ZXLD1371QESTTC datasheet(39/42 Pages) DIODES

39 / 42 page

ZXLD1371

Document number: DS35436 Rev. 1 - 2

39 of 42

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February 2012

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Applications Information (cont.)

Over-Voltage Protection

The ZXLD1371 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 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 ZXLD1371 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. Typical devices for Z1 and Q2 are BZX84C and 2N7002

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