Rainbow Electronics MAX8842 - Manual (Page 7)

MAX8840/MAX8841/MAX8842
Ultra-Low-Noise, High PSRR, Low-Dropout,
150mA Linear Regulators in µDFN
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For design purposes, consider the current limit to be
150mA (min) to 300mA (max). The output can be shorted
to ground for an indefinite amount of time without dam-
aging the part.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation
in the MAX8840/MAX8841/MAX8842. When the junction
temperature exceeds T
J = +160°C, the thermal sensor
signals the shutdown logic, turning off the pass transistor
and allowing the IC to cool down. The thermal sensor
turns the pass transistor on again after the IC’s junction
temperature drops by 10°C, resulting in a pulsed output
during continuous thermal-overload conditions.
Thermal-overload protection is designed to protect the
MAX8840/MAX8841/MAX8842 in the event of a fault
condition. For continual operation, do not exceed the
absolute maximum junction temperature rating of T
J =
+150°C.
Operating Region and Power Dissipation
The MAX8840/MAX8841/MAX8842 maximum power
dissipation depends on the thermal resistance of the
case and circuit board, the temperature difference
between the die junction and ambient, and the rate of
airflow. The power dissipation across the device is:
P = I
OUT
(V
IN
- V
OUT
)
The maximum power dissipation is:
P MAX = (T J - T A ) / ( θ
JC + θ
CA )
where T J - T A is the temperature difference between
the MAX8840/MAX8841/MAX8842 die junction and the
surrounding air,
θ
JC
is the thermal resistance of the
package, and θ
CA is the thermal resistance through the
PC board, copper traces, and other materials to the
surrounding air.
The GND pin of the MAX8840/MAX8841/MAX8842 per-
forms the dual function of providing an electrical con-
nection to ground and channeling heat away. Connect
the GND pin to ground using a large pad or ground
plane.
Noise Reduction
For the MAX8840, an external 0.01µF bypass capacitor
between BP and OUT with an innovative noise bypass
scheme reduces output noises dramatically, exhibiting
11µV
RMS of output voltage noise with C BP = 0.01µF
and C OUT = 1µF. Startup time is minimized by a power-
on circuit that precharges the bypass capacitor.
Applications Information
Capacitor Selection
and Regulator Stability
Use a 1µF capacitor on the MAX8840/MAX8841/
MAX8842 input and a 1µF capacitor on the output.
Larger input capacitor values and lower ESRs provide
better noise rejection and line-transient response.
Reduce output noise and improve load-transient
response, stability, and power-supply rejection by
using large output capacitors. Note that some ceramic
dielectrics exhibit large capacitance and ESR variation
with temperature. With dielectrics such as Z5U and
Y5V, it may be necessary to use a 2.2µF or larger output
capacitor to ensure stability at temperatures below
-10°C. With X7R or X5R dielectrics, 1µF is sufficient at all
operating temperatures. A graph of the Region of Stable
C
OUT ESR vs. Load Current is shown in the Typical
Operating Characteristics.
Use a 0.01µF bypass capacitor at BP (MAX8840) for
low-output voltage noise. The leakage current going into
the BP pin should be less than 10nA. Increasing the
capacitance slightly decreases the output noise. Values
above 0.1µF and below 0.001µF are not recommended.
Noise, PSRR, and Transient Response
The MAX8840/MAX8841/MAX8842 are designed to
deliver ultra-low noise and high PSRR, as well as low
dropout and low quiescent currents in battery-powered
systems. The MAX8840 power-supply rejection is 78dB
at 1kHz and 54dB at 100kHz. The MAX8841/MAX8842
PSRR is 72dB at 1kHz and 46dB at 100kHz (see the
Power-Supply Rejection Ratio vs. Frequency graph in
the Typical Operating Characteristics).
When operating from sources other than batteries,
improved supply-noise rejection and transient response
can be achieved by increasing the values of the input
and output bypass capacitors, and through passive fil-
tering techniques. The Typical Operating Characteristics
show the MAX8840/MAX8841/MAX8842 line- and load-
transient responses.
Dropout Voltage
A regulator’s minimum dropout voltage determines the
lowest usable supply voltage. In battery-powered sys-
tems, this determines the useful end-of-life battery volt-
age. Because the MAX8840/MAX8841/MAX8842 use a
p-channel MOSFET pass transistor, their dropout voltage
is a function of drain-to-source on-resistance (R
DS(ON) )
multiplied by the load current (see the Typical Ope-
rating Characteristics).