Thermal-Overload Protection
Thermal-overload protection limits total power dissipa-
tion in the MAX1658/MAX1659. When the junction tem-
perature exceeds T
J
= +165°C, the pass transistor
deactivates, allowing the IC to cool. Once it has cooled
by 10°C, the control logic will enable operation. Under
thermal overload, the output of the device will pulse as
the die heats up and then cools to operational levels.
Prolonged operation under these conditions is not rec-
ommended.
Operating Region and Power Dissipation
Maximum power dissipation of the MAX1658/MAX1659
depends on the thermal resistance of the package and
circuit board, the temperature difference between the
die and ambient air, and the rate of air flow. The power
dissipation by the device is P = I
OUT
(V
IN
- V
OUT
). The
maximum power dissipation is:
where (T
J
- T
A
) is the temperature difference between
MAX1658/MAX1659 die junction and the surrounding
air, θ
JB
is the thermal resistance of the package, and
θ
BA
is the thermal resistance through the printed circuit
board, copper traces, and other materials to the
surrounding air. The 8-pin SO package for the
MAX1658/MAX1659 features a special lead frame with
a lower thermal resistance and higher allowable power
dissipation than a standard SO-8. The thermal resis-
tance of this package is θ
JB
= 69°C/W, compared with
θ
JB
= 170°C/W for an SO-8.
The IN pins of the MAX1658/MAX1659 package per-
form the dual function of providing an electrical con-
nection to IN and channeling heat away. Connect all IN
pins to the input voltage using a large pad or power
plane on the surface. Where this is impossible, connect
to a copper plane on an adjacent layer. The pad should
meet the dimensions specified in Figure 4.
Figure 4 assumes the IC is soldered directly to the pad,
has a +125°C maximum junction temperature and a
+25°C ambient air temperature, and has no other heat
sources. Use larger pad sizes for lower junction tem-
peratures, higher ambient temperatures, or conditions
where the IC is not soldered directly to a heat-sinking
IN pad.
The MAX1658/MAX1659 can regulate currents up to
350mA and operate with input voltages up to 16.5V, but
not simultaneously. High output currents can only be
sustained when input-output differential voltage is low,
as shown in the following equation. Maximum power
dissipation depends on packaging, board layout, tem-
perature, and air flow. The maximum output current is:
where P
MAX
is derived from the T
J
= 125°C curve of
Figure 4.
Reverse Battery Protection
The MAX1658/MAX1659 feature reverse battery protec-
tion. Under normal operation, a P-channel MOSFET
connects the substrate of the device to IN. When the
input voltage falls below ground (implying reverse bat-
tery conditions), the P-channel switch turns off and dis-
connects the substrate from IN, disabling the device.
The maximum reverse battery voltage allowed is -17V.
SHDN also withstands reverse battery conditions and
can be connected directly to IN with no loss of protec-
tion.
Polarized input bypass capacitors will be damaged
under reverse battery conditions. To ensure circuit reli-
ability, use a non-polarized capacitor at the input.
The MAX1658/MAX1659 do not provide reverse current
protection. If V
OUT
is greater than V
IN
by more than
300mV, reverse current will flow. Reverse current pro-
tection can be added by connecting a Schottky diode
in series with IN.
