Diodes ZXLD1370 - Manual (Page 31)

ZXLD1370
Document number: DS32165 Rev. 5 - 2
31 of 39
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September 2012
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ZXLD1370
A Product Line of
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Application Information
(cont.)
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.
V R E F
0 V
O v e r
C u r r e n t
2 2 5 m V / R 1
0 A
FLA
G
ST
AT
U
S
C
o
il current
O u t o f
r e g u l a t i o n
1 0 0 u s
Figure 36 Diagnostic During Start-up
Boosting V
AUX
Supply Voltage in Boost and Buck-Boost Mode
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 37, to generate a boosted voltage at V
AUX
when the input supply voltage at V
IN
is
below 8V. This enables the device to operate with full output current when V
IN
is at the minimum value of 6V. In the case of a low voltage
threshold MOSFET, the bootstrap circuit is generally not required.
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.
The Zener diode D3 is used to limit the voltage on the V
AUX
pin to less than 60V.
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.