MAX769 데이터 시트보기 (PDF) - Maxim Integrated

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MAX769

IC PAGER SYSTEM 28-QSOP

Maxim Integrated 

2 or 3-Cell, Step-Up/Down,

Two-Way Pager System IC

Applications Information

Component Selection

The MAX769 requires minimal design calculation and is

optimized for the component values shown in Figure 2.

However, some flexibility in component selection is still

allowed, as described in the following text. A list of suit-

able components is provided in Table 6.

Inductor L1 is nominally 68µH, but values from 47µH to

100µH should be satisfactory. The inductor current rat-

ing should be 300mA or more if full output current

(80mA) is needed. If less output current is required, the

inductor current rating can be reduced proportionally

but should never be less than 150mA.

Inductor resistance should be minimized for best effi-

ciency, but since the MAX769 N-channel switch resis-

tance is typically 0.9Ω, efficiency does not improve

significantly for coil resistances below 0.4Ω.

Filter capacitors C1–C4 should be low-ESR types (tan-

talum or ceramic) for lowest ripple and best noise

rejection. The values shown in Figure 2 are optimized

for each output’s rated current. Lower required output

current allows smaller capacitance values.

Resistors at the LBI and RSIN inputs set the voltage at

which the LBO and RSO outputs trigger. The voltage

threshold for both LBI and RSI is 0.6V. The resistors

required to set a desired trip voltage, (Figure 2) VTRIP,

are calculated by:

R1 = R2[(VTRIP(LBO) / 0.6) - 1]

R3 = R4[(VTRIP(LBO) / 0.6) - 1]

To minimize battery drain, use large values for R2 and

R4 (>100kΩ) in the above equations; 470kΩ is a good

starting value.

See the Low-Noise Analog Supply (REG2) section for

information on selecting ROFS.

Since LBO and RSO are open-drain outputs, pull-up

resistors are usually required. Normally these will be

pulled up to REG1. 100kΩ is recommended as a com-

promise between response time and current drain,

although other values can be used. Since LBI and RSO

are high (open circuit) during normal operation, current

normally does not flow in the pull-up resistors until a

low-battery or reset event occurs.

Logic Levels

Note that since the MAX769’s internal logic is powered

from REG1, the input logic levels at the digital inputs

(DR2IN, RUN, SYNC, CS, and SDI) as well as the logic

output level of SDO are governed by the voltage at

REG1. Logic-high inputs at these pins should not

exceed VREG1. Digital inputs should either be driven

from external logic (or a µP) powered from REG1, or by

open-drain logic devices that are pulled up to REG1.

Board Layout and Noise Reduction

The MAX769 makes every effort in its internal design to

minimize noise and EMI. Nevertheless, prudent layout

practices are still suggested for best performance.

Recommendations are as follows:

1) Keep trace lengths at L1, LX1, and LX2, as well as

at PGND, as short and wide as possible. Since LX1

and LX2 toggle between VBATT and VOUT at a fast

rate, minimizing the trace length serves to reduce

excess PC board area that might act as an antenna.

2) Place the filter capacitors at OUT, REG1, REG2,

and REG3 as close to their respective pins as pos-

sible (no more than 0.5mm away).

3) Consider using an inductor at L1. A shielded induc-

tor at L1 will minimize radiated noise, but may not

be essential. Toroids will also exhibit EMI perfor-

mance similar to that of shielded coils.

4) Keep the power components at the uppermost part

of the IC to minimize coupling to other parts of the

circuit. The LX1, LX2, OUT, and PGND pins are

located at the uppermost part of the IC to facilitate

PC board layout. Other pins in this area are digital

and are not affected by close proximity to switching

nodes.

5) Use a separate short, wide ground trace for PGND

and the ground side of the BATT and OUT filter

capacitors. Tie this trace to the ground plane.

Table 6. External Components

SUPPLIER

PART NO.

COMMENTS

INDUCTORS (68µH)

Coilcraft

DT1608C-223,

DT1608C-683

0.58Ω, 3.18mm high,

shielded

Murata

LQH4N680K

1.9Ω, 2.6mm high,

low current, low cost

CD54-680

0.46Ω, 4.5mm high

Sumida

CDR74B-680

0.33Ω, 4.5mm high,

shielded

CD73-680

0.33Ω, 3.5mm high

CAPACITORS

AVX

TPS series

Tantalum

Marcon

THCR series

Ceramic

Sprague

595D series

Tantalum

TDK

C3216 series

Ceramic

STORAGE CAPACITOR (optional at NICD pin)

Polystor

A-10300

1.5 Farads

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