ZXLD1371Q

ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

37 of 42

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September 2015

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ZXLD1371Q

This means that depending on the characteristics of the external MOSFET, the gate voltage may not be enough to fully enhance the

power MOSFET. A boot-strap boosting technique can be used to increase the gate drive voltage at low input voltage. See Figure 17 for

circuit diagram. This can be particularly important for extended use at low input voltages as this is when the switch current will be at its

greatest

– resulting in greatest heat generation within the MOSFET.

Figure 17 Bootstrap Circuit for Boost and Buck-Boost Low Voltage Operations

The Bootstrap circuit guarantees that the MOSFET is fully enhanced reducing both the power dissipation and the risk of thermal runaway

of the MOSFET itself. The bootstrap circuit consists of an extra diode D2 and decoupling capacitor C3 which are used to generate a

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. A typical

value would be 100

Ω. 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 5 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

MOS ET’s power dissipation.

Figure 18 Effect of Bootstrap on LED Current in Buck-Boost Mode

R1

L

ISM

VIN

VAUX

VIN

C3

D1

Drain of

external

switch

D2

LED1..n

R2

D3

Buck-Boost

mode

Boost

mode