MPQ2167 – 6V, 4A, FREQUENCY PROGRAMMABLE, BUCK CONVERTER, AEC-Q100 QUALIFIED

MPQ2167 Rev. 1.0

www.MonolithicPower.com

20

3/21/2019

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider connected to FB

sets the output voltage (see Figure 3). The

feedback resistor R1 must account for both

stability and dynamic response, so it cannot be

too large or too small. R1 is estimated to be

100kΩ. R2 is then given using equation (2):

OUT

R1

R2

V

1

0.606





(2)

The T-type feedback network is highly

recommended (see Figure 3).

Figure 3: Feedback Network

R6+R4 is used to set the loop bandwidth.

Basically,

a

higher

R6+R4

brings

lower

bandwidth. To ensure loop stability, it is strongly

recommended to limit the bandwidth at around

0.1fSW.

Table 1 lists the recommended feedback divider

resistor values for common output voltages.

Check the loop analysis before using in

application. Change the resistance of RT for loop

stability if necessary.

Table 1: Resistor Values for Typical

VOUT

VOUT (V)

R6 (kΩ)

R4 (kΩ)

R5 (kΩ)

1.2

100

100(1%)

100(1%)

1.5

100

100(1%)

66.5(1%)

1.8

100

100(1%)

49.9(1%)

2.5

100

100(1%)

31.6(1%)

3.3

100

100(1%)

22.1(1%)

Selecting the Inductor

The inductor is required to supply constant

current to the output load while being driven by

the switching input voltage. For a default 2.2MHz

application, a 0.47µH to 1.5µH inductor is

recommended. For highest efficiency, chose an

inductor with a DC resistance less than 15mΩ.

When setting the frequency, the inductance may

need to be increased with the frequency

decreasing. A large inductance will result in less

ripple current and a lower output ripple voltage.

However, this also results in a larger inductor,

which will be physically larger and have a higher

series resistance and/or lower saturation current.

A good rule for determining the inductor value is

to allow the inductor ripple current to be

approximately 30% of the maximum load current.

Ensure that the peak inductor current is below

the device peak current limit. The inductance

value can be calculated with equation (3):

 



OUT

OUT

SW

L

IN

VV

L(1

)

fI

V

(3)

Where ∆IL is the peak-to-peak inductor ripple

current.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor current can be calculated with equation

(4):



 



OUT

OUT

LP

OUT

SW

IN

VV

II

(1

)

2f

L

V

(4)

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, and therefore requires a capacitor

to supply the AC current to the step-down

converter while maintaining the DC input voltage.

Use low ESR capacitors for the best performance.

Ceramic capacitors with X5R or X7R dielectrics

are highly recommended because of their low

ESR values and small temperature coefficients.

Other types, including Y5V and Z5U must not be

used as these lose too much capacitance with

frequency, temperature, and bias voltage. Be

sure to place the input capacitors as close to IN

as possible. For most applications, a 22µF

capacitor is sufficient. For higher output voltage,

use 47μF to improve system stability. To get a

small solution size, it is better to choose a proper

package size capacitor with a rating voltage

compliant to the input spec.