As the load current decreases, the converters enter a

pulse-skip mode in which the PWM comparator is dis-

abled. At light loads, efficency is enhanced by a

pulse-skip mode in which switching occurs only as

needed to service the load. Quiescent current in skip

mode is typically 16µA. See the Light-Load Switching

Waveforms and Load Transient graphs in the

Typical

Operating Characteristics.

Load-Transient Response/

Voltage Positioning

The MAX1556/MAX1556A/MAX1557 match the load

regulation to the voltage droop seen during transients.

This is sometimes called voltage positioning. The load

line used to achieve this behavior is shown in Figures 4

and 5. There is minimal overshoot when the load is

removed and minimal voltage drop during a transition

from light load to full load. Additionally, the MAX1556,

MAX1556A, and MAX1557 use a wide-bandwidth feed-

back loop to respond more quickly to a load transient

than regulators using conventional integrating feedback

loops (see Load Transient in the

Typical Operating

Characteristics).

The MAX1556/MAX1556A/MAX1557 use of a wide-band

control loop and voltage positioning allows superior

load-transient response by minimizing the amplitude

and duration of overshoot and undershoot in response

to load transients. Other DC-DC converters, with high

gain- control loops, use external compensation to main-

tain tight DC load regulation but still allow large voltage

droops of 5% or greater for several hundreds of

microseconds during transients. For example, if the

load is a CPU running at 600MHz, then a dip lasting

100µs corresponds to 60,000 CPU clock cycles.

Voltage positioning on the MAX1556/MAX1556A/

MAX1557 allows up to 2.25% (typ) of load-regulation

voltage shift but has no further transient droop. Thus,

during load transients, the voltage delivered to the CPU

remains within spec more effectively than with other

regulators that might have tighter initial DC accuracy. In

summary, a 2.25% load regulation with no transient

droop is much better than a converter with 0.5% load

regulation and 5% or more of voltage droop during load

transients. Load-transient variation can be seen only

with an oscilloscope (see the

Typical Operating

Characteristics), while DC load regulation read by a

voltmeter does not show how the power supply reacts

to load transients.

Dropout/100% Duty-Cycle Operation

The MAX1556/MAX1556A/MAX1557 function with a low

input-to-output voltage difference by operating at 100%

duty cycle. In this state, the high-side p-channel

MOSFET is always on. This is particularly useful in

battery-powered applications with a 3.3V output. The sys-

tem and load might operate normally down to 3V or less.

The MAX1556/MAX1556A/MAX1557 allow the output to

follow the input battery voltage as it drops below the regu-

lation voltage. The quiescent current in this state rises

minimally to only 27µA (typ), which aids in extending bat-

tery life. This dropout/100% duty-cycle operation achieves

long battery life by taking full advantage of the entire bat-

tery range.

The input voltage required to maintain regulation is a

function of the output voltage and the load. The differ-

ence between this minimum input voltage and the out-

put voltage is called the dropout voltage. The dropout

voltage is therefore a function of the on-resistance of

inductor resistance (DCR).

Step-Down Converters

8

_______________________________________________________________________________________

-2.5

-1.5

-2.0

-0.5

-1.0

0.5

0

1.0

0

200

400

800

600

1000

1200

LOAD CURRENT (mA)

Figure 4. MAX1556 Voltage-Positioning Load Line

Figure 5. MAX1557 Voltage-Positioning Load Line

0

200

400

600

LOAD CURRENT (mA)

-1.0

-0.4

-0.6

-0.8

-0.2

0

0.2

0.4

0.6

0.8

1.0