Zxld1371, Applications information – Diodes ZXLD1371 User Manual
Page 37
ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
37 of 42
February 2012
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Applications Information
(cont.)
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.
If V
ADJ
>1.7V, V
SENSE
may be greater than the excess coil current threshold in normal operation and an error will be
reported. Hence, STATUS and FLAG are only guaranteed for V
ADJ
<=V
REF
.
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
will operate down to 5.0V, but for reliable start up V
IN
must be 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 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
MOSFET has a large threshold. One way of overcoming this is to apply a boot-strap network to the V
AUX
pin – see next
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.
V R E F
0 V
O v e r
C u rre n t
2 2 5 m V /R 1
0 A
FLA
G
S
T
AT
US
C
o
il current
O u t o f
re g u la tio n
1 0 0 u s