ZXRD1033NQ16TA 데이터 시트보기 (PDF) - Zetex => Diodes

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ZXRD1033NQ16TA

HIGH EFFICIENCY SIMPLESYNC PWM DC-DC CONTROLLERS

Zetex => Diodes 

ZXRD1000 SERIES

Applications (continued)

Inductor Selection

The inductor is one of the most critical components in

the DC-DC circuit.There are numerous types of devices

available from many suppliers. Zetex has opted to

specify off the shelf encapsulated surface mount

components, as these represent the best compromise

in terms of cost, size, performance and shielding.

The SimpleSyncTM technique uses a main inductor

with an overwinding for the gate drive which is

available as a standard part. However, for engineers

who wish to design their own custom magnetics, this

is a relatively simple and low cost construction

technique. It is simply formed by terminating one of

the multiple strands of litz type wire separately. It is

still wound on the same core as the main winding and

only has to handle enough current to charge the gate

of the synchronous MOSFET. The major benefit is

circuit simplification and hence lower cost of the control

IC. For non-synchronous operation, the overwinding is

not required.

The choice of core type also plays a key role. For

optimum performance, a ’swinging choke’ is often

preferred. This is one which exhibits an increase in

inductance as load current decreases. This has the net

effect of reducing circulating current at lighter load

improving efficiency. There is normally a cost

premium for this added benefit. For this reason the

chokes specified are the more usual constant

inductance type.

Peak current of the inductor should be rated to

minimum 1.2IOUT (max) . To maximise efficiency, the

winding resistance of the main inductor should be less

than the main switch output on resistance.

Schottky Diode

Selection depends on whether a synchronous or

non-synchronous approach is taken. For the

ZXRD1000, the unique approach to the synchronous

drive means minimal dead time and hence a small

SOT23 1A DC rated device will suffice, such as the

ZHCS1000 from Zetex. The device is only designed to

prevent the body diode of the synchronous MOSFET

from conducting during the initial switching transient

until the MOSFET takes over. The device should be

connected as close as possible to the source terminals

of the main MOSFET.

For non-synchronous applications , the Schottky diode

must be selected to allow for the worst case

conditions, when VIN is at its highest and VOUT is

lowest (short circuit conditions for example). Under

these conditions the device must handle peak current

at close to 100% duty cycle.

Frequency Adjustment

The nominal running frequency of the controller is set

to 200kHz in the applications shown. This can be

adjusted over the range 50kHz to 300kHz by changing

the value of capacitor on the CT pin. A low cost

ceramic capacitor can be used.

Frequency = 60000/C3 (pF)

Frequency v temperature is given in the typical

characteristics.

Output Voltage Adjustment

The ZXRD1000 is available as either a fixed 5V, 3.3V or

adjustable output. On fixed output versions, the VFB pin

should be connected to the output. Adjustable operation

requires a resistive divider connected as follows:

The value of the output voltage is determined by the

equation

VOUT = VFB (1 +RRAB ) VFB =1.24V

Note: The adjustable circuit is shown in the following

transient optimisation section. It is also used in the

evaluation PCB. In both these circuits RA is assigned

the label R6 and RB the label R5.

Values of resistor should be between 1k and 20k to

guarantee operation. Output voltage can be adjusted in

the range 2V to 12V for non-synchronous applications.

For synchronous applications, the minimum VOUT is set

by the VGS threshold required for the synchronous

MOSFET, as the swing in the gate using the

SimpleSyncTM technique is approximately VOUT.

8

ISSUE 4 - OCTOBER 2000

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