MPQ4559

– 1.5A, 2MHz, 55V STEP-DOWN CONVERTER AEC-Q100 QUALIFIED

MPQ4559 Rev. 1.01

www.MonolithicPower.com

13

5/24/2016

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2016 MPS. All Rights Reserved.

Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required to

supply the AC current to the step-down converter

while maintaining the DC input voltage. Use low

ESR

capacitors

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

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, 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:

























IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

1

C

f

I

V

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:











































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

MPQ4559 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

MPQ4559 employs current mode control for easy

compensation and fast transient response. The

system stability and transient response are

controlled through the COMP pin. COMP pin is

the output of the internal error amplifier. A series

capacitor-resistor combination 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









400V/V;

is

the

current

sense

resistor value.

The system has two poles of importance. One is

due to the compensation capacitor (C3), the

output resistor of error amplifier. The other is due

to the output capacitor and the load resistor.

These poles are located at:





EA

P1

VEA

G

f

2

π C3 A





P2

LOAD

1

f

2

π C2 R

Where,

is

the

error

amplifier

transconductance, 120

μA/V.

The system has one zero of importance, due to

the

compensation

capacitor

(C3)

and

the

compensation resistor (R3). This zero is located

at:





Z1

1

f

2

π C3 R3