considerably. Under heavy loads, pnp base-drive cur-
rent becomes large, further reducing efficiency; pnp-
based regulators also require considerable current in
dropout when the pass transistor saturates. The
MAX8633–MAX8636 do not suffer from these problems.
With both outputs active, the devices consume only
54µA of quiescent current at no load, and 60µA with
100mA load current for each output (see
Typical
Operating Characteristics
). A pnp-based regulator has a
high dropout voltage that is independent of the load. The
dropout voltage of a p-channel MOSFET is proportional
to load current providing for low-dropout voltage at
heavy loads and extremely low dropout at lighter loads.
Current Limit
The MAX8633–MAX8636 provide independent current
limiting for OUT1 and OUT2. Output current is limited to
500mA (typ) and 400mA (min) for each regulator.
Shutdown (
SHDN1
,
SHDN2
,
SHDN
)
The MAX8634/MAX8635 have independent shutdown
control inputs (SHDN1 and SHDN2) and the MAX8633/
MAX8636 have one shutdown control input (SHDN) for
both outputs. Drive SHDN1 low to shut down OUT1.
Drive SHND2 low to shut down OUT2. Drive both SHDN1
and SHDN2 low to shut down the entire device, reducing
supply current to 1µA max. For the MAX8634, drive
SHDN low to shut down the entire device. Connect
SHDN1, SHDN2, or SHDN to a logic-high or IN to perma-
nently enable the corresponding LDO(s).
Thermal-Overload Protection
Thermal-shutdown circuitry protects the MAX8633–
MAX8636 from damage due to excessive junction tem-
perature. The shutdown circuit disables OUT1 and OUT2
when the junction temperature (T
J
) exceeds +165°C.
Both regulators are reenabled when T
J
falls by 15°C.
Low-Noise Operation (MAX8634/MAX8636)
An external 0.01µF bypass capacitor at BP in conjunc-
tion with an internal resistor creates a lowpass filter. The
MAX8634/MAX8636 exhibit less than 45µV
RMS
of out-
put voltage noise with C
BP
= 0.01µF and C
OUT
= 2.2µF.
These values are shown in the Output Noise Spectral
Density graph in the
Typical Operating Characteristics
section. If output noise is not critical, omit the BP
capacitor to reduce total solution size and cost.
Applications Information
Capacitor Selection and
Regulator Stability
Use a ceramic input capacitor of at least 2.2µF and a
ceramic output capacitor of at least 2.2µF for each out-
put to ensure stable operation over the entire tempera-
ture range. Output capacitors can be reduced to 1µF
for load currents less than 150mA. The MAX8633–
MAX8636 are optimized for ceramic capacitors and
require low equivalent-series resistance (ESR) to
achieve the stated specifications for low-output noise
and power-supply rejection. To ensure proper opera-
tion over the specified temperature range, dielectrics
such as X7R or X5R are recommended. If Z5U or Y5V
dielectrics are used, it may be necessary to increase the
value of the output capacitors to ensure stability at tem-
peratures below -10°C. Tantalum capacitors are not re-
commended due to their higher ESR.
For loads up to 300mA, or for improved load-transient
response, 2.2µF or larger output capacitors can be used.
PSRR and Operation from
Sources Other than Batteries
The MAX8633–MAX8636 deliver low-dropout voltages
and low-quiescent currents in battery-powered systems.
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 capacitors and through passive filtering tech-
niques. Power-supply rejection is 60dB at frequencies
below 1kHz (see the Power-Supply Rejection Ratio vs.
Frequency in the
Typical Operating Characteristics
).
P1 and P2 (P for MAX8634)
External Termination
The pin-programmable inputs (P1 and P2 for
MAX8633/MAX8635/MAX8636, P for MAX8634) should
be connected to IN, GND, or left open. If P_ inputs are
left open, ensure the external capacitance is less than
50pF. If P_ inputs are set high or low, ensure the resis-
tance to IN or GND is less than 1kΩ. The MAX8633–
MAX8636 configure output voltages at OUT1 and OUT2
based on the state of P1 and P2 (P for MAX8634) at
power-on. Subsequent changes to P, or P1 and P2 do
not change the output voltages unless the supply
power is cycled, or all SHDN inputs are simultaneously
driven low to shut down the device.
Load-Transient Considerations
The MAX8633–MAX8636 load-transient response
graphs (see
Typical Operating Characteristics
) show
two components of the output response: a DC step in
the output voltage due to the change in load current,
and the transient response. Increase the value and
decrease the ESR of the output capacitor to attenuate
transient spikes.
MAX8633–MAX8636
Dual 300mA Pin-Programmable LDO
Linear Regulators
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