Detailed Description
The MAX31840 is an average current-mode-control LED
driver IC for boost topology in low-voltage SSL applications.
The IC has an integrated 0.2Ω (max), 36V switching
MOSFET that allows the device to be used in lighting
applications for MR16 and other SSL applications. The
LED driver uses constant-frequency, average current-
mode control to control the duty cycle of the integrated
switching MOSFET. The IC has all the necessary features
required for LED MR16 lighting applications. The IC uses
an input-current-control scheme to achieve power factor
correction. The IC turns on an external bleeder when the
boost operation is off. This feature allows the IC to be
compatible with electronic transformers and trailing-edge
dimmers (for electronic transformers).
The switch current is sensed on CS. The voltage on CS
goes through an internal RC filter. This voltage is then
fed to the negative input of a transconductance amplifier.
The positive input of the gm amplifier is the programmed
input current level. The output voltage of the amplifier is
compared with a ramp at the switching frequency to set the
duty cycle. The switching frequency is set at 700kHz. The
average current-mode controller inside the IC regulates
the input current. The IC also features an internal
overvoltage protection of 36V on IN to protect the internal
switching MOSFET from damage if the LED string is open
or if the voltage is too high.
The IC regulates the average voltage on CS to 200mV.
The average input current is determined by the value of
the resistor connected from the CS pin to ground.
To keep the input power almost constant for line-voltage
variation of ±13%, a constant brightness circuit has been
integrated into the MAX31840. The rectified AC supply after
a RC filter, consists of an internal resistor in conjunction
with the capacitor connected from the AVDET pin to
ground, is used to sense the average voltage level.
The IC has a bleeder gate (BGT) ping and a bleeder
source (BSRC) pin for the control of an external bleeder
N-channel MOSFET. A bleeder is required to draw current
after each AC zero-crossing cycle when the boost converter
just get started. The same bleeder is also used to provide
deep dimming. The bleeder driving circuit inside the IC
maintains the BSRC pin at 0.5V. The bleeder current is
determined by the value of the resistor connected from
the BSRC pin to the ground.
Internal Oscillator
The IC has an internal oscillator with a fixed switching
frequency of 700kHz. 700kHz was chosen to balance
between inductor size and EMI emission.
Input Voltage (IN)
The IC is powered by the voltage on IN and needs to be
maintained within the operating voltage range of from 6.5V
to 35V. An internal UVLO is set at 5.6V. Below 5.4V, there
is no switching of the internal power MOSFET and the
gate driver for the MOSFET is low. The typical hysteresis
of the UVLO threshold is 200mV. There is an internal LDO
of 5V that is used to power all the internal circuitry and
the gate driver for the internal switching MOSFET. The
switching of the internal MOSFET ceases once the rising
voltage on IN exceeds 35V and stays off until the voltage
on IN drops by 1.2V.
Rectied AC (ACREC)
The ACREC pin senses the zero crossings for the external
bleeder driver. The ACREC pin is also used to detect the
dimmer phase to drive the same shared external bleeder
FET. The zero crossing bleeding happens at every AC
zero crossing and lasts a very short time duration. For
deep dimming, an internal circuit detects the time duration
of the dimming phase per a half AC period. It then turns
off the the boost controller at a shorter duration according
to the dimming phase for the next half AC period. The
smaller the dimming phase the shorter the duration. At
around 15° of dimming phase, the deepest dimming is
reached per R4 and C1 values on the ACREC pin. For
both zero crossing and deep dimming, the bleeder FET
is turned on when the LED boost is off, to maintain a
minimum input loading on the electronic transformer. The
deep dimming control is capable of delivering <1% of
maximum power to the LEDs.
External Bleeder Driver (BGT/BSRC)
The BGT and BSRC pins drive an external N-channel
MOSFET for both zero crossing and deep dimming bleeding.
A resistor at the BSRC pin to ground is used to configure
the electronic transformer loading current during either
zero cross and deep dimming. With 0.5V of source voltage,
and a 0.5Ω resistor from source to GND, the bleeder
would draw 1A from the input when the ACREC voltage is
high and the boost regulator is off.
Internal Switching MOSFET
The IC has an integrated switching MOSFET with a maximum
R
DSON
of 200mΩ at +125°C. The typical R
DSON
at
+25°C is 100mΩ. This allows the IC to be used in a boost
LED driver for power levels up to 12W. The maximum
voltage rating of the MOSFET is 36V operation. The drain
of the internal MOSFET is connected to the DRAIN pin
and the source of the internal MOSFET is connected to
the SOURCE pin.
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Maxim Integrated
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MAX31840 MR16 LED Driver with Integrated Control
MOSFET and Deep Dimming
