MP2313 – 24V, 1A SYNC STEP DOWN CONVERTER

MP2313 Rev. 1.0

www.MonolithicPower.com

13

9/1/2014

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APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see Typical Application on page

1). The feedback resistor R1 also sets the

feedback loop bandwidth with the internal

compensation capacitor (see Typical Application

on page 1). R2 is then given by:

OUT

R1

R2

V

1

0.798V

=

−

The feedback network is as Figure 5 shows.

Figure 5: Feedback Network

Table 1 lists the recommended feedback

resistors value for common output voltages.

Table 1—Resistor Selection for Common

Output Voltages

L(µH)

1.05

80.6

255

12

0.68

1.2

80.6

160

12

0.68

1.8

40.2

31.6

12

0.82

2.5

40.2

18.7

12

0.82

3.3

20

6.34

22

1

5

20

3.74

22

1.5

Selecting the Inductor

A 0.47µH to 4.7µH inductor with a DC current

rating of at least 25% percent higher than the

maximum load current is recommended for most

applications. For highest efficiency, the inductor

DC resistance should be less than 15mΩ. For

most designs, the inductance value can be

derived from the following equation.

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×

−

=

×Δ ×

Choose inductor current to be approximately

30% of the maximum load current. The maximum

inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light load conditions below 100mA, larger

inductance

is

recommended

for

improved

efficiency.

Setting the AAM Voltage

The AAM voltage is used to setting the transition

point from AAM to PWM. It should be chosen to

provide the best combination of efficiency,

stability, ripple, and transient.

If the AAM voltage is set lower, then stability and

ripple improves, but efficiency during AAM mode

and transient degrades. Likewise, if the AAM

voltage is set higher, then the efficiency during

AAM and transient improves, but stability and

ripple degrades. So the optimal balance point of

AAM voltage for good efficiency, stability, ripple

and transient should be found out.

Adjust the AAM threshold by connecting a

resistor from AAM pin to ground. Take Figure 6

as reference. An internal 6.2µA current source

charges the external resistor.

Figure 6: AAM Network

Generally, R3 is then given by:

The optimized AAM can be got from Figure 7.