MIC2871 Datasheet, PDF (11/24 Pages) Micrel Semiconductor – 1.2A High-Brightness LED Flash Driver with Single-Wire Serial Interface

Micrel, Inc.
Application Information
The MIC2871 can drive a high-current flash WLED in
either flash mode or torch mode.
Boost Converter
The internal boost converter is turned on/off automatically
when the LED driver is activated/de-activated without any
exception.
The boost converter is an internally-compensated
current-mode PWM boost converter running at 2MHz. It
is for stepping up the supply voltage to a high enough
value at the OUT pin to drive the LED current. If the
supply voltage is high enough, the synchronous switch of
the converter is then fully turned on. In this case, all the
excessive voltage is dropped over the linear LED driver.
Flash Mode
The maximum current level in the flash mode is 1.2A.
This current level can be adjusted through an external
resistor connecting to the FRSET pin according to
Equation 1:
I_LED(MAX)
=
20500
R _FRSET
Eq. 1
Alternatively, the default value of 1A is used when the
FRSET pin is grounded.
The flash mode current can be initiated at this preset
FRSET brightness level by asserting FEN1 or FEN2 pin
HIGH, or by setting the flash control register (address 1),
for the desired flash duration, subjected to the safety
timeout setting. The flash mode current is terminated
when the FEN1 and FEN2 pins are brought LOW and the
flash register is cleared.
Flash mode current can be adjusted to a fraction of the
maximum flash mode current level by selecting the
desired percentage in the flash control register through
the single-wire serial interface. The flash current is the
product of the maximum flash current setting and the
percentage selected in the flash register.
Torch Mode
By default, the maximum torch mode level is one-fourth
(1/4) of the maximum flash mode current. The torch
mode operation is activated by setting the torch control
register (address 2) for the desired duration. The torch
mode current is terminated when the torch register is
cleared or when the configurable safety timer expires.
MIC2871
Like the flash mode current, the torch mode current can
be tuned to a fraction of the maximum torch mode level
by selecting the desired torch current level percentage in
the torch control register (address 2) through the single-
wire serial interface.
The torch current is the product of the maximum Torch
current setting and the percentage selected in the torch
register.
Configurable Safety Timer
The flash safety timeout feature automatically shuts down
the LED current after the safety timer duration is expired
if the programmed LED current exceeds a certain current
threshold. Both the current threshold and the timer
duration are programmable via the safety timer registers
(addresses 3 and 5).
Low-Battery Voltage Detection (LBVD)
When the VIN voltage drops below the LBVD threshold
(default = 3.6V) in flash or torch mode, the LED current
driver is disabled. The LED driver can be resumed by
toggling the corresponding input control signal. The
LBVD threshold is adjustable thru the LBVD control
register (address 4).
Overvoltage Protection
When the output voltage rises above the OVP threshold,
MIC2871 is latched off automatically to avoid permanent
damage to the IC. To clear the latched off condition,
either power cycle the MIC2871 or assert the DC pin
LOW.
Short-Circuit Detection
Each time before enabling the LED driver, the MIC2871
performs the short circuit test by driving the flash LED
with a small (2mA typical) current for 200µs. If (V_OUT –
V_LED) < 1.7V at the end of the short-circuit test, the LED is
considered to be shorted and MIC2871 will ignore the
flash and/or torch mode command. Note that the short-
circuit test is carried out every time prior to flash and
torch mode but the result is not latched.
Thermal Shutdown
When the internal die temperature of MIC2871 reaches
155°C, the LED driver is disabled until the die
temperature falls below 140°C.
May 29, 2013
11
052913-1.0 (while in progress)
Revision 1.0 (final document)