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

Low-Dropout, Step-Down DC-DC Controllers

8

_______________________________________________________________________________________

Modes of Operation

When delivering high output currents, the MAX1649/

MAX1651 operate in continuous-conduction mode. In

this mode, current always flows in the inductor, and

the control circuit adjusts the switch duty cycle to main-

tain regulation without exceeding the switch current

capability (Figure 3). This provides excellent load-tran-

sient response and high efficiency.

In discontinuous-conduction mode, current through the

inductor starts at zero, rises to a peak value, then

ramps down to zero. Although efficiency is still excel-

lent, the output ripple increases slightly, and the switch

waveform exhibits ringing (at the inductor's self-reso-

nant frequency). This ringing is to be expected and

poses no operational problems.

Dropout

The MAX1649/MAX1651 are 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.

__________________Design Procedure

Setting the Output Voltage

The MAX1649/MAX1651 are preset for 5V and 3.3V out-

put voltages, respectively; tie FB to GND for fixed-output

operation. They may also be adjusted from 1.5V (the

reference voltage) to the input voltage, using external

resistors R2 and R3 configured as shown in Figure 4. For

adjustable-output operation, 150k

Ω is recommended for

resistor R3—high enough to avoid wasting energy, yet

low enough to avoid RC delays caused by parasitic

capacitance at FB. R2 is given by:

R2 = R3 x

When using external resistors, it does no harm to con-

nect OUT and the output together, or to leave OUT

unconnected.

Current-Sense Resistor Selection

The current-sense resistor limits the peak switch cur-

the current-sense resistor, and 110mV is the current-

limit trip level (see

Electrical Characteristics).

CIRCUIT OF FIGURE 1, R1 = 75m

Ω

1.5A

0A

1A

2

µs/div

Figure 3. MAX1649 Continuous-Conduction Mode, Heavy

Load-Current Waveform (500mA/div)

(

)

MAX1649

MAX1651

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

0.05

Ω

D1

1N5820

L1

47

µH

P1

Si9430

OUTPUT

@ 1.5A

REF

FB

R2

R3

150k

R2 = R3

– 1

Figure 4. Adjustable-Output Operation