Selecting Critical Components: Components Related to Power Supply IC BO (Brownout) Pins

In succession to the previous discussion of components related to the VCC pin, here we determine the constants for components related to the BO pin of a power supply IC used in the design. The BO pin is used to set the brownout function for the BD7682FJ power supply IC.

What is Brownout Function?

The brownout function is a protective function to stop switching operation if the input voltage VIN goes lower than the voltage required for normal operation. The BO pin is used for this function; for example, at startup, switching operation is not started until the BO pin voltage exceeds the brownout detection voltage value, so that an indefinite output voltage is not generated. Moreover, should the VIN fall during operation, switching operation is halted when the BO pin voltage falls below the brownout detection voltage value. There is no latching when operation is stopped, and when the VIN recovers and the BO pin voltage exceeds the brownout detection voltage value, operation is resumed automatically.

Brownout Setting Resistors R7, R8, R9, R10, R15 and BO Pin Capacitor C8

This circuit diagram shows the resistors and capacitor required by the BO pin, and the internal functional block. The BO pin is connected to the non-inverting input of a voltage comparator within the IC, and a reference voltage of 1.00 V is applied to the inverting input of the comparator. By this means, the brownout detection voltage V_BO is defined as a typical value of 1.00 V, with minimum and maximum values of 0.92 V and 1.08 V.

VIN is divided by resistors R_H and R_L to obtain the voltage to input to the BO pin, to control starting and stopping of switching operation with 1 V as the threshold value. In other words, R_H and R_L set the switching starting and stopping voltages.

Moreover, the following brownout detection hysteresis current I_BO must be considered when calculating the resistance values.

V_BO＜1V（switching-stopped state）：I_BOsink present
V_BO≧1V（switching operation state）：I_BOsink not present

I_BOis defined as 15 μA typical, with minimum and maximum values of 10 μA and 20 μA.

Below, an example of settings for R_H and R_L is shown.
If a VIN that starts switching operation (low→high) is V_INON, and a VIN that stops switching operation (high→low) is V_INOFF, then

From these equations, the following equations for R_H and R_L can be obtained. If V_INON=90 V, V_INOFF=60 V, V_BO=1 V, and I_BO=15 μA, then

In the circuit shown above, we have set
R_H＝R7＋R8＋R9＋R10＝470 kΩ+470 Ω+470 Ω+470 Ω＝1.88 MΩ、R_L=R15＝33 kΩ

The capacitor C8 should always be used, since the BO line has high impedance and so is sensitive to noise. As a rule, it should be 0.01 μF to 1 μF. In the above circuit, a value of 0.1 μF was chosen.