ZXRD1033NQ16TA 데이터 시트보기 (PDF) - Zetex => Diodes

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ZXRD1033NQ16TA

HIGH EFFICIENCY SIMPLESYNC PWM DC-DC CONTROLLERS

Zetex => Diodes 

ZXRD1000 SERIES

Applications

Note: Component names refer to designators shown

in the application circuit diagrams.

Output Capacitors

Output capacitors are a critical choice in the overall

performance of the solution. They are required to filter

the output and supply load transient current. They are

also affected by the switching frequency, ripple

current, di/dt and magnitude of transient load current.

ESR plays a key role in determining the value of

capacitor to be used. Combination of both high

frequency, low value ceramic capacitors and low ESR

bulk storage capacitors optimised for switching

applications provide the best response to load

transients and ripple requirements. Electrolytic

capacitors with low ESR are larger and more

expensive so the ultimate choice is always a

compromise between size, cost and performance.

Care must also be taken to ensure that for large

capacitors, the ESL of the leads does not become an

issue. Excellent low ESR tantalum or electrolytic

capacitors are available from Sanyo OS-CON, AVX,

Sprague and Nichicon.

The output capacitor will also affect loop stability,

transient performance. The capacitor ESR should

preferably be of a similar value to the sense resistor.

Parallel devices may be required.

IRIPPLE(RMS)

=

0.29

VOUT

Lf

(VIN−VOUT)

VIN

where L= output filter inductance

f= switching frequency

For output voltage ripple it is necessary to know the

peak ripple current which is given by:

Ipk−pk

=

VOUT( VIN−VOUT)

L f VIN

Voltage ripple is then:-

VRIPPLE = Ipk −pk ∗ ESR

Input Capacitors

The input capacitor is chosen for its RMS current and

voltage rating. The use of low ESR electrolytic or

tantalum capacitors is recommended. Tantalum

capacitors should have their voltage rating at 2VIN

(max), electrolytic at 1.4VIN(max). IRMS c a n be

approximated by:

IRMS = IOUT

√(V OUT(VIN−VOUT) )

VIN

Underspecification of this parameter can affect long

term reliability. An additonal ceramic capacitor should

be used to provide high frequency decoupling at VIN.

Also note that the input capacitance ESR is effectively in

series with the input and hence contributes to efficiency

losses related to IRMS2 * ESR of the input capacitor.

MOSFET Selection

The ZXRD1000 family can be configured in circuits

where either N or P channel MOSFETs are employed

as the main switch. If an N channel device is used, the

corresponding N phase controller must be chosen.

Similarly, for P channel main switch a P phase

controller must be used. The ordering information has

a clear identifier to distinguish between N and P phase

controllers.

The MOSFET selection is subject to thermal and gate

drive considerations. Care also has to be taken to allow

for transition losses at high input voltages as well as

RDS(ON) l o s s e s f o r t h e m a i n MO SF ET . I t is

recommended that a device with a drain source

breakdown of at least 1.2 times the maximum VIN

should be used.

For optimum efficiency , two N channel low RDS(on)

devices are required. MOSFETs should be selected

with the lowest RDS(ON) consistent with the output

current required. As a guide, for 3-4A output, <50mΩ

devices would be optimum, provided the devices are

low gate threshold and low gate charge. Typically look

for devices that will be fully enhanced with 2.7V VGS

for 4-5A capability.

Zetex offers a range of low RDS(ON)logic level MOSFETs

which are specifically designed with DC-DC power

conversion in mind. Packaging includes SOT23,

SOT23-6 and MSOP8 options. Ideal examples of

optimum devices would be Zetex ZXM64N03X and

ZXM64N02X (N channel). Contact your local Zetex office

or Zetex web page for further information.

ISSUE 4 - OCTOBER 2000

7

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