MAX8880/MAX8881
12V, Ultra-Low-I
Q
, Low-Dropout
Linear Regulators with POK
_______________________________________________________________________________________ 7
where T
J(MAX)
= +150°C, T
A
is the ambient tempera-
ture, θ
JC
is the thermal resistance from the junction to
the case, and θ
CA
is the thermal resistance from the
case through the PC board, copper traces, and other
materials to the surrounding air.
POK Output
The open-drain POK output is useful as a simple error
flag, as well as a delayed reset output. POK sinks cur-
rent when the output voltage is 10% below the regula-
tion point. Connect POK to OUT through a high-value
resistor for a simple error flag indicator. Connect a
capacitor in parallel with the resistor to produce a
delayed POK signal (delay set by the RC time con-
stant). POK is low during out of regulation or in shut-
down and is high impedance during normal operation.
Applications Information
Capacitor Selection and Regulator
Stability
The MAX8880/MAX8881 are designed to be stable with
an output filter capacitor as low as 1µF and an ESR as
high as 1Ω. For general purposes, use a 1µF capacitor
on the device’s input and a 4.7µF capacitor on the out-
put. Larger input capacitor values and lower ESR pro-
vide better supply-noise rejection and transient
response. Use a higher value input capacitor (10µF may
be necessary) if large, fast transients are anticipated
and the device is located several inches from the power
source. Use large output capacitors to improve load-
transient response, stability, and power-supply rejec-
tion. Note that some ceramic dielectric materials (e.g.,
Z5U and Y5V) exhibit a large temperature coefficient for
both capacitance and ESR, and a larger output capaci-
tance may be needed to ensure stability at low tempera-
tures. A 4.7µF output capacitor with X7R or X5R
dielectrics should be sufficient for stable operation over
the full temperature range, with load currents up to
200mA. For load currents up to 40mA, 1µF is accept-
able. A graph of the Region of Stable C
out
ESR vs.
Output Current is shown in the Typical Operating
Characteristics.
Output Voltage Selection
The MAX8881 features a preset output voltage. Internal
precision feedback resistors set the MAX8881EUT18 out-
put to 1.8V, the MAX8881EUT25 output to 2.5V, the
MAX8881EUT33 output to 3.3V, and the MAX8881EUT50
output to 5V. Connect the MAX8881’s FB to OUT for
proper operation.
The MAX8880 features an adjustable output voltage
from 1.25V to 5.5V, using two external resistors con-
nected as a voltage-divider to FB (Figure 3).
The output voltage is set by the following equation:
where typically V
FB
= 1.257V. Choose R4 = 1.2MΩ to
optimize quiescent current, accuracy, and high-fre-
quency power-supply rejection. To simplify resistor
selection:
The total current through the external resistive feedback
and load resistors should be greater than 1µA. Since the
V
FB
tolerance is typically less than ±1.5%, the output
can be set using fixed resistors instead of trim pots.
Power-Supply Rejection and Operation
from Sources Other than Batteries
The MAX8880/MAX8881 are designed to deliver low-
dropout voltages and low quiescent currents in battery-
powered systems. Power-supply rejection is -66dB at
low frequencies and rolls off with frequencies above
100Hz. At high frequencies, the output capacitor is the
major contributor to the rejection of power-supply noise
(see Power-Supply Rejection Ratio vs. Frequency in the
Typical Operating Characteristics).
When operating from sources other than batteries,
improve supply-noise rejection and transient response
by increasing the value of the input and output capaci-
tors and by using passive filtering techniques.
The MAX8880/MAX8881 load-transient response
graphs (see Typical Operating Characteristics) show
the output response due to changing load current.
Reduce overshoot by increasing the output capacitor’s
value up to 10µF and by reducing its ESR.
Chip Information
TRANSISTOR COUNT: 134
