MAX16841_13 Datasheet, PDF (8/18 Pages) Maxim Integrated Products – Controller IC for Dimmable Offline LED Lamps

MAX16841
Controller IC for Dimmable Offline LED Lamps
Input-Current Setting (REFI)
REFI is the external reference for programming the input
current of the LED driver. The input current is proportinal
to the REFI voltage. The IC sources 50FA current out of
this pin and the voltage at the REFI pin can also be set
by connecting a resistor from REFI to GND. Internally, the
REFI signal is downshifted by 100mV and then attenua-
ted by a factor of 5. The attenuated signal is applied to
the positive terminal of the internal error amplifier and this
signal sets the reference for the controller.
Error-Amplifier Output (COMP)
The device includes an internal transconductance
current error amplifier with a typical G_m of 150FS. The
output of the error amplifier is controlled by the TH
comparator output. When the TH comparator is high,
the output of the error amplifier connects to COMP.
When the TH comparator is low, the error amplifier is
disconnected from COMP, preserving the charge on
the compensation capacitor. COMP is connected to the
positive terminal of the PWM comparator.
The device incorporates an average current-mode
control scheme to regulate the input current. The
control loop regulates the average of the CS signal to a
level determined by the REFI voltage. The control loop
consists of the current-sense resistor (R_CS) connected
across CS and GND, the transconductance current error
amplifier, an oscillator providing a 2.4V ramp at switching
frequency, the control voltage on the positive input of the
G_m amplifier, and the PWM comparator.
Overvoltage-Protection Input (OVP)
This is the protection feature in a flyback converter during
an open LED condition. The IN pin is connected to the
auxiliary winding of the flyback transformer. During an
open LED condition, the IN voltage increases and NDRV
is disabled once the IN voltage reaches 22.5V (typ).
When the IN voltage drops by 2V, NDRV is enabled.
Short-Circuit Protection
During an output short condition, the inductor current
keeps increasing with input voltage as there is no nega-
tive voltage across the inductor during the off period of
the switching cycle. During this condition, the CS voltage
signal peak is at a higher level because the inductor
current is at a higher level than during the normal condi-
tion. Once the CS signal exceeds the hiccup threshold
of 2.7V (typ), the internal hiccup block gets activated.
Switching is disabled for 1s (typ) if CS exceeds 2.7V (typ)
for three times.
Thermal Protection
The device enters into thermal-shutdown mode when
junction temperature exceeds +160NC. During thermal
shutdown, NDRV is disabled. The device recovers from
thermal-shutdown mode once the junction temperature
drops by 20NC.
Applications Information
Figure 1 shows a MAX16841-based, triac-dimmable,
PFC, nonisolated-buck offline LED driver. Components
L1, L2, L3, and C1 provide EMI filtering. During the turn-
on instant of triac dimming, there would be significant
ringing due to high inrush current to charge the input
capacitor (C9). The ringing could cause the line current
to fall to zero and this would turn off the triac. R3, R22,
and C14 act as a damper and help to limit the inrush
current and ringing. Due to R3, the efficiency of the sup-
ply decreases. The damper circuit can be omitted in
nondimming applications. The circuit, consisting of D4,
R5, C2, D3, R6, R4, and Q5, bypasses R3 with Q1 after
1ms of dimming instant, thereby reducing the power
dissipation in R3 and improving efficiency. During the
turn-on instant, capacitor C2 is charged by a constant-
current source formed by D3, R6, R4, and Q5. Within
1ms time, sufficient voltage develops across C2 to fire
the SCR Q1. Diode D4 provides fast discharge of C2.
Resistors R8, R9, and R10 program the switching thresh-
old. The rising threshold should be set at a voltage higher
than the maximum LED string voltage. When the input
voltage is below the falling threshold, DIMOUT drives
the Q3 FET on, connecting R7 across the diode-bridge
positive and GND. Thus, a close circuit is formed for the
timing circuit of the triac. Diode D2 blocks capacitors C9
and C14 to discharge through R7. This helps to reduce
the inrush current during the triac turn-on instant.
The circuit consisting of R23, R24, D6, and Q2 is a linear
regulator and provides bias to the device.
The buck-converter circuit is formed by C9, LED+, LED-,
C10, L5, Q4, D10, D11, and R20. Capacitor C9 provides
a path for the switching frequency currents. Maximum
value of this capacitor is limited by the input power-factor
requirements. The higher the value of C9, the lower the
input power factor.
Since the input-voltage waveform to the buck converter is
a rectified sinusoid at line frequency, the LED current has
a ripple at double-line frequency. Electrolytic capacitors
C11 and C12 filter this double-line frequency ripple.
Maxim Integrated
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