MAX1857
shut down, RST is held low independent of the output
voltage. If unused, leave RST grounded or unconnected.
Current Limit
The MAX1857 monitors and controls the pass transis-
tor’s gate voltage, limiting the output current to 1.2A.
This current limit doubles when the output voltage is
within 4% of the nominal value to improve performance
with large load transients. The output can be shorted to
ground for an indefinite period of time without damag-
ing the part.
Thermal Overload Protection
Thermal overload protection limits total power dissipa-
tion in the MAX1857. When the junction temperature
exceeds T
J
= +170°C, a thermal sensor turns off the
pass transistor, allowing the device to cool. The thermal
sensor turns the pass transistor on again after the junc-
tion temperature cools by 20°C, resulting in a pulsed
output during continuous thermal overload conditions.
Thermal overload protection protects the MAX1857 in
the event of fault conditions. For continuous operation,
do not exceed the absolute maximum junction-temper-
ature rating of T
J
= +150°C.
Operating Region and
Power Dissipation
The MAX1857’s maximum power dissipation depends on
the thermal resistance of the IC package and circuit
board, the temperature difference between the die junc-
tion and ambient air, and the rate of air flow. The power
dissipated in the device is P = I
OUT
x (V
IN
- V
OUT
). The
maximum allowed power dissipation is 330mW or:
P
MAX
= (T
J(MAX)
- T
A
) / ( θ
JC
+ θ
CA
)
where T
J
- T
A
is the temperature difference between
the MAX1857 die junction and the surrounding air; 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 materi-
als to the surrounding air.
The MAX1857 delivers up to 0.5A
RMS
and operates with
input voltages up to 5.5V, but not simultaneously. High
output currents can only be sustained when input-output
differential voltages are low, as shown in Figure 3.
Applications Information
Capacitor Selection
and Regulator Stability
Capacitors are required at the MAX1857’s input and
output for stable operation over the full temperature
range and with load currents up to 500mA. Connect a
1µF capacitor between IN and ground and a 3.3µF low
equivalent series resistance (ESR) capacitor between
OUT and ground. For output voltages less than 2V, use
a 4.7µF low-ESR output capacitor. The input capacitor
(C
IN
) lowers the source impedance of the input supply.
Reduce noise and improve load-transient response,
stability, and power-supply rejection by using larger
output capacitors such as 10µF.
The output capacitor’s (C
OUT
) ESR affects stability and
output noise. Use output capacitors with an ESR of
0.1Ω or less to ensure stability and optimum transient
response. Surface-mount ceramic capacitors have very
low ESR and are commonly available in values up to
10µF. Connect C
IN
and C
OUT
as close to the MAX1857
as possible to minimize the impact of PC board trace
inductance.
Noise, PSRR, and Transient Response
The MAX1857 is designed to operate with low dropout
voltages and low quiescent currents in battery-powered
systems while still maintaining good noise, transient
response, and AC rejection. See the Typical Operating
Characteristics for a plot of power-supply rejection ratio
(PSRR) vs. frequency. When operating from noisy
sources, improved supply-noise rejection and transient
response can be achieved by increasing the values of
the input and output bypass capacitors and through
passive filtering techniques.
The MAX1857 load-transient response graphs (see
Typical Operating Characteristics) show two compo-
500mA, Low-Dropout,
Ripple-Rejecting LDO in µMAX
8 _______________________________________________________________________________________
Figure 3. Power Operating Regions: Maximum Output Current
vs. Supply Voltage
