Thermal-overload protection is designed to safeguard
the MAX1793 in the event of fault conditions. For con-
tinuous operation, do not exceed the absolute maxi-
mum junction-temperature rating of T
J
= +150°C.
Operating Region and Power Dissipation
Maximum power dissipation of the MAX1793 depends
on the thermal resistance of the case and circuit board,
the temperature difference between the die junction
and ambient air, and the rate of air flow. The power dis-
sipated by the device is: P = I
OUT
(V
IN
– V
OUT
). The
resulting maximum power dissipation is:
P
MAX
= [(T
J(MAX)
– T
A
)/(θ
JC
+ θ
CA
)]
where (T
J(MAX)
– T
A
) is the temperature difference
between the maximum allowed die junction (+150°C) and
the surrounding air, θ
JC
(junction-to-case) is the thermal
resistance of the package chosen, and θ
CA
is the thermal
resistance from the case through the PCB, copper traces,
and other materials to the surrounding air. Figure 4 shows
allowable power dissipation for typical PCBs at +25°C,
+50°C, and +70°C ambient temperatures.
The MAX1793 TSSOP-EP package features an exposed
thermal pad on its underside. This pad lowers the ther-
mal resistance of the package by providing a direct heat
conduction path from the die to the PCB. Additionally,
the ground pin (GND) also channels heat. Connect the
exposed thermal pad and GND to circuit ground by
using a large pad (minimum 1in
2
recommended), or mul-
tiple vias to the ground plane.
Applications Information
Capacitor Selection
and Regulator Stability
Capacitors are required at the input and output of the
MAX1793. Connect a 4.7µF capacitor between IN and
ground (C
IN
) and a 6.8µF capacitor between OUT and
ground (C
OUT
). C
IN
serves only to lower the source
impedance of the input supply and may be smaller than
4.7µF when the MAX1793 is powered from regulated
power supplies or low-impedance batteries.
The output capacitor’s equivalent series resistance (ESR)
affects stability and output noise. C
OUT
ESR should be
0.5Ω 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. Other low-ESR (< 0.5Ω) capacitors, such as sur-
face-mount tantalum or OS-CON, may also be used.
Connect C
IN
and C
OUT
as close to the IC as possible to
minimize the impact of PCB trace inductance.
Noise, PSRR, and Transient Response
The MAX1793 output noise is typically 115µV
RMS
during
normal operation. This is suitably low for most applica-
tions. See the output noise plot in the
Typical Operating
Characteristics
.
The MAX1793 is designed to achieve low dropout volt-
age and low quiescent current in battery-powered sys-
tems 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 very
noisy sources, supply noise rejection and transient
response can be improved by increasing the values of
the input and output capacitors and employing passive
postfiltering.
Input-Output (Dropout) Voltage
A regulator’s minimum input-to-output voltage differential
(dropout voltage) determines the lowest usable supply
voltage. In battery-powered systems, this determines the
useful end-of-life battery voltage. Since a p-channel
MOSFET is used as the pass device, dropout voltage is
the product of R
DS(ON)
and load current (see the
Electrical Characteristics
and Dropout Voltage vs. Load
Current in the
Typical Operating Characteristics
). The
MAX1793 operating current typically remains below
225µA in dropout.
MAX1793
Low-Dropout, Low I
Q
, 1A Linear Regulator
_______________________________________________________________________________________ 9
MAXIMUM OUTPUT CURRENT vs.
