SM8205 Datasheet, PDF (8/13 Pages) M.S. Kennedy Corporation – High-Power LED Driver with Integrated PWM Dimming MOSFET Driver

SM8205
BASIC OPERATION
Overview
The SM8205 is a current-mode, high-brightness LED
driver designed to control a single-string LED current
regulator with two external N-channel MOSFETs. It
integrates all the building blocks necessary to implement
a fixed-frequency, peak-current-mode control with
programmable slope compensation to control the duty
cycle of the PWM controller. The SM8205 allows
implementation of different converter topologies such as
SEPIC, Boost, Buck-Boost, Buck regulator. Basic
operating discussion that follows it will be helpful to refer
to the block diagram of the IC.
In normal operation with the PWMD pin low level, the
GATE and FAULT pins are driven to GND, the COMP
pin is high impedance to store the previous switching
state on the external compensation capacitor, and the
FDBK pin bias current is reduced to leakage levels.
When the PWM pin transitions high level, the GATE pin
transition high after a short-delay. At the same time, the
internal oscillator generates a pulse to set the SR latch,
turning on the external power MOSFET switch(GATE
goes high), A voltage input proportional to the switch
current, sensed by an external current sense resistor
connected from CS and GND. This voltage is added to a
stabilizing slope compensation ramp and the resulting is
fed into the positive terminal of the PWM comparator.
The current in the external inductor increases steadily
during the time the switch is on. When this non-inversion
input voltage exceeds inversion input voltage of the error
amplifier E.A, the SR latch is reset and the external
power switch is turned off. During the switch off phase,
the inductor current decreases. At the completion of
each oscillator cycle, internal signals such as slope
compensation return to their starting point and a new
cycle begins with the set pulse from the oscillator.
Minimum Input Voltage at Vin Pin
The SM8205 contains two internal high-voltage startup
regulator that allows the VIN pin to be connected directly
to line voltage as high as 46V. The 5V low dropout linear
regulator VCC, is used to power internal PWM controller,
control logic, error amplifier and as the reference for the
LED output current as well as to set switch current limit.
In application, connect a bypass capacitor from this pin
to GND, The recommended capacitance range is from
4.7uF to 10uF. Another 10V LDO regulator VDD, is for
external power MOSFETs driver, on the condition that
VIN≥12V, the regulator generates a 10V supply. If
7V≤VIN≤11V, the VDD is equal to VIN minus drop voltage
across bypass switch. When the voltage on the VDD pin
reaches the rising threshold of 7V, the GATE driver is
enabled. The GATE driver will remain enables until VDD
falls below 6.5V. Connect a bypass capacitor from this
pin to GND, The recommended capacitance range is
from 1.0uF to 10uF.
Gate Driver Voltage (VDD)
The SM8205's 10V LDO regulator powers up the
switching MOSFET driver. Use at least a 1uF low-ESR
ceramic capacitor from VDD to GND for best
performance. Careful choice of a lower Qg FET will allow
higher switching frequency, leading to smaller
magnetics. The VDD pin has it own undervoltage
disable (UVLO) set to 6.5V (typical value) to protect the
external MOSFET from excessive power dissipation
caused by not being fully enhanced.
Control and Reference Voltage (VCC)
The SM8205 has a 2% accurate, 5V reference, VCC. It
provides power to most of the internal circuit blocks
except for the gate drivers. It also can be used as the
reference for the LED output current as well as to set the
switch current limit through a resistor divider. Connect a
at least 4.7uF ceramic capacitor from VCC to GND.
Oscillator and Switching Frequency (RT)
The internal oscillator of the SM8205 is programmable
from 100KHz to 1MHz using a single resistor at RT, Do
not leave this pin open. Use the following formula to
calculate the RT value (1% resistors) :
RT ≈
1
FOSC (KHz) * 11 * 10^{- 9}
Table 1. Operating Frequency Selection
FOSC (KHz)
RT (KΩ)
100
910
200
455
300
303
500
182
600
152
1000
91
Note: Higher frequency operation yields smaller
component size but increases switching losses and gate
driving current, and may not allow sufficiently high or low
duty cycle operation. Lower frequency operation gives
better performance and efficiency but has larger external
component size.
Soft-start (SS)
The soft-start feature allows the LED output current to
gradually reach the LED current’s rate, thereby reducing
start-up stresses and input surge current. At power up,
after the VCC and input under-voltage lockout thresholds
are satisfied, an internal 10uA current source charges an
external capacitor connected to the SS pin. The
capacitor voltage will ramp up slowly and will limit the
COMP pin voltage and the switch current.
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8
Rev. 2.0 @ 2011/10