MAX1674-MAX1676 Maxim, MAX1674-MAX1676 Datasheet - Page 10

• Manufacturer Part Number: MAX1674-MAX1676
• Description: High-Efficiency / Low-Supply-Current / Compact / Step-Up DC-DC Converters
• Manufacturer: Maxim

LBI and LBO. For V
comparator as shown in Figure 8. Calculate the value of
the external resistors R3 and R4 as follows:
Since the low-battery comparator is noninverting, exter-
nal hysteresis can be added by connecting a resistor
between LBO and LBI as shown in Figure 9. When LBO
is high, the series combination of R2 and R7 source
current into the LBI summing junction.
An inductor value of 22µH performs well in most appli-
cations. The MAX1674/MAX1675/MAX1676 will also
work with inductors in the 10µH to 47µH range. Smaller
inductance values typically offer a smaller physical size
for a given series resistance, allowing the smallest
overall circuit dimensions. However, due to higher peak
inductor currents, the output voltage ripple (I
output filter capacitor ESR) also tends to be higher.
Circuits using larger inductance values exhibit higher
output current capability and larger physical dimen-
sions for a given series resistance. The inductor’s incre-
mental saturation current rating should be greater than
the peak switch-current limit, which is 1A for the
High-Efficiency, Low-Supply-Current,
Compact, Step-Up DC-DC Converters
10
Figure 8. Setting Resistor Values for the Low-Battery Indicator
when V
______________________________________________________________________________________
R3
0.1 F
IN
200
R1
R3 = R4(V
< 1.3V
BATT
(MAX1676)
LBI
REF
Applications Information
MAX1674
MAX1675
MAX1676
GND
REF
TRIP
(MAX1676)
- V
CLSEL
SHDN
OUT
LBO
less than 1.3V, configure the
TRIP
LX
FB
47 F
) / (V
Inductor Selection
R2
100k
OUT
LOW-
BATTERY
OUTPUT
0.1 F
V
IN
- V
22 H
REF
)
R4
PEAK
V
OUT
47 F
x
MAX1674, 500mA for the MAX1675, and 1A or 0.5A for
the MAX1676. However, it is generally acceptable to
bias the inductor into saturation by as much as 20%,
although this will slightly reduce efficiency. Table 1 lists
suggested components.
The inductor’s DC resistance significantly affects effi-
ciency. See Table 2 for a comparison of inductor speci-
fications. Calculate the maximum output current as
follows:
where I
Figure 9. Adding External Hysteresis to the Low-Battery
Indicator
I
V
OUT MAX
TRIP
R3
R4
WHERE
V
V = 1.3V
H
L
(V
V
L = inductor value in µH
t
R
I
H
= 1.3V
OUT(MAX)
OFF
LIM
R7
, V
= efficiency (typically 0.9)
BATT
P
L
V
V
)
= resistance of P-channel MOSFET in Ω
H
L
= 0.5A or 1.0A
IS THE LOWER TRIP LEVEL
IS THE UPPER TRIP LEVEL
= LX switch’s off-time in µs
= input voltage
1
1
LBI
V
R
R
R
R
= maximum output current in amps
4
3
BATT LIM
3
7
MAX1674
MAX1675
MAX1676
GND
(
R
R
V
( .
1 3
3
4
OUT
I
V R
) (
OUT
LBO
1 3
2
.
–
V R
)
R
1 2
100k
7
V
/
)
R2
3
OUT
V
OUT
0.1 F
t
OFF
L
–
V
R
BATT
P
47 F
V
OUT