Detailed Description
The MAX16999 is a low-quiescent-current linear regula-
tor designed for applications requiring high reliability,
such as automotive applications. This device can supply
loads up to 100mA and is available in factory-preset out-
put voltages from 0.5V to 3.3V (see the
Selector Guide
).
As illustrated in Figure 1, the MAX16999 linear regulator
consists of a reference, an error amplifier, a p-channel
MOSFET pass transistor, and an internal feedback volt-
age-divider. A power-on reset section signals if the out-
put voltage has come out of regulation. The reset signal
timeout is defined by the charging time of an external
capacitor attached to CRES.
To increase system reliability, the MAX16999 features a
POR reset timeout along with overcurrent and overtem-
perature protection. A power-on reset timeout guaran-
tees startup even with a faulty timing capacitor.
Parameters are guaranteed up to +125°C junction tem-
perature. The EN signal is latched in its last state even
if the signal line becomes disconnected.
Logic-Controlled Enable
The MAX16999 provides a logic-enable input (EN). For
normal operation drive EN to logic-high. When EN is
driven high, the linear regulator starts to regulate by
increasing the output voltage up to the preset value. To
disable the device, drive EN low to set OUT to high
impedance—this enables a pulldown current from
CRES to discharge the capacitor. Once the device is
disabled, the input supply current reduces to less than
0.3µA. The EN input is latched into its last state by a
120kΩ internal resistor. To change state, the latch
needs to be overridden. When EN is low, the RESET
output is high impedance.
POR Timer
Once the output voltage rises above the threshold V
RES
,
two internal timers are simultaneously activated. The
reset timer is realized by means of a pullup current
I
CRES,UP
that charges the capacitor connected to CRES.
As soon as the voltage on C
RES
rises above the thresh-
old of 600mV (typ), RESET goes high impedance.
The internal reset timer is set by the value of the exter-
nal capacitance (C
CRES
). Calculate the reset time
using the following formula:
where C
CRES
is in Farads and the delay to GND is
given in seconds.
The second timer is an internal fault timer and ensures
the regulator does not stay off indefinitely because of a
fault on CRES such as a short. The fault timer runs for a
maximum of 100ms. A logic block monitors both inter-
nal timers to determine the shortest timeout. If the first
timeout is the fault timer, the pullup current is switched
off in order to avoid unnecessary current consumption
and a resistive pulldown is also activated. If
t
POR_DELAY
exceeds 100ms (typ) the fault timer defines
the timeout behavior.
Current Limit
Once the output voltage reaches regulation, the output
current is limited to 150mA (typ). If the output current
exceeds the current limit, the output voltage begins to
decrease.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipa-
tion in the MAX16999. When the junction temperature
exceeds +165°C, a thermal sensor turns off the pass
transistor, allowing the IC to cool. The thermal sensor
turns the pass transistor on again after the junction tem-
perature cools by 15°C, resulting in a pulsed output
during continuous thermal-overload conditions.
Thermal-overload protection safeguards the MAX16999
in the event of fault conditions. For continuous opera-
tion, do not exceed the absolute maximum junction-
temperature rating of +150°C. Table 1 lists maximum
DC output currents (milliamps) that are allowed for
operation at T
A
= +125°C without causing thermal shut-
down of the MAX16999.
