MP2228 – SYNCHRONOUS STEP-DOWN CONVERTER

MP2228 Rev. 1.1

www.MonolithicPower.com

12

8/20/2013

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Setting the AAM Voltage

The AAM voltage sets the transition point from

AAM to CCM. Select a voltage to balance

efficiency, stability, ripple, and transient.

A low AAM voltage improves stability and ripple,

but degrades transient and efficiency during

AAM. Likewise, a high AAM voltage improves

the transient and efficiency during AAM, but

degrades stability and ripple.

The AAM voltage comes from the tap of a

shown in Figure 4.

VCC(4.9V)

R3

R4

AAM

Figure 4: AAM Network

Generally, choose R4 to be around 10kΩ, then

R3 is:

⎟

⎠

⎞

⎜

⎝

⎛

−

=

1

AAM

VCC

R4

R3

0

0.1

0.2

0.3

0.4

0.5

0.6

024

6

8

10

12

Figure 5: AAM Values for Common Output

Selecting the Output Capacitor

The output capacitor (C2) maintains the DC

output voltage. Use ceramic, tantalum, or low-

ESR electrolytic capacitors. For best results,

use low ESR capacitors to keep the output

voltage ripple low. The output voltage ripple can

be estimated as:

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2

⎛⎞

⎛⎞

Δ=

× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠

equivalent series resistance (ESR) value of the

output capacitor.

For

ceramic

capacitors,

the

capacitance

dominates the impedance at the switching

frequency, and the capacitance causes the

majority of the output voltage ripple. For

simplification, the output voltage ripple can be

estimated as:

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated as:

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP2228 can be optimized for a wide range of

capacitance and ESR values.

External Bootstrap Diode

An external bootstrap diode can enhance the

efficiency of the regulator given the following

conditions:

Duty cycle is high: D=

IN

OUT

V

V

>65%

In these cases, add an external BST diode from

the VCC pin to BST pin, as shown in Figure 6.

MP2228

Figure 6: Optional External Bootstrap Diode to

Enhance Efficiency