and remains high over a wide range

of operating currents. Programmable soft start can also be

adjusted via the capacitor, C7, in the compensation loop.

What is SimpleSyncTM?

Conventional Methods

In the conventional approach to the synchronous

DC-DC solution, much care has to be taken with the

timing constraints between the main and synchronous

switching devices. Not only is this dependent upon

individual MOSFET gate thresholds (which vary from

device to device within data sheet limits and over

temperature), but it is also somewhat dependent upon

magnetics, layout and other parasitics. This normally

means that significant ‘dead time’ has to be factored

in to the design between the main and synchronous

devices being turned off and on respectively.

Incorrect application of dead time constraints can

potentially lead to catastrophic short circuit conditions

between VIN and GND. For some battery operated

systems this can not only damage MOSFETs, but also

the battery itself. To realise correct ‘dead time’

implementation takes complex circuitry and hence

implies additional cost.

The ZETEX Method

Zetex has taken a different approach to solving these

problems. By looking at the basic architecture of a

synchronous converter, a novel approach using the

main circuit inductor was developed. By taking the

inverse waveform found at the input to the main

inductor of a non-synchronous solution, a

synchronous drive waveform can be generated that is

always relative to the main drive waveform and

inverted with a small delay. This waveform can be

used to drive the synchronous switch which means no

complex circuitry in the IC need be used to allow for

shoot-through protection.

Implementation

Implementation was very easy and low cost. It simply

meant peeling off a strand of the main inductor

winding and isolating it to form a coupled secondary

winding. These are available as standard items

referred to in the applications circuits parts list.The use

of a small, surface mount, inexpensive ’square loop’

ferrite bead provides an excellent method of

eliminating shoot-through due to variation in gate

thresholds. The bead essentially acts as a high

impedance for the few nano seconds that

shoot-through would normally occur. It saturates very

quickly as the MOSFETs attain steady state operation,

reducing the bead impedance to virtually zero.

Benefits

The net result is an innovative solution that gives

additional benefits whilst lowering overall

implementation costs. It is also a technique that can

be simply omitted to make a non-synchronous

controller, saving further cost, at the expense of a few

efficiency points.

ISSUE 4 - OCTOBER 2000

5

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