ZXLD1370_11 DIODES [Diodes Incorporated], ZXLD1370_11 Datasheet - Page 27

• Manufacturer Part Number: ZXLD1370_11
• Description: 60V HIGH ACCURACY BUCK/BOOST/BUCK-BOOST LED DRIVER CONTROLLER
• Manufacturer: DIODES [Diodes Incorporated]

ZXLD1370
Document number: DS32165 Rev. 3 - 2
Applications Information
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 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.
Boosting V
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
used, as shown below in figure 36, to generate a boosted voltage at V
This enables the device to operate with full output current when V
voltage threshold MOSFET, the bootstrap circuit is generally not required.
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.
The Zener diode D3 is used to limit the voltage on the V
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.
AUX
supply voltage in Boost and Buck-Boost mode
Fig 36. Bootstrap circuit for Boost and Buck-boost low voltage operations
(Continued)
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AUX
27 of 35
pin to less than 60V.
V R E F
IN
0 V
0 A
is at the minimum value of 6V. In the case of a low
AUX
Fig 35. Diagnostic during Start-up
when the input supply voltage at V
C u r r e n t
O v e r
1 0 0 u s
Diodes Incorporated
A Product Line of
O u t o f
r e g u l a t i o n
© Diodes Incorporated
ZXLD1370
IN
February 2011
is below 8V.
2 2 5 m V / R 1