ZXLB1600 datasheet(11/16 Pages) ZETEX

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APPLICATION NOTES

Adjusting output voltage

When connected as shown in the typical application

circuit, the ZXLB1600 will produce a nominal output

voltage of 28V. This can be adjusted by one of the three

methods described below.

1) Output voltage adjustment by external resistors

The internal reference and resistor divider network R1

and R2 set a nominal output of 28V. However, this

network is accessible at the FB pin and can be shunted

by means of external resistors to set different nominal

output voltages. The potential divider defines output

voltage according to the relationship:

VOUT(dc) = (R1+R2)/R1 x 1.23V

When using external resistors, these should be chosen

with lower values than the internal resistors to

minimize errors caused by the ±25% absolute value

variation of the internal resistors. The internal resistors

have high values in order to minimize these errors.

The following table gives suggested E24/E96 resistor

values for various output voltages.

Required

output

voltage

External resistor

across R2

External resistor

across R1

280k

91k

12V

715k

82k

18V

1 M

75k

20V

1.15 M

75k

22V

1.15M

68.1k

25V

1.2 M

62k

2) Output adjustment by external voltage

The internal voltage reference (Pin ADJ) may be

overdriven by an external control voltage to set the

output voltage. The relationship between applied

voltage (VADJ) and output voltage (VOUT) is:

VOUT = 22.86 x VADJ

Note that the output can be set to any value between

the input voltage and the maximum operating voltage

in this way. However, some non-linearity in the above

expression may occur at values of VADJ below

approximately 0.5V.

Also note that when driving the ADJ pin, the control

voltage must have sufficiently low impedance to sink

the bias current of the internal reference.(10 A max).

3) PWM output adjustment

A Pulse Width Modulated (PWM) signal can be applied

to the EN pin in order to adjust the output voltage to a

value below the value set in in 1) or 2). This method of

adjustment permits the device to be turned on and the

output voltage set by a single logic signal applied to the

EN pin. No external resistors or capacitors are required

and the amplitude of the control signal is not critical,

providing it conforms to the limits defined in the

electrical characteristics.

Two modes of adjustment are possible as described

below:

Filtered 'DC' mode

If a PWM signal of 10kHz or higher is applied to the EN

pin, the device will remain active when the EN pin is

low. However, the input to the internal low pass filter

will be switched alternately from VREF to ground, with a

duty cycle (D) corresponding to that of the PWM signal.

This will present a filtered dc voltage equal to the duty

cycle multiplied by VREF to the control loop and will

produce a dc output voltage lower than the maximum

set value. This voltage is given by:

VOUT = 28 x D

A square wave signal applied to the EN pin, for

example, will turn the device on and produce a nominal

regulated output of 14V.

ZXLB1600

ISSUE 3 - SEPTEMBER 2003

SEMICO NDUC TORS

ZXLB1600 Datasheet(PDF) 11 Page - Zetex Semiconductors

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