LM3561 Datasheet, PDF (16/36 Pages) Texas Instruments – LM3561 Synchronous Boost Converter with 600mA High Side LED Driver and I 2 C-Compatible Interface

during high battery current situations. For example, when the
LM3561 is engaged in a Flash event and TX2 is pulled high
(active high polarity) the current source (LED) is forced into
torch mode at the programmed Torch current setting. If TX2
is then pulled low before the flash pulse terminates, the LED
current will step back to the previous flash current level. At the
end of the flash timeout, whether the TX2 pin is high or low,
the LED current will turn off. In addition to forcing torch mode
with a TX2 event, the TX2 input can be set to force shutdown.
Configuration Register 2 bit[0] sets this mode (see Figure 8).
In TX2 shutdown mode, a TX2 event will shutdown the flash
pulse. Once shut down, the flash pulse must be re-enabled
via STROBE or the flash enable bits in the Enable Register.
GPIO2 Mode
The TX2/GPIO2/INT pin is configured as a general purpose
logic input/output by setting GPIO Register bit[3] = '1'. In
GPIO2 mode this pin can be either a logic input or output de-
pending on the bit settings for GPIO Register bit [4] (see Table
4).
Interrupt Output Mode
The TX2/GPIO2/INT pin is configured as an interrupt output
by setting the TX2/GPIO2/INT as a GPIO output and setting
bit [6] of the GPIO register to '1'. When in INT mode, the TX2/
GPIO2/INT pin will pull low when either of the following occur:
1. The LM3561 is in NTC Mode, the LED current source is
enabled, and V_NTC falls below V_TRIP.
2. The LM3561's Input Voltage Monitor is enabled and V_IN
falls below V_{IN_TH}.
INDICATOR LED/THERMISTOR (LEDI/NTC)
The LEDI/NTC pin serves a dual function, either as an LED
indicator driver or as a threshold detector for a negative tem-
perature coefficient (NTC) thermistor circuit.
Led Indicator Mode (LEDI)
LEDI/NTC is configured as an LED indicator driver by setting
Configuration Register 1 bit [4] = '0' (default). The indicator
current source is enabled by setting Enable Register bits [1:0]
= '01'. In Indicator mode there are 8 different current levels
available (2.25mA - 18mA) which are programmed through
the Indicator Register (see Table 3).
Led Thermal Comparator (NTC Mode)
Writing a '1' to Configuration Register 1 bit [4] disables the
indicator current source and configures LEDI/NTC as a com-
parator input for monitoring an NTC thermistor circuit. In this
mode LEDI/NTC becomes the negative input of an internal
comparator, with the positive input internally connected to an
internal reference (V_TRIP = 1V). Additionally, Configuration
Register 2 bit [1] determines the action NTC Mode takes if the
voltage at LEDI/NTC falls below V_TRIP . With Configuration
register 2 bit [1] = '0', the LED current source will be forced
into Torch mode when the voltage at LEDI/NTC falls below
V_TRIP. With Configuration Register 2 bit [1] = '1' the device will
shut down the current source (LED) when V_LEDI/NTC falls be-
low V_TRIP. When the LM3561 is forced from Flash to Torch,
normal LED operation (during the same Flash pulse) can only
be re-started by reading from the Flags Register and ensuring
the voltage at V_LEDI/NTC is above V_TRIP. When V_LEDI/NTC falls
below V_TRIP and the Flags register is cleared, the LM3561 will
go through a 250µs deglitch time before the flash current falls
to either torch mode or goes into shutdown. This deglitch time
prevents noise from inadvertently tripping the NTC compara-
tor. For a more detailed description of this mode and design-
ing the NTC circuit (see NTC THERMISTOR SELECTION
section in the Applications Information section of this
datasheet).
In NTC mode the NTC flag (see Flags Register and Fault In-
dicators section) can be output on the TX2/GPIO2/INT pin.
This is accomplished by making the TX2/GPIO2/INT an in-
terrupt output (see Interrupt Output Mode section).
ALTERNATE EXTERNAL TORCH (AET MODE)
With Configuration Register 2 bit [2] set to '1' the operation of
TX1/TORCH becomes dependent on its occurrence relative
to STROBE. In this mode if TX1/TORCH goes high first, fol-
lowed by STROBE going high, the LEDs are forced into torch
mode with no timeout. In this mode if TX1/TORCH goes high
after STROBE has gone high, then the TX1/TORCH pin op-
erates as a normal flash interrupt, and the LEDs will turn off
at the end of the timeout duration. (See LM3561 Timing Dia-
grams: Figure 9 and Figure 10). AET mode is can only be
used with STROBE configured as a level sensitive input.
Configuring STROBE for edge sensitive operation (STR bit =
1), will force TX1 to act as a simple flash interrupt.
INPUT VOLTAGE MONITOR
The LM3561 has an internal comparator that monitors the
voltage at IN and can force the LED current into torch mode
or into shutdown if V_IN falls below the programmable VIN
Monitor Threshold (V_{IN_TH}). Bit [0] in the VIN Monitor register
enables or disables this feature. When enabled, Bits [2:1]
program the 4 adjustable thresholds of 2.9V, 3V, 3.1V, and
3.2V. Bit [3] in Configuration Register 2 selects whether an
under-voltage event forces Torch mode or forces the LED
current source off. (See Table 5 and Table 11.)
When the VIN Monitor is active and V_IN falls below the pro-
grammed threshold (V_{IN_TH}) the LEDs will either turn off or
their current will be reduced to the programmed Torch current
setting. To reset the LED current to its previous level, two
things must occur. First, V_IN must go above V_{IN_TH,} and the
Flags register must be read back.
In VIN Monitor mode the VIN Monitor Flag (see Flags Register
and Fault Indicators section) can be output on the TX2/
GPIO2/INT pin. This is accomplished by making the TX2/
GPIO2/INT an interrupt output (see Interrupt Output Mode
section).
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