MAX1672 Maxim, MAX1672 Datasheet - Page 8

• Manufacturer Part Number: MAX1672
• Description: Step-Up/Down DC-DC Converter in QSOP Package
• Manufacturer: Maxim

For fixed output voltages of 3.3V or 5V, connect 3/5 to
PS or GND and connect FB to GND (Table 2).
Alternatively, adjust the output voltage from 1.25V to
5.5V by connecting two resistors, R1 and R2 (Figure 2),
which form a voltage divider between OUT and FB.
Choose resistor values as follows:
where V
has a maximum value of 50nA, R1 and R2 can be large
with no significant accuracy loss. Choose R2 in the
100kΩ to 270kΩ range and calculate R1 using the
above formula. For 1% error, the current through R1
should be at least 100 times FB’s bias current.
Whenever the voltage at FB exceeds 70mV above
GND, the state of the 3/5 pin is ignored. Connect 3/5 to
GND when adjusting V
leave 3/5 unconnected.
The MAX1672 contains a comparator for low-battery
detection. If the voltage at PGI falls below V
ly 1.25V), the open-drain comparator output (PGO)
goes high. Hysteresis is typically 30mV. Set the low-
battery detector’s threshold with resistors R3 and R4
(Figure 2) using the following equation:
where V
detector and V
at PGI has a maximum value of 50nA, R3 and R4 can
be large to minimize input loading with no significant
accuracy loss. Choose R4 in the 100kΩ to 270kΩ range
and calculate R3 using the above formula. For 1%
error, the current through R3 should be at least 100
times PGI’s bias current.
The PGO output is open-drain and should be pulled
high with external resistor R5 for normal operation. If the
low-battery comparator is not used, connect PGI and
PGO to GND.
Step-Up/Down DC-DC Converter
in QSOP Package
Table 2. Output Voltage Control
8
__________________Design Procedure
_______________________________________________________________________________________
3/5 5
REF
PGT
0
1
X
= 1.25V. Since the input bias current at FB
is the desired threshold of the low-battery
REF
R1 = R2[(V
R3 = R4[(V
= 1.25V. Since the input bias current
OUT
Output Voltage Selection
>70mV
GND
GND
FB
OUT
Low-Battery Detection
PGT
with a resistor divider. Never
/ V
/ V
REF
REF
) -1]
) -1]
+1.25 to +5.5
V
OUT
REF
+3.3
+5
(V)
(typical-
A 10µH inductor performs well in most MAX1672 appli-
cations. Smaller inductor values typically offer a smaller
physical size for a given series resistance, but may
increase switching losses. Larger inductor values
exhibit higher output current capability and larger phys-
ical dimensions for a given series resistance. For opti-
mum performance, choose an inductor value from
Table 3 or by using the following equation:
where I
0.8A for I
The inductor’s incremental saturation current rating
should also be greater than the peak switch-current
limit. However, it is generally acceptable to bias most
inductors into saturation by as much as 20% with slight-
ly reduced efficiency. The inductor’s DC resistance sig-
nificantly affects efficiency. See Tables 4 and 5 for a list
of suggested inductors and suppliers.
Figure 2. Adjustable Output Voltage Configuration
1.8V TO 11V
100 F
0.5A
INPUT
ON
ON
C1
0.1 F
LIM
LIM
C3
0.8A
OFF
OFF
V
is the peak switch-current limit, which is
= PS and 0.5A for I
OUT
V
R3
R4
IN(min)
PGI
3/5
ILIM
I
ONA
ONB
REF
+ V
LIM
IN
PGND
MAX1672
I
DIODE
LIM
+ V
10 H
L1
SWITCH
GND
t
LX
OUT
PG0
OFF
PS
FB
Inductor Selection
LIM
2
= GND.
t
L
ON
R1
R2
C2
100 F
(max)
R5
1M
1.25V TO
5.5V OUTPUT
LOW-BATTERY-
DETECTOR
OUTPUT
C4
4.7 F