Rainbow-electronics MAX863 Uživatelský manuál stáhnout pdf (Strana 14)

MAX863

Dual, High-Efficiency, PFM, Step-Up

DC-DC Controller

14 ______________________________________________________________________________________

Set Feedback Compensation

External voltage feedback to the MAX863 should be

compensated for stray capacitance and EMI in the

feedback network. Proper compensation is achieved

when the MAX863 switches evenly, rather than in wide-

ly spaced bursts of pulses with large output ripple.

Typically, lead compensation consisting of a 10pF to

220pF ceramic capacitor (C1 in Figure 7) across the

upper feedback resistor is adequate. Circuits with

OUT

or V

greater than 7.5V may require a second

capacitor across the lower feedback resistor. Initially,

choose this capacitor so that R2C2 = R1C1. Set the

final values of the compensation capacitors based on

empirical analysis of a prototype.

PC Board Layout and Routing

High switching speeds and large peak currents make

PC board layout an important part of design. Poor lay-

out can cause excessive EMI and ground-bounce, both

of which can cause instability or regulation errors by

corrupting the voltage and current-feedback signals.

Place power components as close together as possi-

ble, and keep their traces short, direct, and wide. Keep

the extra copper on the board and integrate it into

ground as an additional plane. On multi-layer boards,

avoid interconnecting the ground pins of the power

components using vias through an internal ground

plane. Instead, place the ground pins of the power

components close together and route them in a “star”

ground configuration using component-side copper,

then connect the star ground to the internal ground

plane using multiple vias.

The current-sense resistor and voltage-feedback net-

works should be very close to the MAX863. Noisy

traces, such as from the EXT pins, should be kept away

from the voltage-feedback networks and isolated from

them using grounded copper. Consult the MAX863

evaluation kit manual for a full PC board example.

MAX863

EXT2

CS2

OUT2

= 24V, 35mA

OUT1

= 5V

= 1.8V TO V

OUT1

N1B

IRF7103

0.1µF

49.9k

15pF

22µF

35V

0.1Ω

100mΩ

909k

N1A

50mΩ

100k

220µF

10V

≤0.1Ω

MBRS340T3

MBRS140

10µH

100µF

10V

≤0.1Ω

100µF

10V

≤0.1Ω

0.1µF

ON/OFF

FB2

SHDN1

EXT1

CS1

LBO

LOW-BATTERY

DETECTOR OUTPUT

LBI

SENSE1

BOOT

GND

SHDN2

REF

PGNDFB1

270pF

Figure 8. Bootstrapped 3.3V Logic and 24V LCD Bias Supply

1 2 ... 9 10 11 12 13 14 15 16

DC-DC Controller 1

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