ZXLD1371QESTTC datasheet(38/42 Pages) DIODES

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ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

38 of 42

www.diodes.com

September 2015

ZXLD1371Q

Application Information (cont.)

Overvoltage Protection

The ZXLD1371Q 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 Overvoltage-Protection (OVP) network should be provided externally to

the MOSFET to avoid damage due to open circuit conditions. This is shown in Figure 19 below, highlighted in the dotted blue box.

Figure 19 OVP Circuit

The zener voltage is determined according to: Vz = VLEDMAX +10% where VLEDMAX is maximum LED chain voltage.

If the LEDA voltage exceeds VZ 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 Vz. If the voltage at LEDA remains above VZ for longer than 20ms then the ZXLD1371Q will enter

into a shutdown state.

Care should be taken to ensure that the maximum gate voltage of the Q2 MOSFET is not exceeded.

Take care of the max voltage drop on the Q2 MOSFET gate. Typical devices for Z1 and Q2 are BZX84C and 2N7002

ZXLD1371QESTTC Datasheet(PDF) 38 Page - Diodes Incorporated