MP9928—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER

MP9928 Rev. 1.0

www.MonolithicPower.com

19

5/20/2016

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage.

FB

VOUT

R8

R9

If R8 is determined, then R9 can be calculated

with below formula:

1

0.8V

V

R

R

OUT

8

9

−

=

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 MP9928 has three fixed options for current

limit setting: when ILIM pin is connected to GND,

the current sense voltage is set to 25mV; when

ILIM pin is connected to VCC1, the current sense

voltage is set to 50mV and when ILIM pin is

floating, the current limit sense voltage is set to

75mV.

inductor current. Its value is chosen based on the

current limit threshold. The relationship between

Ipk

ILIMIT

V

R

(5)

5m

Ω to 50mΩ.

Programmable Switching Frequency

There are a number of variables to consider

when choosing the switching frequency. A high

frequency will increase switching losses and gate

charge losses, while a lower frequency requires

more inductance and capacitance, which results

in larger real estate and also higher cost. It is a

trade off between power loss and passive

component size. Additionally, in noise-sensitive

applications, the switching frequency should be

out of a sensitive frequency band.

The MP9928’s frequency can be programmed

from 100kHz to 1000kHz with a resistor from

operating frequency can be calculated by:

FREQ

s

20000

R

(k )

1

f (kHz)

Ω=

−

Ω.

Table 2: Frequency vs. Resistor

Resistor (

kΩ)

Frequency (kHz)

65

300

39

500

19

1000

VCC1 can be powered from both VIN and VCC2.

If connecting VCC2 to an external power supply

to improve the overall efficiency, this VCC2

should be larger than 4.7V but smaller than 12V.

MP9928

Internal

LDO

4.7V

Ext .

Power

Supply

VCC1

VCC2

≤12V, VCC2 can