Zxld1374, Application information – Diodes ZXLD1374 User Manual
Page 28
ZXLD1374
Document number: DS35032 Rev. 3 - 2
28 of 39
September 2012
© Diodes Incorporated
ZXLD1374
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Over-Voltage Protection
The ZXLD1374 is inherently protected against open-circuit load when used in Buck configuration. However care has to be taken with open-
circuit load conditions in Buck-boost or Boost configurations. This is because in these configurations there is only an over-voltage FLAG but no
internal open-circuit protection mechanism for the internal MOSFET. In this case an Over-Voltage-Protection (OVP) network should be provided
to the MOSFET to avoid damage due to open circuit conditions. This is shown in Figure 35 below, highlighted in the dotted blue box.
Figure 35 OVP Circuit
The zener voltage is determined according to: V
Z
= V
LEDMAX
+10%. If the LX pin voltage exceeds V
Z
the gate of MOSFET Q1 will rise turning Q1
on. This will pull the PWM pin low and disable the LX output until the voltage on the LX falls below Vz. If the LX pin remains above V
Z
for longer
than 20ms then the ZXLD1374 will enter into a shutdown state.
Care should be taken such that the maximum gate voltage of the Q1 MOSFET is not exceeded.
An alternative solution for OVP function is to use the diagnostic section of the ZXLD1374 to initiate the disabling of the LX pin. For example, a
microcontroller could be used to respond to the FLAG and the status pins, and if an over-voltage state is indicated, the microcontroller could
switch the device off by pulling the PWM signal low.