ZXLD1370

Document number: DS32165 Rev. 6 - 2

31 of 40

www.diodes.com

December 2016

© Diodes Incorporated

ZXLD1370

Diagnostic signals should be ignored during the device start-

up for 100μs. 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 100μs the diagnostic is signaling

an overcurrent then an out-of-regulation status. These two

events are due to the charging of the inductor and are not true

fault conditions.

VREF

0V

Over

Current

225mV/R1

0A

Out of

regulation

100us

Figure 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

threshold MOSFET, the bootstrap circuit is generally not required.

R1

L

ISM

VIN

VAUX

VIN

C3

D1

Drain of

external

switch

D2

LED1..n

R2

D3

Buck-Boost

mode

Boost

mode

Figure 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.