off time reduces transient input current when the LCD is

activated.

Shutdown: ON and LCDON

A logic-low level at ON shuts down all MAX1677 cir-

cuits including the LCD converter, reference, and LBI

comparator. A logic-high level at LCDON activates the

LCD boost converter. The LCD boost converter can

only be activated when ON is high. When ON is low,

the MAX1677 draws 1µA.

Low-Battery Comparator

The MAX1677 has an on-chip comparator for low-bat-

tery detection. If the voltage at LBI falls below 614mV,

LBO (an open-drain output) sinks current to GND. The

low-battery trip level is set by two resistors (Figure 6).

Since the LBI input current is less than 50nA, large

resistor values (R6

≤ 130kΩ) can be used to minimize

input loading. Calculate R5 as follows:

Connect a pullup resistor (R8) to

LBO when driving

CMOS logic.

LBO is an open-drain output and can be

pulled as high as 6V regardless of the voltage at OUT.

When LBI is above 0.614V,

LBO is high impedance. If

the LBI comparator is not used, ground LBI.

Since the low-battery comparator is noninverting, hys-

teresis can be added by connecting a resistor (R7)

from LBI to

LBO as shown in Figure 7. When LBO is

high, the series combination of R8 and R7 source cur-

rent into the summing node at LBI (no current flows into

the IC).

Design Procedure

The MBC feedback pin (FB) features Dual Mode opera-

tion. With FB grounded, the MBC output is preset to

3.3V. It can also be adjusted from 2.5V to 5.5V with

external resistors, R3 and R4, as shown in Figure 8. To

set the output voltage externally, select resistor R4 in

the 10k

Ω to 200kΩ range. Calculate R3 using:

Setting the LCD Output Voltage

For either positive or negative LCD output voltages, set

the voltage with two external resistors, R1 and R2, as

shown in Figures 2 and 3. Since the input current at FB

has a maximum of 50nA, large resistors can be used

without significant accuracy loss. Begin by selecting R2

MAX1677

POUT

LBO

LOGIC POWER

LOW-BATTERY OUTPUT

LBI

R5

R6

R8

Figure 6. Setting the Low-Battery Trip Threshold

MAX1677

POUT

LBO

LBI

R5

R6

R8

100k

R7

R5

+

R7 R6

0.614V

R6

(

)

)

R7 + R8

Figure 7. Adding External Hysteresis to the LBI Comparator

MAX1677

POUT

FB

MBC OUTPUT

R3

R4

GND

Figure 8. Setting the MBC Output Voltage Externally

Compact, High-Efficiency, Dual-Output

Step-Up and LCD Bias DC-DC Converter

12

______________________________________________________________________________________

Table 3. Setting LCD Output Polarity and

Peak Inductor Current

225

GND through 50k

Ω

Negative

225

OUT through 50k

Ω

Positive

350

350

INDUCTOR CURRENT

(mA)

LCDPOL

CONNECTED TO:

OUT

GND

Negative

Positive

LCD

OUTPUT

POLARITY