10

LR8

Comparison with Discrete Startup Implementations

The LR8 provides several advantages when compared with discretely implemented start-up circuits.

Zener Implementation

Transistor Implementation

LR8

LR8 Implementation

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

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

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

off threshold.

10mA

0mA

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4

2

3

1

Input voltage is applied. The LR8 begins operating (in current

amount of current.

The output voltage of the LR8 reaches the PWM controller’s

turn-on threshold. Controller begins operating, drawing current.

current drawn by the controller exceeds the LR8’s current limit.

Bootstrap voltage reaches the level of the LR8’s output and

takes over. LR8 current drops to zero.

Power supply reaches steady-state operation.

3

4

2