Table 2. Recommended Inductors
output and storing energy in the inductor’s magnetic
field. The current-mode feedback system regulates the
peak inductor current as a function of the output voltage
error signal. Since the average inductor current is nearly
the same as the peak inductor current (assuming that
the inductor value is relatively high to minimize ripple
current), the circuit acts as a switch-mode transconduc-
tance amplifier. This pushes the output LC filter pole,
normally found in a voltage-mode PWM, to a higher fre-
quency. To preserve inner-loop stability and eliminate
inductor staircasing, an internal slope-compensation
ramp is summed into the main PWM comparator. During
the second half of the switching cycle (off-time), the
internal high-side P-channel MOSFET turns off and the
internal low-side N-channel MOSFET turns on. Now the
inductor releases the stored energy as its current ramps
down while still providing current to the output. The output
capacitor stores charge when the inductor current
exceeds the load current and discharges when the
inductor current is lower, smoothing the voltage across
the load. Under overload conditions, when the inductor
current exceeds the current limit, the high-side MOSFET
is turned off and the low-side MOSFET remains on for
the remainder of the cycle to let the inductor current
ramp down.
Pulse-Group Mode
Pulse-group mode is used to minimize the supply cur-
rent with a light load. In pulse-group mode, the IC shuts
off most internal circuitry when V
OUT
is +0.8% above
nominal regulation. When V
OUT
drops below +0.8% of
the nominal regulation voltage, the IC powers up its cir-
cuits and resumes switching.
Pulse-Skip Mode
Pulse-skip mode is also used to minimize the supply
current with a light load. The difference between pulse-
group and pulse-skip modes is that when V
OUT
rises
above the +0.8% regulation point, pulse-group mode
stops switching and completely turns off a number of
circuits. Under the same conditions, pulse-skip mode
stops switching but leaves all circuits on. The delay
coming out of pulse-skip mode is shorter than with
pulse-group mode. In pulse-skip mode, the output volt-
age ripple is lower, and the load-transient response
faster. However, the quiescent current is higher than in
pulse-group mode.
Forced-PWM Mode
In forced-PWM mode, the MAX1572 operates at a con-
stant 2MHz switching frequency without pulse skipping.
This is desirable in noise-sensitive applications, since the
output ripple is minimized and has a predictable noise
spectrum. Forced-PWM mode requires higher supply
current with light loads due to constant switching.
100% Duty-Cycle Operation
The MAX1572 can operate at 100% duty cycle. In this
state, the high-side P-channel MOSFET is turned on (not
switching). This occurs when the input voltage is close to
the output voltage. The dropout voltage is the voltage
drop due to the output current across the on-resistance
of the internal P-channel MOSFET (R
DS(ON)P
) and the
inductor resistance (R
L
):
V
DROPOUT
= I
OUT
× ( R
DS(ON)P
+ R
L
)
R
DS(ON)P
is given in the Electrical Characteristics sec-
tion. R
L
, for a few recommended inductors, is given in
Table 2.
Load-Transient Response/
Voltage Positioning
The MAX1572 uses voltage positioning that matches
the load regulation to the voltage droop seen during
load transients. In this way, the output voltage does not
overshoot when the load is removed, which results in
the total output-voltage variation being half as wide as
in a conventional design. Figure 2 shows an example of
a voltage-positioned and a nonvoltage-positioned load
transient. Additionally, the MAX1572 uses a wide-band-
width feedback loop to respond more quickly to a load
transient than regulators using conventional integrating
feedback loops.
The load line used to achieve voltage positioning is
shown in Figure 3. This assumes a nominal operating
point of 3.6V input at 300mA load.
