MP5403B

– DUAL BUCK AND ONE LOAD SWITCH PMIC

MP5403B Rev. 1.0

www.MonolithicPower.com

20

12/5/2017

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© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

Output Voltage Setting

The output voltage of the two switchers can be

adjusted with the external resistor dividers (see

Figure 3). The typical reference voltage of both

FB1 and FB2 is 600mV. The maximum allowed

voltage for the outputs is close to the input

voltage minus the voltage drop when the high-

side switch is 100% turned on.

MP5403

SW2

FB1

FB2

SW1

R1

R2

R3

R4

Figure 3: Feedback Resistor Dividers to Set the

Output Voltages

The divider current is recommended to be higher

than 500nA to avoid influence from the feedback

node leakage current (which is in the 10nA

level). Considering control loop optimization, the

pull-high resistor is recommended to be between

100 -

500kΩ. Then, the pull-down resistor can be

calculated with Equation (2):

1

V

6

.

0

V

)

3

orR

(

1

R

)

4

orR

(

2

R

OUT





(2)

Table 2 shows some typical output voltages and

their

corresponding

recommended

resistor

divider values.

Table 2: Output Voltage vs. Resistor Values

R1

R2

1.2V

300k

Ω

300k

Ω

1.5V

300k

Ω

200k

Ω

1.8V

300k

Ω

150k

Ω

2.5V

300k

Ω

95.3k

Ω

3.3V

300k

Ω

66.5k

Ω

.

Selecting the Inductor

The inductor has a great impact on several key

performances for the step-down switcher, such as

inductor

current

ripple,

output

voltage

ripple,

efficiency, and load transient response.

Calculate the inductor current ripple with Equation (3):

SW

IN

OUT

IN

OUT

L

f

L

V

)

V

V

(

V

I











(3)

Calculate the inductor peak current with Equation (4):

2

L

Load

Lpk

I

I

I







(4)

Choosing the inductance is a trade-off between the

output ripple, efficiency, and transient response. The

larger the inductance is, the smaller the output ripple,

but the slower the response. Choose an inductance

that makes the ripple current 30 - 40% of the max load

current.

The inductor saturation current must be higher than

the inductor peak current.

The inductor also impacts the solution efficiency in

terms of conduction loss and frequency- and coil-

related loss. Generally, the DC resistance provides

DC conduction loss information. For AC conduction

loss and coil-related loss, refer to the vendor

’s

datasheet for more detailed information.

Input Capacitor Selection

The input capacitor reduces the surge current

drawn from the input and the switching noise

from the device. Select an input capacitor with

switching frequency impedance that is less than

the input source impedance to prevent high-

frequency switching current from passing to the

input source. Use low ESR ceramic capacitors

with

X5R

or

X7R

dielectrics

with

small

temperature coefficients. For most applications,

a

22μF capacitor is sufficient.