MBR0540 datasheet(13/20 Pages) ONSEMI

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turns P1 on and current is routed to the internal bias circuitry

from the BIAS pin.

Here is an example of the power savings:

The input voltage range for Vin is 4.5 V to 40 V. The input

voltage range for BIAS is 3.3 V to 6 V. The quiescent current

specification is 3 mA (min), 4 mA (typ), and 6.25 mA (max).

Using a typical battery voltage of 14 V and the typical

quiescent current number of 4 mA, the power would be:

P + V

I + 14

4e−3 + 56 mW

We’ll assume the BIAS pin is connected to an external

regulator at 5 V instead of the output voltage. The BIAS pin

would normally be connected to the output voltage, but

adding an added switching regulator efficiency number here

would cloud this example. Now the internal BIAS circuitry

is being powered via 5 V. The resulting on chip power being

dissipated is:

P + V

I + 5

4e−3 + 21 mW

The power savings is 35 mW.

Now, to demonstrate more notable savings using the

maximum battery input voltage of 40 V, the maximum

quiescent current of 6.25 mA, and the lowest allowed BIAS

voltage for proper operation of 3.3 V;

Powered from Vin:

P + 40

6.25e−3 + 250 mW

Powered from the BIAS pin:

P + 3.3

6.25e−3 + 21 mW

The power savings is 229 mW.

Minimum Load Requirement

As pointed out in the previous section, a minimum load is

required for this regulator due to the predriver current

feeding the output. Placing a resistor equal to VO divided by

12 mA should prevent any voltage overshoot at light load

conditions. Alternatively, the feedback resistors can be

valued properly to consume 12 mA current.

COMPONENT SELECTION

Input Capacitor

In a buck converter, the input capacitor witnesses pulsed

current with an amplitude equal to the load current. This

pulsed current and the ESR of the input capacitors determine

the VIN ripple voltage, which is shown in Figure 19. For VIN

ripple, low ESR is a critical requirement for the input

capacitor selection. The pulsed input current possesses a

significant AC component, which is absorbed by the input

capacitors.

The RMS current of the input capacitor can be calculated

using:

IRMS + IO D(1 * D)

where:

D = switching duty cycle which is equal to VO/VIN.

IO = load current.

Figure 19. Input Voltage Ripple in a Buck Converter

To calculate the RMS current, multiply the load current

with the constant given by Figure 20 at each duty cycle. It is

a common practice to select the input capacitor with an RMS

current rating more than half the maximum load current. If

multiple capacitors are paralleled, the RMS current for each

capacitor should be the total current divided by the number

of capacitors.

Figure 20. Input Capacitor RMS Current can be

Calculated by Multiplying Y Value with Maximum Load

Current at any Duty Cycle

0.2

0.4

1.0

DUTY CYCLE

0.1

0.3

0.4

0.5

0.6

0.2

0.6

0.8

Selecting the capacitor type is determined by each

design’s constraint and emphasis. The aluminum

electrolytic capacitors are widely available at lowest cost.

Their ESR and Equivalent Series Inductor (ESL) are

relatively high. Multiple capacitors are usually paralleled to

achieve lower ESR. In addition, electrolytic capacitors

usually need to be paralleled with a ceramic capacitor for

filtering high frequency noises. The OS−CON are solid

aluminum electrolytic capacitors, and therefore has a much

lower ESR. Recently, the price of the OS−CON capacitors

has dropped significantly so that it is now feasible to use

them for some low cost designs. Electrolytic capacitors are

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