Rainbow-electronics MAX1845 Manual do Utilizador descarregar pdf (Página 22)

MAX1845

Dual, High-Efficiency, Step-Down

Controller with Accurate Current Limit

22 ______________________________________________________________________________________

mount all of the power components on the top side of

the board, with connecting terminals flush against one

another.

Keep the high-current paths short, especially at the

ground terminals. This practice is essential for stable,

jitter-free operation. Short power traces and load con-

nections are essential for high efficiency. Using thick

copper PC boards (2oz vs. 1oz) can enhance full-load

efficiency by 1% or more. Correctly routing PC board

traces is a difficult task that must be approached in

terms of fractions of centimeters, where a single mil-

liohm of excess trace resistance causes a measurable

efficiency penalty.

Place the current-sense resistors close to the top-side

star-ground point (where the IC ground connects to the

top-side ground plane) to minimize current-sensing

errors. Avoid additional current-sensing errors by using

a Kelvin connection from CS_ pins to the sense resis-

tors.

The following guidelines are in order of importance:

• Keep the space between the ground connection of

the current-sense resistors short and near the via to

the IC ground pin.

• Minimize the resistance on the low-side path. The

low-side path starts at the ground of the low-side

FET, goes through the low-side FET, through the

inductor, through the output capacitor, and returns

to the ground of the low-side FET. Minimize the resis-

tance by keeping the components close together

and the traces short and wide.

• Minimize the resistance in the high-side path. This

path starts at V

, goes through the high-side FET,

DL_

GND

OUT_

CS_

DH_

FB_

BATT

OUT

MAX1845

Figure 9. Setting V

OUT

with a Resistor-Divider

MAX1845

TO ERROR

AMP1

TO ERROR

AMP2

OUT2

FB2

0.1V

FB1

FIXED

2.5V

FIXED

1.5V

FIXED

1.8V

OUT1

Figure 8. Feedback Mux

AGND PLANE

PGND PLANE

VIA TO OUT1

VIA TO PGND PLANE AND IC GND

VIA TO CS1

NOTCH

USE AGND PLANE TO:

- BYPASS V

AND REF

- TERMINATE EXTERNAL FB, ILIM,

OVP DIVIDERS, IF USED

- PIN-STRAP CONTROL

INPUTS

USE PGND PLANE TO:

- BYPASS V

- CONNECT IC GROUND

TO TOP-SIDE STAR GROUND

VIA TO TOP-SIDE

GROUND

TOP-SIDE GROUND PLANE

Figure 10. PC Board Layout Example

1 2 ... 17 18 19 20 21 22 23 24

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