Rainbow Electronics MAX15011 - Manual (Page 15)

Applications Information
Load Dump
Most automotive applications run off a multicell 12V
lead-acid battery with a nominal voltage that swings
between 9V and 16V, depending on load current,
charging status, temperature, and battery age, etc. The
battery voltage is distributed throughout the automobile
and is locally regulated down to voltages required by
the different system modules. Load dump occurs when
the alternator is charging the battery and the battery
becomes disconnected. Power in the alternator (behav-
ing now essentially as an inductor) flows into the dis-
tributed power system and elevates the voltage seen at
each module. The voltage spikes have rise times typi-
cally greater than 5ms and decay within several hun-
dred milliseconds but can extend out to 1s or more
depending on the characteristics of the charging sys-
tem. These transients are capable of destroying semi-
conductors on the first fault event.
The MAX15009/MAX15011 feature load-dump transient
protection up to +45V.
Setting the Output Voltage
The MAX15009/MAX15011 feature dual-mode opera-
tion: these devices operate in either a preset voltage
mode or an adjustable mode. In preset voltage mode,
internal feedback resistors set the linear regulator out-
put voltage (V
OUT_LDO ) to 5V. To select the preset 5V
output voltage, connect FB_LDO to SGND.
To select an adjustable output voltage between 1.8V
and 11V, use two external resistors connected as a
voltage-divider to FB_LDO (Figure 3). Set the output
voltage using the following equation:
V OUT_LDO = V FB_LDO x (R 1 + R 2 ) / R 2
where V FB_LDO = 1.235V and R 2 ≤ 50kΩ.
Setting the
RESET Timeout Period
The reset timeout period is adjustable to accommodate
a variety of applications. Set the reset timeout period by
connecting a capacitor, C
RESET , between CT and
SGND. Use the following formula to select the reset
timeout period, t
RESET :
t RESET = C RESET x V CT_TH / I CT
where t
RESET
is in seconds and C
RESET
is in µF.
V CT_TH is the CT ramp threshold in volts and I CT is the
CT ramp current in µA, as described in the Electrical
Characteristics table.
MAX15009/MAX15011
Automotive 300mA LDO Regulators with
Switched Output and Overvoltage Protector
______________________________________________________________________________________ 15
IN
R1
R2
V
IN
FB_LDO
SGND
OUT_LDO
MAX15009
MAX15011
Figure 3. Setting the LDO Output Voltage
Table 1. EN_LDO/H HO OL LD D Truth/State Table
OPERATION STATE EN_LDO HOLD OUT_LDO COMMENT
Initial State Low Don’t care OFF
EN_LDO is pulled to SGND through an internal pulldown. HOLD
is unconnected and is internally pulled up to OUT_LDO. The
regulator is disabled.
Turn-On State High Don’t care ON
EN_LDO is externally driven high turning regulator on. HOLD is
pulled up to OUT_LDO.
Hold Setup State High Low ON
HOLD is externally pulled low while EN_LDO remains high
(latches EN_LDO state).
Hold State Low Low ON
EN_LDO is driven low or left unconnected. HOLD remains
externally pulled low keeping the regulator on.
Off State Low
High or
unconnected
OFF
HOLD is driven high or left unconnected while EN_LDO is low.
The regulator is turned off and EN_LDO/
HOLD logic returns to the
initial state.