ZXLB1600 datasheet(5/16 Pages) DIODES

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Device description

The device is a flyback boost converter, working in

discontinuous mode.

With reference to the chip block diagram and typical

application circuit, the operation of the device is as

follows:

Control loop

When 'EN' is high, the control circuits become active. The

high side of the coil is connected to the input via a large

PMOS isolating switch (MP) and the low side to ground via

NDMOS transistor (MN). The current in the coil is allowed

to build up to an internally defined level (nominally 200 to

300mA) before MN is turned off. The energy stored in the

coil is then transferred to the output capacitor (C2) via

diode (D1). The output voltage is sensed at pin 'SENSE' by

internal resistors R1 and R2 (which may be shunted

externally at pin 'FB') and compared to a reference voltage

(1.23V nominal). A comparator senses when the output

voltage is above that set by the reference and its output is

used to control the 'off' time of the output switch. The

control loop is self-oscillating, producing pulses of 10 s

maximum duration (switch 'on'), at a frequency that varies

in proportion to the output load current. The minimum

'off' time of the output switch is fixed at 1.25 s nominal, to

allow time for the coil's energy to be dissipated before the

switch is turned on again. This maintains stable and

efficient operation.

Setting output voltage

With external voltage

The output voltage is equal to the voltage present on

the 'ADJ' pin of the device multiplied by the internal

resistor network factor (R1+R2)/R1. The ADJ pin is

connected to the internal reference voltage of value

VADJ(nom) and by default will give a nominal output of

28V. However, if required, the ADJ pin may be

overdriven with an external dc voltage VADJ, in order to

adjust the output voltage to a value lower than the

default value.

ZXLB1600

ISSUE 3 - SEPTEMBER 2003

SEMICO NDUC TORS

BLOCK DIAGRAM

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