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

_______________________________________________________________________________________

9

inductor, and the control circuit adjusts the switch duty

cycle to maintain regulation without exceeding the

switch current capability (Figure 3a). This provides

excellent load-transient response and high efficiency.

In discontinuous-conduction mode (DCM), current

through the inductor starts at zero, rises to a peak

value, then ramps down to zero. Although efficiency is

still excellent, the output ripple increases slightly, and

the switch waveforms exhibit ringing (the self-resonant

frequency of the inductor). This ringing is to be expect-

ed and poses no operational problems.

Dropout

The MAX649/MAX651/MAX652 are said to be in

dropout when the input voltage (V+) is low enough

that the output drops below the minimum output

voltage specification (see

Electrical Characteristics).

The dropout voltage is the difference between the

input and output voltage when dropout occurs.

See the

Typical Operating Characteristics for the

Dropout Voltage vs. Load Current and Dropout Voltage

vs. Temperature graphs.

CIRCUIT OF FIGURE 1, R1 = 150m

Ω

1.5A

0A

1A

2

µs/div

Figure 3a. MAX649 Continuous-Conduction Mode, Heavy

Load-Current Waveform (500mA/div)

CIRCUIT OF FIGURE 1, R1 = 100m

Ω

2.5A

0A

1.5A

0.5A

1.0A

2.0A

5

µs/div

Figure 3b. MAX649 Light/Medium Load-Current Waveform

(500mA/div)

(

)

MAX649

MAX651

MAX652

V+

CS

GND

5

6

2

8

3

C2

330

µF

7

1

EXT

OUT

SHDN

4

C3

0.1

µF

C4

0.1

µF

C1

100

µF

R1

0.1

Ω

D1

1N5820

L1

22

µH

P1

Si9430

OUTPUT

@ 1.5A

REF

FB

R2

R3

150k

R2 = R3

– 1

Figure 4. Adjustable-Output Operation