MAX1677EEE+T Datasheet MAX1677EEE+T, MAX1677EEE, MAX1677EEE-T | P13-P15

MAX1677

Compact, High-Efficiency, Dual-Output

Step-Up and LCD Bias DC-DC Converter

______________________________________________________________________________________ 13

in the 10kΩ to 200kΩ range and calculate R1 using one

of the following two equations (for positive or negative

output).

For a positive LCD output, connect LCDPOL to OUT as

shown in Figure 2. This sets the threshold at LCDFB to

1.25V. Select R2 and the desired output voltage

LCD

), and calculate R1:

For positive LCD output: R1 = R2 [(V

LCD

/ 1.25V) - 1]

Figure 3 shows the standard circuit for generating a

negative LCD supply. This connection limits V

LCD

values between -V

and -28V. If a smaller negative

output voltage is required, D2’s cathode can be con-

nected to V

rather than ground. This alternate con-

nection permits output voltages from 0 to -28 - V

.

For a negative LCD output voltage, connect LCDPOL to

GND. The feedback threshold voltage of LCDFB is set

to 0. Select R2 and the desired output voltage (V

LCD

and calculate R1:

For negative LCD output: R1 = R2 ×



LCD

 / 1.25V

To minimize ripple in the LCD output and prevent sub-

harmonic noise caused by switching pulse grouping, it

may be necessary in some PC board layouts to con-

nect a small capacitor in parallel with R1. For R1 values

in 500kΩ to 2MΩ range, 22pF is usually adequate.

Many LCD bias applications require an adjustable out-

put voltage. In Figure 9, an external control voltage

(generated by a potentiometer, DAC, filtered PWM con-

trol signal, or other source) is coupled to LCDFB

through the resistor R

ADJ

. The output voltage of this cir-

cuit, for both positive and negative outputs, is given by:

OUT

= V

INIT

+ (R1 / R

ADJ

)(V

LCDFB

- V

ADJ

)

where V

INIT

is the initial output obtained without the

added adjust voltage, as calculated in one of the pre-

ceding two equations. V

LCDFB

is 1.25V for the positive

configuration, and 0 for the negative configuration.

ADJ

sets the output adjustment span, which is

1.25V × R1 / R

ADJ

for either polarity output. Note that

raising V

ADJ

lowers V

OUT

in positive output designs,

while in negative output designs, raising V

ADJ

increas-

es the magnitude of the negative output.

Higher LCD Output Voltages

If the application requires LCD output voltages greater

than +28V, use the connection in Figure 10. This circuit

adds one capacitor-diode charge pump stage to

increase the output voltage without increasing the volt-

age stress on the LCDLX pin. The maximum output

voltage of the circuit is +55V and output current is

slightly less than half that available from the standard

circuit in Figure 2. In Figure 10, diodes D1, D2, and D3

should be at least 30V-rated Schottky diodes such as

1N5818 or MBR0530L or equivalent. Capacitors C1

and C2 should also be rated for 30V, while C3 must be

rated for the maximum set output voltage.

Applications Information

Inductor Selection

The MAX1677’s high switching frequency allows the

use of small surface-mount inductors. The 10µH values

shown in Figures 2 and 3 are recommended for most

applications, although values between 4.7µH and 47µH

are suitable. Smaller inductance values typically offer a

smaller physical size for a given series resistance,

allowing the smallest overall circuit dimensions. Larger

inductance values exhibit higher output current capa-

bility, but larger physical dimensions.

MAX1677

ADJ

LCD

GND

(REF)

Figure 9. Adjusting LCD Output Voltage

MAX1677

LCDLX

OUT

LCDPOL

10µH

1µF

30V

2.2µF

30V

2.2µF

+40V/5mA

(SET TO

NO MORE

THAN 55V)

D1, D2, D3 = 30V RATED SCHOTTKY DIODES:

MBR0530L OR EQUIVALENT.

65k

LCDFB

Figure 10. Higher LCD Output Voltage

Use inductors with a ferrite core or equivalent; powder

iron cores are not recommended for use with the

MAX1677’s high switching frequencies. The inductor’s

incremental saturation rating ideally should exceed the

selected current limit, however it is generally accept-

able to bias most inductors into saturation by as much

as 20% (although this may reduce efficiency).

For best efficiency, select inductors with resistance no

greater than the internal N-channel FET resistance in

each boost converter (220mΩ for the MBC, and 1Ω for

the LCD). The inductor is effectively in series with the

input at all times, so inductor wire losses can be rough-

ly approximated by I

× R

. See Table 4 for a list of

inductor suppliers.

The LCD boost converter (LCD) features selectable

inductor/switch current limit of 350mA or 225mA. The

higher current setting provides the greatest output cur-

rent, while the lower setting allows the smallest inductor

size.

External Diodes

The MAX1677’s on-chip synchronous rectifier allows

the normally required external Schottky diode to be

omitted from the MBC in designs whose input exceeds

1.4V. In circuits that need to operate below 1.4V (1-cell

inputs for example), connecting a Schottky diode in

parallel with the internal synchronous rectifier (from LX

to POUT) provides the lowest start-up voltage. Suitable

devices are the 1N5817 or MBR0520L, however the

diode current rating need not match the peak switch

current, since most of the current is handled by the on-

chip synchronous rectifier.

Since the LCD boost converter (LCD) does not have

synchronous rectification, an external diode is always

needed. High switching speed demands a high-speed

rectifier. For best efficiency, Schottky diodes such as

the 1N5818 and MBR0530L are recommended. Be

sure that the diode current rating exceeds the peak

current set by LCDPOL, and that the diode voltage rat-

ing exceeds the LCD output voltage. In particularly

cost-sensitive applications, and if the LCD’s 225mA

peak current is set, a high-speed silicon signal diode

(such as an 1N4148) may be used instead of a

Schottky diode, but with reduced efficiency.

Input Bypass Capacitors

A low-ESR input capacitor connected in parallel with

the battery will reduce peak currents and input-reflected

noise. Battery bypassing is especially helpful at low input

voltages and with high-impedance batteries (such as

alkaline types). Benefits include improved efficiency

and lower useful end-of-life voltage for the battery.

100µF is typically recommended for 2-cell applications.

Small ceramic capacitors may also be used for light

loads or in applications that can tolerate higher input

ripple. Only one input bypass capacitor is typically

needed for both the MBC and LCD.

Output Filter Capacitors

For most applications, a 100µF, 10V, low-ESR output fil-

ter capacitor is recommended for the MBC output. A

surface-mount tantalum capacitor typically exhibits

30mV ripple when the MBC is stepping up from 1.2V to

3.3V at 100mA. OS-CON and ceramic capacitors offer

lowest ESR, while low-ESR tantalums offer a good bal-

ance between cost and performance.

The LCD output typically exhibits less than 1% peak-to-

peak ripple with 4.7µF of filter capacitance. This can be

either a ceramic or tantalum type, but be sure that the

capacitor voltage rating exceeds the LCD output volt-

age. If the LCD’s 225mA peak switch current setting is

used, the designer can choose lower output ripple or

MAX1677

Compact, High-Efficiency, Dual-Output

Step-Up and LCD Bias DC-DC Converter

14 ______________________________________________________________________________________

Table 4. Component Suppliers

PHONE FAX

Sanyo: OS-CON

and GX series

619-661-6835 619-661-1055

Murata: LQH4 and

LQH3C series

814-237-1431 814-238-0490

SUPPLIER

INDUCTORS

CAPACITORS

AVX: TPS series 803-946-0690 803-626-3123

TDK: NLC Series 847-390-4373 847-390-4428

Matsuo:

267 series

714-969-2591 714-960-6492

Sprague: 595D

series

603-224-1961 603-224-1430

Motorola:

MBR0520

602-303-5454 602-994-6430

Nihon: EC11 FS1

series

805-867-2555 805-867-2698

Coilcraft: DO and

DT series

847-639-6400 847-639-1469

Sumida: CD, CDR,

and RCH series

847-956-0666 847-956-0702

INDUCTORS

CAPACITORS

DIODES

MAX1677

Compact, High-Efficiency, Dual-Output

Step-Up and LCD Bias DC-DC Converter

______________________________________________________________________________________ 15

reduce the output filter to 2.2µF. Ceramic capacitors will

exhibit lower ripple than equivalent value (or even higher

value) tantalums due to lower ESR.

Layout Considerations

The MAX1677’s high-frequency operation makes PC

board layout important for minimizing ground bounce

and noise. Protect sensitive analog grounds by using a

star ground configuration. Minimize ground noise by

connecting PGND, the input bypass-capacitor ground

terminal, and the output filter-capacitor ground terminal

to a single point (star ground configuration). Also, mini-

mize lead lengths to reduce stray capacitance and

trace resistance. Where an external resistor-divider is

used to set output voltage, the trace from FB or LCDFB

to the feedback resistors should be extremely short to

minimize coupling from LX and LCDLX. To maximize

efficiency and minimize output ripple, use a ground

plane and connect the MAX1677 GND and PGND pins

directly to the ground plane. Consult the MAX1677

evaluation kit for a full PC board example.

Chip Information

TRANSISTOR COUNT: 1221

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P16

MAX1677EEE+T

Mfr. #:

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Manufacturer:

Maxim Integrated

Description:

Switching Voltage Regulators Compact Step-Up & LCD Bias DC/DC

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MAX1677EEE+T

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