ZXLD1371

ZXLD1371

Document number: DS35436 Rev. 1 - 2

37 of 42

www.diodes.com

February 2012

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In the event of more than one fault/warning condition occurring, the higher severity condition will take precedence. E.g.

‘Excessive coil current’ and ‘Out of regulation’ occurring together will produce an output of 0.9V on the STATUS pin.

Diagnostic signals should be ignored during the device

start – up for 100

μs. The device start up sequence will

be initiated both during the first power on of the device

or after the PWM signal is kept low for more than

15ms, initiating the standby state of the device.

In particular, during the first 100

μs the diagnostic is

signaling an over-current then an out-of-regulation

status. These two events are due to the charging of

the inductor and are not true fault conditions.

Figure 16. Diagnostic during Start-up

Reduced Input Voltage Operation

To facilitate operation in automotive and other applications, that have large transient reductions in system supply voltage,

the ZXLD1371 is now capable of operating down to input voltages as low as 5.0V. Care must be taken when operating at

these lower supply voltages to ensure that the external MOSFET is correctly enhanced and that the boosting ratio is not

increased to excessive amounts where both the duty cycle and peak-switch current limits are not exceeded. The device

account any noise that may occur on the supply lines.

In Buck-boost and Boost modes (most common topologies for applications likely to require transient operation down to

supply voltages approaching 5.0V) as the input voltage reduces then the peak switch current will increase the ZXLD1371

compensates for this by allowing the sense voltage to increase while maintaining regulation of the LED current. However if

the boost ratio (switch output voltage/input voltage) is increased too much then the sense voltage could be increased too

much causing an over-current flag to be triggered and/or loss of regulation.

In addition to this, increased power dissipation will occur in the external MOSFET switch – especially if the external

section.

If the ZXLD1371 is used in buck mode at low voltages then the boot-strap network cannot be implemented and so a low

threshold MOSFET with low gate capacitance should be used. Some loss of regulation is expected to occur at voltages

below 6V – see Buck mode Typical Characteristics Section.

When using the ZXLD1371 in applications with transient input voltage excursions we recommend using the web calculator

to optimize operation over the normal operating band. Then change the input range to include the transient excursion while

keeping the optimized component selection to check expected function during the transient input voltage conditions.

VR EF

0V

Ov e r

Cu rre n t

225 m V /R 1

0A

Ou t o f

re gulation

100us