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

MPQ2167 Rev. 1.0

www.MonolithicPower.com

21

3/21/2019

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2019 MPS. All Rights Reserved.

Since the input capacitor absorbs the input

switching current, it requires an adequate ripple

current rating, which should be greater than the

converter’s maximum input ripple current. The

input ripple current can be estimated with

equation (5):



 

OUT

OUT

CIN

OUT

IN

IN

VV

II

(1

)

VV

(5)

The worst case condition occurs at VIN = 2VOUT,

where:

OUT

CIN

I

I

2



For simplification, choose an input capacitor

whose RMS current rating is greater than half of

the maximum load current.

The input capacitor can be electrolytic, tantalum,

or ceramic. When using electrolytic or tantalum

capacitors, use a small high-quality ceramic

capacitor (0.1μF) placed as close to the IC as

possible. The input capacitance value determines

the input voltage ripple of the converter. If there

is an input voltage ripple requirement in the

system design, choose an input capacitor that

meets the specification.

The input voltage ripple caused by capacitance

can be estimated with equation (6):





 



OUT

OUT

OUT

IN

SW

IN

IN

IN

IV

V

V(1

)

fC

V

V

(6)

The worst-case condition occurs at VIN = 2VOUT,

where:

 



OUT

IN

SW

IN

I

1

V

4f

C

Selecting the Output Capacitor

The output capacitor maintains the output DC

voltage. Ceramic capacitors with low ESR are

recommended for keeping the output voltage

ripple low and their smaller size. Electrolytic and

polymer capacitors may also be used. The output

voltage ripple can be estimated with equation (7):



 







OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V(1

) (R

)

fL

V

8f

C

(7)

RESR is the equivalent series resistance of the

output capacitor.

For ceramic capacitors, the impedance at the

switching

frequency is

dominated by the

capacitance. The output voltage ripple is mainly

caused by the capacitance. For simplification, the

output voltage ripple can be estimated with

equation (8):



 

 

OUT

OUT

OUT

2

SW

OUT

IN

VV

V(1

)

8f

L C

V

(8)

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with equation (9):



 





OUT

OUT

OUT

ESR

SW

IN

VV

V(1

) R

fL

V

(9)

Another consideration of the output capacitance

is the allowable overshoot in VOUT if the load is

suddenly removed. In this case, energy stored in

the inductor will be transferred to COUT causing its

voltage to rise. In order to achieve a desired

overshoot relative to the regulated voltage, the

output capacitance can be estimated with

equation (10):









2

OUT

OUT

22

OUT

OUTMAX

OUT

IL

C

V((V

/ V

)

1)

(10)

Where VOUTMAX/VOUT is the allowable maximum

overshoot. After calculating the capacitance

required for both the ripple and overshoot,

choose the larger of the calculated values.

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MPQ2167 can be optimized for a wide range of

capacitance and ESR values.

Layout Recommendation

Efficient PCB layout is critical for stable operation.

Refer to Figure 4 and follow the guidelines below

for optimal design layout:

1. Place high-current paths (GND, IN, and SW)

very close to the device with short, direct, and

wide traces.

2. Place input capacitors as close to IN as

possible to minimize high frequency noise.

3. Place the feedback resistor divider as close

as possible to FB. And keep the FB trace

away from switching node.