MPQ4558 – 1A, 2MHz, 55V STEP-DOWN CONVERTER, AEC-Q100 QUALIFIED

MPQ4558 Rev. 1.01

www.MonolithicPower.com

14

5/17/2013

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© 2013 MPS. All Rights Reserved.

Input Capacitor

The input current to the step-down converter is

discontinuous and therefore requires a capacitor

to supply the AC current to the step-down

converter while maintaining the DC input voltage.

Use capacitors with low equivalent series

resistance (ESR) for the best performance.

Ceramic capacitors are preferred, but tantalum or

low-ESR electrolytic capacitors may also suffice.

For simplification, choose the input capacitor with

an RMS current rating greater than half of the

maximum load current. The input capacitor (C1)

can be electrolytic, tantalum or ceramic.

When using electrolytic or tantalum capacitors,

include a small, high-quality ceramic capacitor—

i.e. 0.1μF—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 the input. The input voltage ripple

caused by the capacitance can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

Δ

IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

1

C

f

I

V

Output Capacitor

The output capacitor (C2) maintains the DC

output voltage. Use ceramic, tantalum, or low-

ESR electrolytic capacitors for best results. 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

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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

×

=

IN

OUT

2

S

OUT

OUT

V

V

1

2

C

L

f

8

V

ΔV

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated as:

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

MPQ4558 can be optimized for a wide range of

capacitances and ESR values.

Compensation Components

MPQ4558 employs current-mode control for easy

compensation and fast transient response. The

COMP pin controls the system stability and

transient response—the COMP pin is the output

of the internal error amplifier. A capacitor-resistor

combination

in

series

sets

a

pole-zero

combination to control the characteristics of the

control system. The DC gain of the voltage

feedback loop is given by:

OUT

FB

VEA

CS

LOAD

VDC

V

V

A

G

R

A

×

×

×

=

Where

400V/V,

is

the

current

sense

transconductance,5.6A/V, and

The system has two poles of importance: One is

caused by the compensation capacitor (C3) and

the output resistor of error amplifier; the other is

caused by the output capacitor and the load

resistor. These poles are located at:

=

××

EA

P1

VEA

G

f

2π C3 A

=

××

P2

LOAD

1

f

2 π C2 R

Where,

is

the

error

amplifier

transconductance, 120μA/V.

The system has one zero of importance from C3

and the compensation resistor (R3). This zero is

located at:

=

××

Z1

1

f

2 π C3 R 3