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

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

external power MOSFETs driver, on the condition that

across bypass switch. When the voltage on the VDD pin

reaches the rising threshold of 7V, the GATE driver is

falls below 6.5V. Connect a bypass capacitor from this

pin to GND, The recommended capacitance range is

from 1.0uF to 10uF.

The SM8205's 10V LDO regulator powers up the

switching MOSFET driver. Use at least a 1uF low-ESR

ceramic

capacitor

from

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.

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

The internal oscillator of the SM8205 is programmable

not leave this pin open. Use the following formula to

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Table 1. Operating Frequency Selection

100

200

300

1000

600

500

910

455

303

91

152

182

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,

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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Rev. 2.0 @ 2011/10