MP2918

—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER

MP2918 Rev. 1.02

www.MonolithicPower.com

21

5/31/2017

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© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see Figure 5).

FB

VOUT

R8

R9

Figure 5: External Resistor Divider

If R8 is known, then R9 can be calculated with

Equation (4):

8

9

OUT

R

R

V

1

0.8V





(4)

Table

1

lists

the

recommended

feedback

resistor values for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

R8

(kΩ)

R9

(kΩ)

3.3

37.4 (1%)

12 (1%)

5

63.4 (1%)

12 (1%)

12

169 (1%)

12 (1%)

Setting Current Sensing

The MP2918 has three fixed current limit

options: 25mV, when ILIM is connected to

SGND; 50mV, when ILIM is connected to VCC1;

and 75mV, when ILIM is floating. Ensure that

the application can deliver a full load of current

over the full operating temperature range when

setting ILIM.

inductor current. Its value is chosen based on

the current limit threshold. The relationship

Ipk

ILIMIT

V

R

(5)

across

it.

Considering

the

output

current,

efficiency,

and

ILIM

threshold,

the

and 50m

Ω.

Programmable Switching Frequency

Consider different variables when choosing the

switching

frequency.

A

high

frequency

increases switching losses and gate charge

losses, while a low frequency requires more

inductance and capacitance, resulting in larger

real

estate

and

higher

cost.

Setting

the

switching frequency is a trade-off between

power loss and passive component size. In

noise-sensitive

applications,

the

switching

frequency

should

be

out

of

a

sensitive

frequency band.

The MP2918

’s frequency can be programmed

from 100kHz to 1000kHz with a resistor from

FREQ to SGND (see Table 2). The value of

calculated with Equation (6):

FREQ

s

20000

R

(k )

1

f (kHz)

 



(6)

Table 2: Frequency vs. Resistor

Resistor (

kΩ)

Frequency (kHz)

65

300

45.3

430

39

500

19

1000

VCC Regulator Connection

If connecting VCC2 to an external power supply

to improve the overall efficiency, VCC2 should

be larger than 4.7V but smaller than 12V (see

Figure 6).

MP2918

INTERNAL

LDO

4.5V

IN

Ext .

Power

Supply

VCC1

VCC2

Figure 6: Internal Circuitry of VCC2