LM3560 Datasheet, PDF (13/42 Pages) Texas Instruments – LM3560 Synchronous Boost Flash Driver w/ Dual 1A High-Side Current Sources (2A Total Flash Current)

Overview
The LM3560 is a high-power white LED flash driver capable
of delivering up to 2A of LED current into a single LED, or up
to 1A into two parallel LEDs. The device incorporates a 2MHz
constant frequency, synchronous boost converter, and two
high-side current sources to regulate the LED current over the
2.5V to 5.5V input voltage range.
During operation when the output voltage is greater than V_IN
– 150 mV the boost converter switches and maintains at least
300 mV across both current sources (LED1 and LED2). This
minimum headroom voltage ensures that the current sources
remain in regulation. When the input voltage rises above the
LED voltage + current source headroom voltage, the device
stops switching and turns the PFET on continuously (Pass
mode). In Pass mode the difference between (V_IN - I_LED x
R_{ON_P}) and the voltage across the LED's is dropped across
the current sources.
Four hardware control pins provide control of the LM3560.
These include a hardware Flash Enable (STROBE), Dual
Flash Interrupt inputs (TX1 and TX2) designed to interrupt the
flash pulse during high battery current conditions, and a logic
high hardware enable (HWEN) that can be pulled low to rapid-
ly place the device into shutdown. Additional features of the
LM3560 include an internal 4-bit ADC for LED voltage moni-
toring, an internal comparator for LED thermal sensing via an
external NTC thermistor, an input voltage monitor that can
reduce the Flash current during input undervoltage condi-
tions, a low-power message indicator LED current source,
and a mode for utilizing the flash LEDs as a privacy indicator.
Control of the LM3560 is done via an I²C-compatible inter-
face. This includes adjustment of the Flash and Torch current
levels, adjustment of the indicator LED currents, changing the
Flash Timeout Duration, changing the switch current limit, and
reading back the ADC results. Additionally, there are 8 flag
bits that indicate flash current time-out, LED over-tempera-
ture, LED failure (LED short or output OVP condition), device
thermal shutdown, V_IN under voltage condition, V_IN Flash
Monitor undervoltage condition, and the occurrence of a TX1
or TX2 interrupt.
STARTUP (ENABLING THE DEVICE)
Turn-on of the LM3560 is done through bits [1:0] of the Enable
Register. These bits enable the device in Torch mode, Flash
mode, or Privacy Indicate mode. Additionally, bit 6 of the En-
able Register enables the message indicator at the LEDI/NTC
pin. On startup, when V_OUT is less than V_IN, the internal syn-
chronous PFET turns on as a current source and delivers 350
mA to the output capacitor. During this time both current
sources (LED1, and LED2) are off. When the voltage across
the output capacitor reaches 2.2V the current sources can
turn on. At turn-on the current sources step through each
FLASH and TORCH level until their target LED current is
reached (32 µs/step). This gives the device a controlled turn-
on and limits inrush current from the V_IN supply.
INDEPENDENT LED CONTROL
Bits [4:3] of the Enable register provide for independent turn-
on and turn-off of the LED1 or LED2 current sources. Once
enabled, the LED current is adjusted by writing to the Torch
Brightness or Flash Brightness Registers depending on
whether Flash or Torch mode is selected. Both the Torch
Brightness and the Flash Brightness Registers provide for in-
dependent current programming for the LED currents in either
LED1 or LED2. (See TORCH BRIGHTNESS REGISTER DE-
SCRIPTIONS (Address 0xA0) and FLASH BRIGHTNESS
REGISTER (Address 0xB0).)
PASS MODE
On turn-on when the output voltage charges up to ( V_IN – 150
mV), the LM3560 will decide if the part operates in Pass Mode
or Boost mode. If the voltage difference between V_OUT and
V_LED is less then 300 mV, the device operates in Boost Mode.
If the difference between V_OUT and V_LED is greater then 300
mV, the device operates in Pass Mode. In Pass Mode the
boost converter stops switching and the synchronous PFET
turns fully on bringing V_OUT up to (V_IN – I_IN x R_PMOS) whwer
R_PMOS = 80 mΩ. In Pass Mode the inductor current is not lim-
ited by the peak current limit. In this situation the output
current must be limited to 3A.
OVER-VOLTAGE PROTECTION
The output voltage is limited to typically 5V (5.075V max). In
situations such as the current source open, the LM3560 will
raise the output voltage to try to keep the LED current at its
target value. When V_OUT reaches 5V the over-voltage com-
parator will trip and turn off both the internal NFET and PFET.
When V_OUT falls below 4.8V (typical), the LM3560 will begin
switching again.
CURRENT LIMIT
The LM3560 features 4 selectable current limits of 1.6A, 2.3A,
3.0A, and 3.6A. These are programmable through the I²C-
compatible interface via bits [6:5] of the Flash Duration Reg-
ister. When the current limit is reached, the LM3560 stops
switching for the remainder of the switching cycle.
Since the current limit is sensed in the NMOS switch there is
no mechanism to limit the current when the device operates
in Pass Mode. In situations where there could potentially be
large load currents at OUT, and the LM3560 is operating in
Pass mode, the load current must be limited to 3A. In Boost
mode or Pass mode if V_OUT falls below approximately 2.3V,
the part stops switching and the PFET operates as a current
source, limiting the current to typically 350 mA. This prevents
damage to the LM3560 and excessive current draw from the
battery during output short-circuit conditions.
THERMAL SHUTDOWN
The LM3560 features a thermal shutdown threshold of typi-
cally +150°C. When the devices die temperature reaches
+150°C the active current sources (LED1 and/or LED2) will
shut down, and the TSD flag in the Flags register is written
high. The device cannot be started up again until the Flags
register is read back. Once the Flags register is read back
either current source can be re-enabled into Privacy, Torch,
or Flash Mode. The thermal shutdown (TSD) circuitry has an
internal 250 µs de-glitch timer which helps prevent unwanted
noise from falsely triggering a TSD event. However, when the
LM3560 is in boost mode at higher flash currents, the de-glitch
timer can get reset by the high currents in the LM3560's GND.
As a result the thermal shutdown's internal de-glitch timer can
be reset before the TSD event can get latched in. This pre-
vents a TSD event from being triggered until the LM3560's
flash pulse reaches the end of the flash duration, when the
noisy currents have dropped to a lower level. However, once
the noise is lower, and a TSD event is triggered, the next flash
pulse is not allowed until the flags register is read back. In
pass mode the boost switcher is off and the lower noise en-
vironment allows the devices TSD circuitry to shut down
immediately when the die temperature reaches +150°C.
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