MPQ4462 – 3.5A, 4MHz, 36V, STEP-DOWN CONVERTER, AEC-Q100 QUALIFIED

MPQ4462 Rev. 1.11

www.MonolithicPower.com

13

7/17/2013

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© 2013 MPS. All Rights Reserved.

For simplicity, choose an 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. Electrolytic or tantalum

capacitors will need a small, high-quality

ceramic capacitor (0.1μF) placed as close to

the IC as possible. Ceramic capacitors must

have enough capacitance to prevent excessive

input voltage ripple. The capacitor-incurred

input voltage ripple is approximately:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

Output Capacitor

The output capacitor (C2) maintains the output

DC voltage. Use ceramic, tantalum, or low-ESR

electrolytic capacitors. Low-ESR capacitors are

best at limiting the output voltage ripple. The

output voltage ripple is approximately:

OUT

OUT

OUT

ESR

SIN

S

VV

1

V1

R

fL

V

8 f

C2

⎛⎞

⎛⎞

Δ=

× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠

capacitor’s equivalent series resistance.

If using ceramic capacitors, the capacitance

dominates the impedance at the switching

frequency and contributes to the majority of the

output voltage ripple. The output voltage ripple

is approximately:

OUT

OUT

OUT

2

SIN

VV

ΔV1

8f

L C2

V

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

If using either tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. The output ripple is

approximately:

OUT

OUT

OUT

ESR

IN

S

VV

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

The output capacitor also affects the regulatory-

system’s stability. The MPQ4462 can be

optimized for a wide range of capacitances and

ESR values.

Compensation Components

The MPQ4462 employs current-mode control

for easy compensation and fast transient

response. The COMP pin—the output of the

internal

error

amplifier—controls

system

stability and transient response. A series RC

combination adds a pole-zero pair to the control

system . The DC gain of the voltage feedback

loop is:

FB

VDC

LOAD

CS

VEA

OUT

V

AR

G

A

V

=×

×

×

Where

(9A/V), and

The system has two important poles: the

compensation capacitor (C3) and the error

amplifier’s output resistor; and the output

capacitor and the load resistor. These poles are

located at:

EA

P1

VEA

G

f

2C3 A

=

π×

×

P2

LOAD

1

f

2C2 R

=

π×

×

Where,

is

the

error

amplifier’s

transconductance, 60μA/V.

The system has one important zero due to the

compensation

capacitor

(C3)

and

the

compensation resistor (R3). This zero is located

at:

Z1

1

f

2C3 R3

=

π×

×

The system may have another important zero if

the output capacitor is large and/or has a high-

ESR. The zero is located at:

ESR

ESR

1

f

2C2 R

=

π×

×

In case requires a third pole set by the

compensation

capacitor

(C6)

and

the