Low-Noise switching power supply is ideal for AM and FM bands in automotive radio systems

Output Voltage and Switching Frequency

To have 8V regulated on the OUTB node, it is necessary to choose the proper feedback resistor-divider (composed of the R₂₂ and R₂₁ resistors). Note first that the data sheet for IC2 recommends low-side resistors of less than 100kΩ. Choosing a 51kΩ low-side resistor-divider for R₂₂, the high-side resistor-divider must be selected using the Equation 1:

(Equation 1)

Where V_FB = 1V (typ).

Selecting a standard resistor value of 360kΩ for R₂₂, the typical resulting output voltage value is:

(Equation 2)

Assuming that the resistors have a 1% tolerance, the minimum and maximum voltage values for the entire switching power supply (OUTB) are:

(Equation 3)

(Equation 4)

Where V_FB(MIN) is 0.985V and V_FB(MAX) is 1.015V.

As recommended in the data sheet, the external frequency must be higher than 110% of the IC’s internal selected frequency. Since we are synchronizing IC2’s switching frequency with an external 2MHz signal, we must choose an R₁₆ internal oscillator resistance that imposes an internal switching frequency less than 1.8MHz.

For this reason, a 30kΩ resistor was selected for R_16. To make IC2 switch at a 2MHz fixed frequency, it is necessary to avoid the dropout condition. This IC avoids dropout until the turn-off time (t_OFF) is higher than 100ns (typ). This implies that the system can never go over a maximum duty cycle of:

    (Equation 5)

Considering an efficiency (Eff) of 90% for the step-down regulator IC2, the minimum input voltage (OUTA) that is able to ensure 2MHz fixed frequency switching is:

(Equation 6)

This means that the OUTA voltage must never be less than the 11.11V threshold. To guarantee an OUTA voltage that is always above 11.11V, IC1 must be enabled when the battery voltage (IN node) goes below 11.5V. This leaves a margin of approximately 390mV for voltage drop across the L₁ inductor and D₂ Schottky diode.

During the 40V load-dump peak, the OUTA voltage reaches its higher voltage value and IC2 must regulate the 8V on its output. Thus, during load-dump peak, IC2 should operate with a duty cycle of:

                                                                             (Equation 7)

The device’s minimum turn-on time (t_ON) is 80ns (typ), which enables it to reach a minimum duty cycle of:

                                                                              (Equation 8)

at a 2MHz switching frequency.

A minimum duty cycle of 0.16 ensures the 8V regulation during the 40V load dump.

Inductor and Current Sense

 

Figure 3. The inductor current for IC2 (MAX16952).

If you reduce the inductor peak current by using a large inductance value, you increase the efficiency of the IC2. Accomplishing this, however, requires more printed-circuit board (PCB) area and degrades the load regulation. As an acceptable compromise, choose the inductor value so you obtain an LIR (the ratio of the inductor peak-to-peak AC current to the DC average current) equal or lower than 0.3. Consider the following equations based on Figure 3:

(Equation 9)

(Equation 10)

(Equation 11)

Merging these equations together, a single formula can indicate the L value:

(Equation 12)

Thus, the minimum inductor value to reach an LIR factor equal or lower than 0.3 during a normal condition (OUTA = 12V) is:

  (Equation 13)

Using a standard inductor L₂ of 2.2µH results in a LIR factor of 0.24 with a peak inductor current of:

   (Equation 14)

The current limit is triggered when the voltage across the R₂₀ sense resistor reaches 68mV (min). Leaving margin for the tolerance on the inductor, the sense resistor has been sized to have a voltage drop of 60% of the current-limit threshold when the inductor current reaches its peak value (I_PEAK):

(Equation 15)

Therefore, a standard 15mΩ resistor has been chosen for R₂₀.