MAX1672 Datasheet, PDF (8/12 Pages) Maxim Integrated Products – Step-Up/Down DC-DC Converter in QSOP Package

Step-Up/Down DC-DC Converter
in QSOP Package
__________________Design Procedure
Output Voltage Selection
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:
R1 = R2[(V_OUT / V_REF) -1]
where V_REF = 1.25V. Since the input bias current at FB
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_OUT with a resistor divider. Never
leave 3/5 unconnected.
Low-Battery Detection
The MAX1672 contains a comparator for low-battery
detection. If the voltage at PGI falls below V_REF (typical-
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:
R3 = R4[(V_PGT / V_REF) -1]
where V_PGT is the desired threshold of the low-battery
detector and V_REF = 1.25V. Since the input bias current
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.
Table 2. Output Voltage Control
3/5
FB
V_OUT (V)
0
GND
+5
1
GND
+3.3
X
>70mV
+1.25 to +5.5
INPUT
1.8V TO 11V
L1
10µH
C1
R3
100µF
IN
LX
PGI
PS
R4
MAX1672
ON OFF
ONA
OUT
ON OFF
ONB
3/5
0.5A 0.8A
ILIM
PG0
C3
0.1µF
REF
FB
PGND
GND
C2
100µF
1.25V TO
5.5V OUTPUT
R1 R5
1M
C4
4.7µF
LOW-BATTERY-
DETECTOR
OUTPUT
R2
Figure 2. Adjustable Output Voltage Configuration
Inductor Selection
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:
( ) ^V_OUT ^{+ V}_DIODE
I_LIM
t_OFF < L
( ) _<
V_IN(min) + V_SWITCH
I_LIM
2t_ON(max)
where I_LIM is the peak switch-current limit, which is
0.8A for I_LIM = PS and 0.5A for I_LIM = GND.
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.
8 _______________________________________________________________________________________