MP38870 – 3A, 14V, 600KHz STEP-DOWN WITH SYNCHRONOUS GATE DRIVER

MP38870 Rev. 1.1

www.MonolithicPower.com

9

10/22/2009

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© 2009 MPS. All Rights Reserved.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, a small, high quality ceramic

capacitor, i.e. 0.1μF, should be placed 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 input. The

input voltage ripple caused by capacitance can

be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

Δ

IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

1

C

f

I

V

Selecting the Output Capacitor

The output capacitor (C2) is required to

maintain the DC output voltage. Ceramic,

tantalum, or low ESR electrolytic capacitors are

recommended.

Low

ESR

capacitors

are

preferred to keep the output voltage ripple low.

The output voltage ripple can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

Δ

2

C

f

8

1

R

V

V

1

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

equivalent series resistance (ESR) value of the

output capacitor.

In the case of 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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

×

=

IN

OUT

2

S

OUT

OUT

V

V

1

2

C

L

f

8

V

ΔV

In the case of tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the

output ripple can be approximated to:

ESR

IN

OUT

S

OUT

OUT

R

V

V

1

L

f

V

ΔV

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP38870 can be optimized for a wide range of

capacitance and ESR values.

PC Board Layout

The high current paths (GND, IN and SW)

should be placed very to the device with short,

direct and wide traces. The input capacitor

needs to be as close as possible to the IN and

GND pins. The external feedback resistors

should be placed next to the FB pin. Keep the

switching node SW short and away from the

feedback network.

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency of the regulator, the applicable

conditions of external BST diode are:

Duty cycle is high: D=

IN

OUT

V

V

>65%

In these cases, an external BST diode is

recommended from the output of the voltage

regulator to BST pin, as shown in Fig.3

MP38870

SW

BST

C

L

BST

C

5V or 3.3V

OUT

External BST Diode

IN4148

Figure 3—Add Optional External Bootstrap

Diode to Enhance Efficiency

The recommended external BST diode is

IN4148, and the BST cap is 0.1~1µF.