ZXLB1600 datasheet(6/16 Pages) DIODES

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Filtered PWM operation

The input of an internal low pass filter is switched to

VREF when the EN pin is high and switched to ground

when the EN pin is low. The output of this filter drives

the comparator within the control loop. A continuous

high state on EN therefore provides a filtered voltage of

value Vref to the comparator and the control loop

regulates the output to a nominal value of 28V.

However, by varying the duty cycle (D) of the EN signal

at a suitably high frequency (f>10kHz), the control loop

will see a voltage, that has an average value equal to

the duty cycle multiplied by VREF. This provides a

means of adjusting the output voltage to a lower value.

It also allows the device to be both turned on and

adjusted with a single signal at the 'EN' pin. The output

during this mode of operation will be a dc voltage equal

to D x 28V

Gated PWM operation

The internal circuitry of the ZXLB1600 is turned off

when no signal is present on the 'EN' pin for more than

120 s (nominal). A low frequency signal applied to the

EN pin will therefore gate the device 'on' and 'off' at the

gating frequency and the duty cycle of this signal can

be varied to provide a 'chopped' output voltage equal

to D x 28V. For best accuracy, the gating frequency

should be made as low as possible (e.g. below 1kHz),

such that the turn off delay of the chip is only a small

proportion of the gating period.

Further details of setting output current are given in the

application notes.

Low battery detection

A comparator driving an open drain NMOS output

transistor performs the low battery flag function. The

detection threshold (battery voltage falling) is set to

1.98V nominal with internal resistors R3 and R4, but

this can be changed by shunting the internal potential

divider with two external resistors at pin 'LBT'.

Operation is such that the output transistor will be

turned on when the battery voltage falls below the

detection threshold.

VBLT = 1.21 x (R3+R4)/R4

A small amount (nominal 20mV) of hysteresis is

provided to aid clean switching. The low battery

detection circuit remains active when 'EN' is low.

ZXLB1600

ISSUE 3 - SEPTEMBER 2003

SEMICO NDUC TORS

ZXLB1600 Datasheet(PDF) 6 Page - Diodes Incorporated

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