MP9186 – 20V, 6A SYNCHRONOUS STEP-DOWN CONVERTER

MP9186 Rev. 1.0

www.MonolithicPower.com

13

2/4/2013

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© 2013 MPS. All Rights Reserved.

For simplification, choose the input capacitor

whose RMS current rating is greater than half of

the maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, place a small, high-quality ceramic

capacitor (e.g. 0.1µF) as close to the IC as

possible. When using ceramic capacitors, make

sure that they have enough capacitance to

provide sufficient charge to prevent excessive

voltage ripple at the input. The input voltage

ripple caused by the capacitance can be

estimated by:

LOAD

OUT

OUT

IN

IN

S

IN

I

V

V

V

1

f

C1

V

V





∆

=

×

×

−





×





Selecting the Output Capacitor

The device requires an output capacitor (C2) to

maintain the DC output voltage. Use ceramic,

tantalum, or low ESR electrolytic capacitors.

Use low-ESR capacitors to limit the output

voltage ripple. Estimate the output voltage

ripple with:

OUT

OUT

OUT

ESR

S

1

IN

S

V

V

1

V

1

R

f

L

V

8 f

C2









∆

=

×

−

×

+









×

× ×



 



equivalent series resistance (ESR) value of the

output capacitor.

For ceramic capacitors, the impedance at the

switching

frequency

is

dominated

by

the

capacitance. The output voltage ripple is mainly

caused by the capacitance. For simplification,

the output voltage ripple can be estimated by:

OUT

OUT

OUT

2

IN

S

1

V

V

∆V

1

V

8 f

L

C2





=

×

−





×

×

×





For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated to:

OUT

OUT

OUT

ESR

IN

S

1

V

V

∆V

1

R

f

L

V





=

×

−

×





×





The characteristics of the output capacitor also

affect the stability of the regulatory system. The

MP9186 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%

Use an external BST diode from the VCC pin to

the BST pin—as shown in Figure 6—for such

cases.

MP9186

BST

SW

VCC

L

External BST Diode

IN4148

Figure 6: Optional Bootstrap Diode

The recommended external BST diode is

IN4148, and the BST capacitor is 0.1µF to 1µF.