ZXLD1371Q

ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

36 of 42

www.diodes.com

September 2015

© Diodes Incorporated

ZXLD1371Q

In the event of more than one fault/warning condition occurring, the higher severity condition will take precedenc

e. 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 device start-

up for 100μs. 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 100μs the diagnostic is signaling an overcurrent 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 ZXLD1371Q

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

higher than 5.4V. The designer should also take into 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 ZXLD1371Q 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 MOSFET has a large

– see next section.

If the ZXLD1371Q 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 ZXLD1371Q 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.

VREF

0V

Over

Current

225mV/R1

0A

Out of

regulation

100us