Zxld1371, Applications information – Diodes ZXLD1371 User Manual
Page 38
ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
38 of 42
February 2012
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Applications Information
(cont.)
Boosting V
AUX
Supply Voltage in Boost and Buck-Boost Mode
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 boosted voltage at V
AUX
. This enables the device to operate with full output current when V
IN
is at the minimum value of 5V. 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. A typical value would be 100 ohms. 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 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 MOSFET’s power dissipation.
Figure 18. Effect of Bootstrap on LED Current in Buck-Boost Mode
6
5
7
8
10
9
11
12
13
14
15
16
17
18
V (V)
IN
T = 25 C, L = 33µH
R = 150m , R9 = 120k
R10 = 36k
A
S
°
Ω
Ω
Ω
0.25
0.27
0.29
0.33
0.31
0.35
0.37
0.39
0.41
0.43
0.45
LE
D
C
U
R
R
E
N
T
(A
)
5 LEDs
5 LEDs Bootstrap