quiescent current at no load and 220µA with 100mA
load current on both outputs (see the Typical Operating
Characteristics). A PNP-based regulator has a high
dropout voltage that is independent of the load. A P-
channel MOSFET’s dropout voltage is proportional to
load current, providing for low dropout voltage at heavy
loads and extremely low dropout at lighter loads.
Current Limit
The MAX8559 contains two independent current lim-
iters, one for each regulator output, monitoring and
controlling the pass transistor’s gate voltage and limit-
ing the output current to 310mA (min). The outputs can
be shorted to ground continuously without damaging
the part.
Low-Noise Operation
An external 0.01µF bypass capacitor at BP in conjunc-
tion with an internal resistor creates a lowpass filter.
The MAX8559 exhibits less than 32µV
RMS
of output
voltage noise with C
BP
= 0.01µF and C
OUT
= 10µF. The
Typical Operating Characteristics show a graph of
Output-Noise Spectral Density with these values. If out-
put noise is not critical, the BP capacitor can be
removed to reduce total solution size and cost.
Shutdown
The MAX8559 has independent shutdown control
inputs (SHDNA and SHDNB). Drive SHDNA low to shut
down OUTA. Drive SHNDB low to shut down OUTB.
Drive both SHDNA and SHDNB low to shut down the
entire chip, reducing supply current to 0.01µA. Connect
SHDNA or SHDNB to a logic high or IN_ for always-on
operation of the corresponding LDO. Each LDO output
is internally discharged to ground through a 385Ω
resistor in shutdown mode.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipa-
tion in the MAX8559. Each regulator has its own inde-
pendent thermal detector. When one of the regulators’
junction temperature exceeds T
J
= +160°C, that regu-
lator’s pass transistor is turned off allowing the IC to
cool. The thermal sensor turns the pass transistor on
again after the IC’s junction temperature cools by 10°C.
This results in a pulsed output during continuous ther-
mal-overload conditions.
Operating Region and Power Dissipation
The MAX8559 maximum power dissipation depends on
the thermal resistance of the case and the circuit board,
the temperature difference between the die junction and
ambient air, and the rate of airflow. The power dissipa-
tion across the device is P = I
OUT
x (V
IN
- V
OUT
).
The maximum power dissipation allowed is:
P
MAX
= (T
J
- T
A
) / (R
θJB
+ R
θBA
)
where T
J
- T
A
is the temperature difference between
the MAX8559 die junction and the surrounding air,
R
θJB
(R
θJC
) is the thermal resistance of the package,
and R
θBA
is the thermal resistance through the printed
circuit board, copper traces, and other materials to the
surrounding air.
The exposed paddle of the TDFN package performs
the function of channeling heat away. Connect the
exposed paddle to the board ground plane.
Applications Information
Capacitor Selection and
Regulator Stability
For load currents up to 150mA, use a single 2.2µF
capacitor to bypass both inputs of the MAX8559 and a
2.2µF capacitor to bypass each output. Larger input-
capacitor values and lower ESRs provide better supply-
noise rejection and line-transient response. To reduce
output noise and improve load-transient voltage dips,
use larger output capacitors up to 10µF. For stable oper-
ation over the full temperature range with load currents
up to 300mA, input and output capacitors should be a
minimum of 4.7µF.
Note that some ceramic dielectrics exhibit large capaci-
tance and ESR variation with temperature. With
dielectrics such as Z5U and Y5V, it may be necessary to
use 4.7µF or more for up to 150mA load current to
ensure stability at temperatures below -10°C. With X7R
or X5R dielectrics, 2.2µF is sufficient at all operating tem-
peratures. These regulators are optimized for ceramic
capacitors. Tantalum capacitors are not recommended.
Use a 0.01µF bypass capacitor at BP for low-output volt-
age noise. Increasing the capacitance slightly decreas-
es the output noise, but increases the startup time.
PSRR and Operation from Sources Other
than Batteries
The MAX8559 is designed to deliver low-dropout volt-
ages and low quiescent currents in battery-powered
systems. Power-supply rejection ratio is 70dB at 10kHz
(see Power-Supply Rejection Ratio vs. Frequency in
the Typical Operating Characteristics). When operat-
ing from sources other than batteries, improved sup-
ply-noise rejection and transient response is achieved
by increasing the values of the input and output
bypass capacitors and through passive RC or CRC fil-
tering techniques.
MAX8559
Dual, 300mA, Low-Noise Linear Regulator
with Independent Shutdown in UCSP or TDFN
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