Rainbow Electronics MAX16841 - Manual (Page 7)

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MAX16841
Controller IC for Dimmable Offline LED Lamps
Detailed Description
The MAX16841 is a fixed-frequency offline LED driver
IC that is compatible with both leading-edge triac dim-
mers and trailing-edge transistor dimmers. The device
uses a fixed-frequency average current-mode control
scheme to control the switching current in the MOSFET.
In addition, a peak-limit comparator is used to limit the
peak switching current during overload and transient
conditions. The peak-limit comparator has a threshold of
2.2V. For the active PFC, the device uses a proprietary
current-control scheme where the averaged switch cur-
rent on a cycle-by-cycle basis is set to a programmed
DC value. This maximizes the efficiency of the converter
by operating in continuous-conduction mode (CCM) at
low AC line voltage (100V to 120V) and in discontinuous-
conduction mode (DCM) at high AC line voltage (220V
to 240V). Switching is initiated when the voltage on the
TH pin exceeds a threshold of 1.24V. In the case of the
buck configuration, the V
TH
falling threshold should be
set in such a way so that the input voltage exceeds the
maximum forward voltage of the LED string. In the case
of the buck-boost or flyback configuration, this threshold
can be set lower.
The device also uses a proprietary current-sense scheme
to regulate the LED current.
The device switching frequency is adjustable from 50kHz
to 300kHz using a single resistor from NDRV to ground.
The device operates over a wide 11V to 20V supply
voltage. The device’s switching MOSFET gate driver
sources and sinks up to 1A, making it capable of driving
high-voltage MOSFETs in offline LED driver applica-
tions for power ranges up to 25W. The device allows for
dimming with leading-edge and trailing-edge dimmers.
Additional features include thermal shutdown and
overvoltage protection.
IN
The device is powered up by the voltage at IN. All the
internal regulators derive power from IN. The operational
voltage is between 11V and 20V.
TH
TH sets the threshold for switching. Switching is initi-
ated once TH crosses 1.24V. The TH comparator has a
150mV hysteresis. In a buck configuration, the V
TH
falling
threshold should be set in such a way so that the input
voltage is equal to the maximum forward voltage of the
LED string. In a buck-boost configuration, the V
TH
falling
threshold can be set to a lower level.
DIMOUT
For proper operation with triac dimmers, the load con-
nected to the dimmer should draw at least the startup
current when the dimmer is in the off state. For proper
operation of the timing circuit of the dimmer, there should
always be a close-current path. To ensure this, a bleeder
resistor is connected across IN and GND with the help
of an external FET. DIMOUT drives this external FET on
when V
TH
goes below the falling threshold. The bleeder
resistor is disconnected when V
TH
crosses its rising
threshold, resulting in better performance and efficiency.
Internal Oscillator
The internal oscillator of the device is programmable
from 50kHz to 300kHz. Connect a single resistor from
NDRV to GND to set the oscillator frequency. Upon
power-up, an 8FA of current sinks into this resistor. An
internal ramp is then compared against the voltage on
NDRV to determine the oscillator frequency.
Frequency Dithering
The device incorporates a frequency-dithering feature.
This feature helps to reduce EMI.
n-Channel MOSFET Switch Driver (NDRV)
The NDRV driver drives the gate of an external n-channel
switching MOSFET. NDRV switches between IN and
GND. NDRV is capable of sourcing/sinking 1A of peak
current, allowing the device to switch MOSFETs in an
offline LED driver application. The average current drawn
from the supply to drive the external MOSFET depends
on the MOSFET gate charge and switching frequency.
Use the following equation to calculate the MOSFET
driver supply current:
NDRV G SW
I Q f = ×
Switching MOSFET Current Sense (CS)
The switching MOSFET current-sense resistor should be
connected to the CS pin of the device. The device con-
trols the average of the CS signal to a level determined
by the REFI voltage. Internal leading-edge blanking of
200ns (typ) is provided to avoid premature turn-off of
the switching MOSFET in each switching cycle. A peak-
limit comparator is used to limit the peak switch current
during overload and transient conditions. The peak-limit
comparator has a threshold of 2.2V (typ).