Diodes ZXLD1370 - Manual (Page 32)

ZXLD1370
Document number: DS32165 Rev. 5 - 2
32 of 39
www.diodes.com
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Over-Voltage Protection
The ZXLD1370 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 no internal open-circuit protection
mechanism for the external MOSFET. In this case an Over-Voltage-Protection (OVP) network should be provided externally to the MOSFET to
avoid damage due to open circuit conditions. This is shown in figure 38 below, highlighted in the dotted blue box.
Figure 38 OVP circuit
The zener voltage is determined according to: Vz = V
LEDMAX
+10% where V
LEDMAX
is maximum LED chain voltage.
If the LEDA voltage exceeds V
Z
the gate of MOSFET Q2 will rise turning Q2 on. This will pull the PWM pin low and switch off Q1 until the voltage
on the drain of Q1 falls below V
Z
. If the voltage at LEDA remains above V
Z
for longer than 20ms then the ZXLD1370 will enter into a shutdown
state.
Care should be taken such that the maximum gate voltage of the Q2 MOSFET is not exceeded.
Take care of the max voltage drop on the Q2 MOSFET gate.