LM3561 Datasheet, PDF (14/36 Pages) Texas Instruments – LM3561 Synchronous Boost Converter with 600mA High Side LED Driver and I 2 C-Compatible Interface

Circuit Description
OVERVIEW
The LM3561 is a high power white LED flash driver capable
of delivering up to 600mA of LED current into a single white
LED. The device incorporates a 2MHz constant frequency,
synchronous boost converter, and a high side current source
to regulate the LED current over the 2.5V to 5.5V input voltage
range.
When the LM3561 is enabled and the output voltage is greater
than V_IN – 150mV, the PWM converter switches and main-
tains at least 250mV (V_HR) across the current source (V_OUT -
V_LED). This minimum headroom voltage ensures that the
current sinks remain in regulation. When the input voltage is
above V_LED + V_HR the device operates in pass mode with the
device not switching and the PFET on continuously. In pass
mode the difference between (V_IN - I_LED×R_{ON_P}) and V_LED is
dropped across the current source. If the device is operating
in pass mode and V_IN drops to a point that forces the device
into switching, the LM3561 will make a one-time decision to
jump into switching mode. The LM3561 remains in switching
mode until the device is shutdown and re-enabled. This is true
even if V_IN were to rise back above V_LED + 250mV during the
active Flash or Torch cycle. This prevents the LED current
from oscillating back and forth between pass and boost mode
when V_IN is close to V_OUT.
The main features of the LM3561 include: dual TX inputs (TX1
and TX2) for forcing the device into a lower current state dur-
ing high battery current conditions, a hardware flash enable
input (STROBE), an active low shutdown input (HWEN), an
input voltage monitor for detecting low battery voltage condi-
tions, and a dual function pin that can be configured as a low
power indicator LED current source or as a comparator input
for LED thermal sensing via an external NTC thermistor.
Control of the LM3561 is done via an I²C-compatible inter-
face. This includes: adjustment of the LED current in TORCH
and FLASH mode, adjustment of the indicator LED current,
programming the flash LED current timeout duration, and
programming of the switch current limit. Additionally, there are
7 flag bits that can be read back indicating that the pro-
grammed flash current timeout has expired, a device over
temperature condition has happened, an LED failure (open or
short), an LED thermal failure (tripping of the internal NTC
comparator), an input under voltage fault (VIN Monitor), and
a separate flag for each TX input.
STARTUP
Turn on of the LM3561 via the I²C-compatible interface is
done through bits [1:0] of the Enable Register. The device can
be enabled in either Indicate mode, Torch mode, or Flash
mode (see Table 2). On startup in Flash or Torch mode, when
V_OUT is less than V_IN, the internal synchronous PFET turns on
as a current source and delivers typically 200mA to the output
capacitor. During this time the flash LED current source (LED)
is off. When the voltage across the output capacitor reaches
2.3V the current source can turn on. At turn-on, the current
source steps through each FLASH and TORCH level until the
target LED current is reached (32µs/step). This gives the de-
vice a controlled turn-on and limits inrush current from the
V_IN supply.
PASS MODE
On turn on, when the output voltage charges up to ( V_IN – 150
mV), the LM3561 will decide if the part operates in Pass Mode
or Boost mode. If the voltage difference between V_OUT and
V_LED is less than 250mV, the device will transition into Boost
Mode. If the difference between V_OUT and V_LED is greater than
250mV, the device will operate 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×R_PMOS)
where (R_PMOS = 240mΩ). In Pass Mode the inductor current
is not limited by the peak current limit. In this situation the
output current must be limited to 1.5A.
OVER-VOLTAGE PROTECTION
The output voltage is limited to typically 5V (4.9V min). In sit-
uations such as the current source open, the LM3561 will
raise the output voltage in order to try and keep the LED cur-
rent at its target value. When V_OUT reaches 5V, the over-
voltage comparator will trip and turn off both the internal NFET
and PFET switches. When V_OUT falls below 4.8V (typical) the
LM3561 will begin switching again.
CURRENT LIMIT
The LM3561 features 2 selectable current limits — 1A and
1.5A. These are selectable through the I²C-compatible inter-
face via bit [5] of the Flash Duration Register. When the
current limit is reached the LM3561 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 LM3561 is operating in
Pass mode, the load current must be limited to 1.5A. 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 200mA. This prevents
damage to the LM3561 and excessive current draw from the
battery during output short circuit conditions.
THERMAL SHUTDOWN
The LM3561 features a thermal shutdown threshold of typi-
cally +150°C. When the die temperature reaches +150°C, the
active current source (LED) will shutdown, 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 the current source can be re-
enabled into Torch, or Flash Mode. The thermal shutdown
(TSD) circuitry has an internal 250µs de-glitch timer which
helps prevent unwanted noise from falsely triggering a TSD
event. However, when the LM3561 is in boost mode at higher
flash currents, the de-glitch timer can get reset by the high
currents in the LM3561's GND. As a result the thermal
shutdown's internal de-glitch timer can get reset before the
TSD event can get latched in. This causes a TSD event to not
get triggered until the LM3561'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 environment allows the devices TSD
circuitry to shutdown immediately when the die temperature
reaches +150°C.
FLASH MODE
In Flash mode the LED current source (LED) provides 16 dif-
ferent current levels from typically 36mA to 600mA. The Flash
currents are set by writing to bits [3:0] of the Flash Brightness
Resister. Flash mode is activated by either writing a (1, 1) to
bits [1:0] of the Enable Register, or by pulling the STROBE
pin high. Once the Flash sequence is activated the current
source (LED) will ramp up to the programmed Flash current
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