ZXLD1370

ZXLD1370

Document number: DS32165 Rev. 4 - 2

28 of 36

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March 2011

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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.

VR E F

0V

Ov e r

C ur r ent

22 5m V / R 1

0A

Ou t o f

r egu l at i o n

100 us

Fig 36. Diagnostic during Start-up

When the input voltage is lower than 8V, the gate voltage will also be lower 8V. This means that depending on the

characteristics of the external MOSFET, the gate voltage may not be enough to fully enhance the power MOSFET. This

boosting technique is particularly important when the output MOSFET is operating at full current, since the boost circuit

allows the gate voltage to be higher than 12V. This guarantees that the MOSFET is fully enhanced reducing both the power

dissipation and the risk of thermal runaway of the MOSFET itself. An extra diode D2 and decoupling capacitor C3 can be

voltage threshold MOSFET, the bootstrap circuit is generally not required.

Fig 37. Bootstrap circuit for Boost and Buck-boost low voltage operations

The resistor R2 can be used to limit the current in the bootstrap circuit in order to reduce the impact of the circuit itself on the

LED accuracy. The impact on the LED current is usually a decrease of maximum 5% compared to the nominal current

value set by the sense resistor.

Due to the increased number of components and the loss of current accuracy, the bootstrap circuit is recommended only

when the system has to operate continuously in conditions of low input voltage (between 6 and 8V) and high load current.

Other circumstances such as low input voltage at low load current, or transient low input voltage at high current should be

evaluated keeping account of the external MOSFET power dissipation.