MAX8529 データシートの表示（PDF） - Maxim Integrated

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MAX8529

1.5MHz Dual 180° Out-of-Phase PWM Step-Down Controller with POR

Maxim Integrated 

1.5MHz Dual 180° Out-of-Phase

PWM Step-Down Controller with POR

mind that transient response performance of buck regula-

tors operated close to dropout is poor, and bulk output

capacitance must often be added (see the VSAG equa-

tion in the Design Procedure section).

The absolute point of dropout is when the inductor cur-

rent ramps down during the minimum off-time (ΔIDOWN)

as much as it ramps up during the maximum on-time

(ΔIUP). The ratio h = ΔIUP / ΔIDOWN is an indicator of

the ability to slew the inductor current higher in

response to increased load, and must always be

greater than 1. As h approaches 1, the absolute mini-

mum dropout point, the inductor current cannot

increase as much during each switching cycle and

VSAG greatly increases unless additional output capac-

itance is used.

A reasonable minimum value for h is 1.5, but adjusting

this up or down allows tradeoffs between VSAG, output

capacitance, and minimum operating voltage.

For a given value of h, the minimum operating voltage

can be calculated as:

VIN(MIN)

=

⎡

⎢

⎣⎢1

VOUT + VDROP1

- hfSWtOFF(MIN)

⎤

⎥

⎦⎥

+

VDROP2

-

VDROP1

where VDROP1 is the sum of the parasitic voltage drops

in the inductor discharge path, including synchronous

rectifier, inductor, and PCB resistances; VDROP2 is the

sum of the resistances in the charging path, including

high-side switch, inductor, and PCB resistances; and

tOFF(MIN) is from the Electrical Characteristics. The

absolute minimum input voltage is calculated with h = 1.

If the calculated V+(MIN) is greater than the required min-

imum input voltage, then reduce the operating frequency

or add output capacitance to obtain an acceptable

VSAG. If operation near dropout is anticipated, calculate

VSAG to be sure of adequate transient response.

Dropout Design Example:

VOUT = 5V

fSW = 600kHz

tOFF(MIN) = 250ns

VDROP1 = VDROP2 = 100mV

h = 1.5

VIN(MIN)

=

⎡

⎣⎢1

-

5V +100mV

⎤

1.5(600kHz)(250ns)

⎥

⎦

+100mV − 100mV = 6.58V

Calculating again with h = 1 gives the absolute limit of

dropout:

VIN(MIN)

=

⎡

⎣⎢1

-

5V +100mV ⎤

(600kHz)(250ns)

⎥

⎦

+100mV − 100mV = 6V

Therefore, VIN must be greater than 6V, even with very

large output capacitance, and a practical input voltage

with reasonable output capacitance would be 6.58V.

Improving Noise Immunity

Applications where the MAX8529 must operate in noisy

environments can typically adjust their controller’s com-

pensation to improve the system’s noise immunity. In par-

ticular, high-frequency noise coupled into the feedback

loop causes jittery duty cycles. One solution is to lower

the crossover frequency (see the Compensation section).

PCB Layout Guidelines

Careful PCB layout is critical to achieve low switching

losses and clean, stable operation. This is especially

true for dual converters where one channel can affect

the other. Refer to the MAX8529 EV kit data sheet for a

specific layout example.

If possible, mount all of the power components on the

top side of the board with their ground terminals flush

against one another. Follow these guidelines for good

PCB layout:

1) Isolate the power components on the top side from

the analog components on the bottom side with a

ground shield. Use a separate PGND plane under

the OUT1 and OUT2 sides (referred to as PGND1

and PGND2). Avoid the introduction of AC currents

into the PGND1 and PGND2 ground planes. Run the

power-plane ground currents on the top side only.

2) Use a star ground connection on the power plane to

minimize the crosstalk between OUT1 and OUT2.

3) Keep the high-current paths short, especially at the

ground terminals. This practice is essential for sta-

ble, jitter-free operation.

4) Connect GND and PGND together close to the IC.

Do not connect them together anywhere else.

Carefully follow the grounding instructions under

step 4 of the Layout Procedure section.

5) Keep the power traces and load connections short.

This practice is essential for high efficiency. Use

thick copper PCBs (2oz vs. 1oz) to enhance full-

load efficiency by 1% or more.

______________________________________________________________________________________ 17

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