+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

output voltage will be the battery voltage minus the

drop across diode D1.

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.

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.

The MAX630’s low-battery detector monitors the line-

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.

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.

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

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.

CMOS Micropower Step-Up

Switching Regulator

10

______________________________________________________________________________________

MAX630

47

μF

25V

2mH

3

2

57

47pF

1N4148

SHUTDOWN

6

4

GND

3V

LITHIUM

CELL

OPERATE

499k

Ω

47.6k

Ω

SECTION 1

SECTION 2

+12V

MAX630

3

2

7

5

4

6

GND

47pF

10k

Ω

280k

Ω

100k

Ω

1/6 4069

33

μH

IRF543

1N5817

470

μF

+5V AT 0.5A

Figure 4. High-Power 3V to 5V Converter with Shutdown