Rainbow Electronics MAX1677 - Manual (Page 12)

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:
R5 = R6 [(V TRIP / 0.614V) - 1]
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:
R3 = R4 [(V OUT / 1.25V) - 1]
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
V IN (V TRIP )
R5
R6
R8
Figure 6. Setting the Low-Battery Trip Threshold
MAX1677
POUT
LBO
LBI
V IN (V TRIP : V H , V L )
R5
R6
R8
100k
R7
WHERE V H IS THE RISING V TRIP LEVEL
AND V L IS THE FALLING V TRIP LEVEL.
V
H
= 0.614V
[
1+
R5
+
R5 ]
R7 R6
0.614V
R6
V L = 0.614V + R5
( )
)
V
POUT
- 0.614V
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
MAX1677
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
L CDLX PEAK
INDUCTOR CURRENT
(mA)
LCDPOL
CONNECTED TO:
OUT
GND Negative
Positive
LCD
OUTPUT
POLARITY