CMOS Micropower Step-Up
Switching Regulator
The MAX630’s low-battery detector monitors the line-
+3V Battery to +5V DC-DC Converter
A common power-supply requirement involves conver-
sion of a 2.4V or 3V battery voltage to a 5V logic sup-
ply. The circuit in Figure 5 converts 3V to 5V at 40mA
powered +5V, and the LBD output can be used to shut
down unnecessary sections of the system during power
failures. Alternatively, the low-battery detector could
monitor the NiCad battery voltage and provide warning
of power loss when the battery is nearly discharged.
with 85% efficiency. When I (pin 6) is driven low, the
C
output voltage will be the battery voltage minus the
drop across diode D1.
Unlike battery backup systems that use 9V batteries,
this circuit does not need +12V or +15V to recharge the
battery. Consequently, it can be used to provide +5V
backup on modules or circuit cards that only have 5V
available.
The optional circuitry using C1, R3, and R4 lowers the
oscillator frequency when the battery voltage falls to
2.0V. This lower frequency maintains the output-power
capability of the circuit by increasing the peak inductor
current, compensating for the reduced battery voltage.
9V Battery Life Extender
Figure 7’s circuit provides a minimum of 7V until the 9V
battery voltage falls to less than 2V. When the battery
Uninterruptable +5V Supply
In Figure 6, the MAX630 provides a continuous supply
of regulated +5V, with automatic switchover between
line power and battery backup. When the line-powered
input voltage is at +5V, it provides 4.4V to the MAX630
and trickle charges the battery. If the line-powered
input falls below the battery voltage, the 3.6V battery
supplies power to the MAX630, which boosts the bat-
tery voltage up to +5V, thus maintaining a continuous
supply to the uninterruptable +5V bus. Since the +5V
output is always supplied through the MAX630, there
are no power spikes or glitches during power transfer.
voltage is above 7V, the MAX630’s I pin is low, putting
C
it into the shutdown mode that draws only 10nA. When
the battery voltage falls to 7V, the MAX8212 voltage
detector’s output goes high, enabling the MAX630. The
MAX630 then maintains the output voltage at 7V, even
as the battery voltage falls below 7V. The LBD is used
to decrease the oscillator frequency when the battery
voltage falls to 3V, thereby increasing the output cur-
rent capability of the circuit.
+12V
47μF
25V
10kΩ
1N4148
SHUTDOWN
6
5
I
+V
C
S
3
2
MAX630
L
X
280kΩ
100kΩ
OPERATE
7
C
V
X
FB
499kΩ
GND
4
47pF
6
2mH
33μH
I
3
5
2
C
L
X
1N5817
+5V AT 0.5A
7
MAX630
+V
C
V
S
FB
3V
LITHIUM
CELL
X
GND
4
1/6 4069
IRF543
47.6kΩ
47pF
470μF
SECTION 1
SECTION 2
Figure 4. High-Power 3V to 5V Converter with Shutdown
10 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
