Forced and Automatic Bypass Mode
Invoke forced-bypass mode by driving HP high or
invoke automatic bypass by applying a high voltage to
REFIN (VREFIN > 2.1V with a Li-ion (Li+) battery at IN).
To prevent excessive output ripple as the step-down
converter approaches dropout, the MAX8581/MAX8582
preemptively enter bypass mode automatically when
VREFIN > 0.465 VIN (see Figure 1).
Shutdown Mode
Connect SHDN to GND or logic-low to place the
MAX8581/MAX8582 in shutdown mode and reduce
supply current to 0.1µA. In shutdown, the control cir-
cuitry, internal switching MOSFET, and synchronous
rectifier turn off and LX becomes high impedance.
Connect SHDN to IN or logic-high for normal operation.
Fast Soft-Start
The MAX8581/MAX8582 have internal fast soft-start cir-
cuitry that limits inrush current at startup, reducing tran-
sients on the input source. Soft-start is particularly
useful for supplies with high output impedance such as
Li+ and alkaline cells. See the Soft-Start Waveforms in
the Typical Operating Characteristics.
Analog REFIN Control
The MAX8581/MAX8582 use REFIN to set the output
voltage and to switch to bypass mode. The output volt-
age is two times the voltage applied at REFIN minus
half the IR voltage drop caused by the inductor’s DC
resistance for the MAX8581/MAX8582. This allows the
converter to operate in applications where dynamic
voltage control is required.
Applications Information
The MAX8581/MAX8582 are optimized for use with a tiny
inductor and small ceramic capacitors. The correct
selection of external components ensures high efficiency,
low output ripple, and fast transient response.
Setting the Output Voltage
The MAX8581/MAX8582 output voltages are set by the
voltage applied to REFIN. The output voltage is 2 VREFIN
minus half the IR voltage drop caused by the inductor’s
DC resistance for the MAX8581/MAX8582.
Inductor Selection
The MAX8581/MAX8582 use 1.5µH and 3.3µH, respec-
tively. Low inductance values are physically smaller
but require faster switching, which results in some effi-
ciency loss (see the Typical Operating Characteristics
for efficiency).
The inductor’s DC current rating only needs to match the
maximum load of the application because the
MAX8581/MAX8582 feature zero current overshoot during
startup and load transients. For optimum transient
response and high efficiency, choose an inductor with
DC series resistance in the 50mΩto 150mΩrange.
Output Capacitor Selection
The output capacitor is required to keep the output volt-
age ripple small and to ensure regulation loop stability.
The output capacitor must have low impedance at the
switching frequency. Ceramic capacitors with X5R or
X7R dielectric are highly recommended due to their
small size, low ESR, and small temperature coefficients.
Due to the unique feedback network, the output capa-
citance can be very low. In most applications, 2.2µF
works well. For optimum load-transient performance
and very low output ripple, the output capacitor value
can be increased.
Input Capacitor Selection
The input capacitor reduces the current peaks drawn
from the battery or input power source and reduces
switching noise in the MAX8581/MAX8582. The imped-
ance of the input capacitor at the switching frequency
should be kept very low. Ceramic capacitors with X5R
or X7R dielectric are highly recommended due to their
small size, low ESR, and small temperature coefficients.
Due to the MAX8581/MAX8582s’ fast soft-start, the input
capacitance can be very low. In most applications,
2.2µF works well. For optimum noise immunity and low
input ripple, the input capacitor value can be increased.
MAX8581/MAX8582
2.5MHz/1.5MHz Step-Down Converters
with 60mΩBypass in TDFN for CDMA PA Power
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