MP4566―36V, 1MHz, 0.6A, STEP-DOWN CONVERTER WITH 35μA QUIESCENT CURRENT

MP4566 Rev. 1.01

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12

9/22/2011

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© 2011 MPS. All Rights Reserved.

For simplification, choose the input capacitor

whose RMS current rating greater than half of the

maximum load current. 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:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

Where C1 is the input capacitance value.

Selecting the Output Capacitor

The output capacitor 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:

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2

⎛⎞

⎛⎞

Δ=

× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠

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:

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

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:

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

MP4566 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

MP4566 employs current mode control for easy

compensation and fast transient response. It

already integrates the compensation components

inside. Thus, just choosing appropriate feedback

resistor divider is enough for obtaining a stable

and fast transient performance. Generally,

choosing R1 to be around 150k to 200k is

recommended. In order to adjust load transient

dynamic performance, slightly changing the feed-

forward capacitor from 15pF to 150pF range is

recommended.

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 VCC pin to BST pin, as

shown in Figure 2.

BST

SW

0.1u-1uF

External BST Diode

IN4148

L

Cout

MP4566

Figure 2—Add Optional External Bootstrap

Diode to Enhance Efficiency

The recommended external BST diode is IN4148,

and the BST cap is 0.1~1uF.

PC Board Layout

PCB layout is very important to achieve stable

operation. It is highly recommended to duplicate

EVB layout for optimum performance.

If change is necessary, please follow these

guidelines.