MP2321 Rev. 1.0

www.MonolithicPower.com

16

10/26/2016

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage. First, choose a value for R2. R2

should be chosen reasonably, since a small R2

leads to considerable quiescent current loss,

but a large R2 makes the FB noise sensitive.

Then R1 can be determined with Equation (11):

OUT

REF

REF

VV

R1

R2

V





(11)

circuit is shown in Figure 8.

Figure 8: Feedback Network

Table 2 lists recommended resistor values for

common output voltages.

Table 2: Resistor Selection for Common Output

Voltages

(10)

VOUT (V)

R1 (kΩ)

R2 (kΩ)

1.0

27

40.2

1.2

40.2

40.2

1.5

60.4

40.2

1.8

80.6

40.2

2.5

127

40.2

3.3

182

40.2

5

294

40.2

NOTE:

10) The feedback resistors in Table 2 are optimized for 500kHz of

switching frequency. The detailed schematics are shown in

the typical application circuit section.

Setting the Frequency

Set forced PWM mode switching frequency by

connecting a resistor (R6) from VIN to

FREQ/MODE and leaving R7 not stuffed (NS)

(see Figure 9). Refer to the MODE Selection

section on page 12 for more detail.

Determine

R6

with

Equation

(12):



6

Delay _ PWM

IN

SW

IN

Vo 10

T

(ns)

V

0.4

F(kHz) V

R6(k )

14.5

















Figure 9: R6 vs. Forced PWM Mode Switching

Frequency

Set the auto PFM/PWM mode switching

frequency by connecting a resistor (R7) from

FREQ/MODE pin to ground and leaving R6 not

stuffed (NS) (see Figure 10). Determine R7 with

Equation (13):



6

Delay _ PFM

IN

SW

IN

Vo 10

T

(ns)

V

0.4

F(kHz) V

R7(k )

13

















(13)

Figure 10: R7 vs. Auto PFM/PWM Mode

Switching Frequency

Equation (12) and Equation (13) show the

typical switching frequency calculation formulas.

The actually frequency changes slightly at

different load currents and different input

voltages.