Figure 2 shows what happens to the ideal circuit of

Figure 1 if the switch turns on with a 66% duty cycle and

than it falls because the magnitude of the voltage

Varying the duty cycle and switching frequency keeps

the peak current constant as input voltage varies. The

These three equations ensure constant peak currents for

a given inductor value, across all input voltages (ignoring

the voltage drop across the diode (D1) and the resistive

losses in the switch and inductor). The variable duty

cycle also ensures that the current through the inductor

discharges to zero at the end of each pulse.

Figure 3 shows the MAX639/MAX640/MAX653 block dia-

gram and a typical connection in which 9V is converted

to 5V (MAX639), 3.3V (MAX640), or 3.0V (MAX653). The

sequence of events in this application is as follows:

When the output dips:

(1) The error comparator switches high.

(2) The internal oscillator starts (15µs start-up time)

and connects to the gate of the LX output driver.

charging and discharging the inductor, and sup-

plying current to the output (as described above).

When the output voltage recovers:

(1) The comparator switches low.

(2) LX turns off.

(3) The oscillator shuts down to save power.

Fixed or Adjustable Output

For operation at the preset output voltage, connect VFB

to GND; no external resistors are required. For other

output voltages, use an external voltage divider. Set the

output voltage using R3 and R4 as determined by the

following formula:

where R4 is any resistance in the 10k

Ω to 1MΩ range (typ-

ically 100k

Ω), and the VFB threshold is typically 1.28V.

5V/3.3V/3V/Adjustable, High-Efficiency,

8

_______________________________________________________________________________________

MAX639

MAX640

MAX653

VARIABLE

FREQUENCY

AND

DUTY-CYCLE

OSCILLATOR

ERROR

COMPARATOR

LOW-BATTERY

COMPARATOR

+1.28V

BANDGAP

REFERENCE

MODE-SELECT

COMPARATOR

100

µF

1N5817

L = 100

µH

5V, 3.3V OR 3.0V

AT 100mA

5

1

LX

VOUT

50mV

GND

4

VFB

7

R2

2

3

LBO

LBI

R1

33

µF

INPUT, +5.5V TO +11.5V (MAX639),

+3.8V TO +11.5V (MAX640), +3.5V TO +11.5V (MAX653)

8

6

SHDN

V+

Figure 3. Block Diagram