AS1331
Datasheet - D e t a i l e d D e s c r i p t i o n
Start-Up Mode
At start-up the switch SW D is disabled and its diode is used to transfer current to the output capacitor until V
OUT
reaches approximately 2.15V. The inductor current is controlled by an alternate algorithm during start-up.
Note:
Do not apply loads >1mA until V
OUT
= 2.3V is reached.
Other AS1331 Features
Shutdown
The part is in shutdown mode while the voltage at pin EN is below 0.4V and is active when the voltage is higher than
1.4V.
Note:
EN can be driven above V
IN
or V
OUT
, as long as it is limited to less than 5.5V.
Output Disconnect and Inrush Limiting
During shutdown V
OUT
is going to 0V so that no current from the input source is running thru the device. The inrush
current is also limited at turn-on mode to minimize the surge currents seen by the input supply. These features of the
AS1331 are realized by opening both P-channel MOSFETs of the rectifiers, allowing a true output disconnect.
Power-OK and Low-Battery-Detect Functionality
LBO goes low in startup mode as well as during normal operation if:
1) The voltage at the LBI pin is below LBI threshold (1.25V). This can be used to monitor the battery voltage.
2) LBI pin is connected to GND and V
OUT
is below 92.5% of its nominal value. LBO works as a power-OK signal in this
case.
The LBI pin can be connected to a resistive-divider to monitor a particular definable voltage and compare it with a
1.25V internal reference. If LBI is connected to GND an internal resistive-divider is activated and connected to the
output. Therefore, the Power-OK functionality can be realised with no additional external components.
The Power-OK feature is not active during shutdown and provides a power-on-reset function that can operate down to
V
IN
= 1.8V. A capacitor to GND may be added to generate a power-on-reset delay. To obtain a logic-level output,
connect a pull-up resistor from pin LBO to pin VOUT. Larger values for this resistor will help to minimize current
consumption; a 100k
Ω
resistor is perfect for most applications
For the circuit shown in the left of
the input bias current into LBI is very low, permitting large-
value resistor-divider networks while maintaining accuracy. Place the resistor-divider network as close to the device as
possible. Use a defined resistor for R
2
and then calculate R
1
as:
V
IN
-
(EQ 1)
R
1
=
R
2
⋅ ⎛
------------------- –
1
⎞
⎝
V
SENSE
⎠
Where:
V
SENSE
(the internal sense reference voltage) is 1.25V.
R
2
(the predefined resistor in the resistor devider) has to be
≤
270k
Ω
.
In case of the LBI pin is connected to GND, an internal resistor-devider network is activated and compares the output
voltage with a 92.5% voltage threshold. For this particular Power-OK application, no external resistive components are
necessary.
Thermal Shutdown
To prevent the AS1331 from short-term misuse and overload conditions the chip includes a thermal overload
protection. To block the normal operation mode all switches will be turned off. The device is in thermal shutdown when
the junction temperature exceeds 145°C. To resume the normal operation the temperature has to drop below 135°C.
A good thermal path has to be provided to dissipate the heat generated within the package. Otherwise it’s not possible
to operate the AS1331 at its useable maximal power. To dissipate as much heat as possible away from the package
into a copper plane with as much area as possible, it’s recommended to use multiple vias in the printed circuit board.
It’s also recommended to solder the Exposed Pad (pin 11) to the GND plane.
Note:
Continuing operation in thermal overload conditions may damage the device and is considered bad practice.
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