LR8K4 Ver la hoja de datos (PDF) - Supertex Inc
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LR8K4 Datasheet PDF : 11 Pages
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LR8
Comparison with Discrete Startup Implementations
The LR8 provides several advantages when compared with discretely implemented start-up circuits.
Zener Implementation
VIN
VOUT
Transistor Implementation
VIN
VOUT
LR8 Implementation
VIN
LR8
VOUT
Disadvantages
• Continues to draw current from high voltage source after
supply has bootstrapped, resulting in inefficiencies
• Bias must be set for minimum input voltage, resulting in
high current drain at high input voltages
• Poor regulation
• No current limit
• No overtemperature protection
• In the Zener implementation, requires large power resistor
and Zener
Advantages
• LR8 goes into standby mode after supply has
bootstrapped, drawing no current from high voltage
input
• Good regulation
• Built-in current limiting
• Overtemperature protection
Exceeding LR8’s Current Limit for Startup Applications
The LR8 has a built-in current limit of 10mA minimum. If the current drawn by the PWM controller exceeds this limit, the LR8 may still be
used. To do this, the LR8’s output capacitor supplies a portion of the current until the power supply can bootstrap itself and the LR8 is no
longer needed. The following figure graphically illustrates how this is accomplished.
Most PWM controllers have an undervoltage lockout (UVL) circuit or programmable start/stop voltages. When the voltage supplied to the
PWM controller reaches the turn-on threshold, the controller begins operating and consuming current. If current exceeds the current limit
for the LR8, the voltage at VOUT begins to decay. With a large enough capacitor, the supply will bootstrap before voltage decays to the turn-
off threshold.
VIN 1
VOUT
4
VBOOT
IPWM
ILR8
tBOOT
2
3
10mA
VHYS
0mA
1 Input voltage is applied. The LR8 begins operating (in current
limiting mode since COUT appears as a short). VOUT begins to
rise as COUT charges. The PWM controller draws a small
amount of current.
2 The output voltage of the LR8 reaches the PWM controller’s
turn-on threshold. Controller begins operating, drawing current.
Bootstrap voltage begins climbing while VOUT decays since
current drawn by the controller exceeds the LR8’s current limit.
3 Bootstrap voltage reaches the level of the LR8’s output and
takes over. LR8 current drops to zero.
4 Power supply reaches steady-state operation.
10
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