MAX1556_10 Datasheet, PDF (8/12 Pages) Maxim Integrated Products – 16μA IQ, 1.2A PWM Step-Down DC-DC Converters

16µA I_Q, 1.2A PWM DC-DC
Step-Down Converters
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
1.0
0.5
0 _{VIN = 3.6V}
-0.5
V_IN = 5.5V
-1.0
V_IN = 2.6V
-1.5
-2.0
-2.5
0
200 400 600 800 1000 1200
LOAD CURRENT (mA)
Figure 4. MAX1556 Voltage-Positioning Load Line
1.0
0.8
0.6
0.4
0.2 ^{VIN = 3.6V}
0
V_IN = 5.5V
-0.2
-0.4
V_IN = 2.6V
-0.6
-0.8
-1.0
0
200
400
600
LOAD CURRENT (mA)
Figure 5. MAX1557 Voltage-Positioning Load Line
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
the internal p-channel MOSFET (R_DS(ON)P) and the
inductor resistance (DCR).
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