LED 节能灯 电源驱动芯片 OS9910 资料(3)

LED 节能灯 日光灯 电源驱动芯片 OS9910 资料

Application Information

The OS9910 is optimized to drive buck LED drivers using open-loop peak current mode control. This method of control enables fairly accurate LED current control without the need for high side current sensing or the design of any closed loop controllers. The IC uses very few external components and enables both Linear and PWM dimming of the LED current. A resistor connected to the RT pin programs the frequency of operation (or the off-time). The oscillator produces pulses at regular intervals. These pulses set the SR ip- op in the OS9910 which causes the gate driver to turn on. The same pulses also start the blanking timer which inhibits the reset input of the SR ip op and prevent false turn-offs due to the turn-on spike. When the FET turns on, the current through the inductor starts ramping up. This current ows through the external sense resistor RCS and produces a ramp voltage at the CS pin. The comparators are constantly comparing the CS pin voltage to both the voltage at the LD pin and the internal 250mV. Once the blanking timer is complete, the output of these comparators is allowed to reset the ip op. When the output of either one of the two comparators goes high, the ip op is reset and the gate output goes low. The gate goes low until the SR ip op is set by the oscillator. Assuming a 30% ripple in the inductor, the current sense resistor RCS can be set using:0.25V (or VLD)

RCS =

1.15 ILED

(A)

●

7.5V at the VDD pin. This voltage is used to power the IC and any external resistor dividers needed to control the IC. The VDD pin must be bypassed by a low ESR capacitor to provide a low impedance path for the high frequency current of the output gate driver.

The OS9910 can also be operated by supplying a voltage at the VDD pin greater than the internally regulated voltage. This will turn off the internal linear regulator of the IC and the OS9910 will operate directly off the voltage supplied at the VDD pin. Please note that this external voltage at the VDD pin should not exceed 12V.

Although the VIN pin of the OS9910 is rated up to 450V, the actual maximum voltage that can be applied is limited by the power dissipation in the IC. For example, if an 8-pin SOIC (junction to ambient thermal resistance Rθ,j-a = 128°C/W) OS9910 draws about IIN = 2mA from the VIN pin, and has a maximum allowable temperature rise of the junction temperature limited to about ΔT = 100°C, the maximum volt-age at the VIN pin would be:

In these cases, to operate the OS9910 from higher input

Constant frequency peak current mode control has an in-voltages, a Zener diode can be added in series with the VIN herent disadvantage – at duty cycles greater than 0.5, the pin to divert some of the power loss from the OS9910 to control scheme goes into subharmonic oscillations. To pre-the Zener diode. In the above example, using a 100V zener vent this, an arti cial slope is typically added to the current diode will allow the circuit to easily work up to 450V.sense waveform. This slope compensation scheme will af-fect the accuracy of the LED current in the present form. The input current drawn from the VIN pin is a sum of the However, a constant off-time peak current control scheme 1.0mA current drawn by the internal circuit and the current does not have this problem and can easily operate at duty drawn by the gate driver (which in turn depends on the switch-cycles greater then 0.5 and also gives inherent input volt-ing frequency and the gate charge of the external FET).age rejection making the LED current almost insensitive to

input voltage variations. But, it leads to variable frequency IIN ≈ 1.0mA + QG fS operation and the frequency range depends greatly on the input and output voltage variation.OS9910 makes it easy In the above equation, fS is the switching frequency and QG to switch between the two modes of operation by changing is the gate charge of the external FET (which can be ob-one connection (see oscillator section).tained from the datasheet of the FET).

Input Voltage Regulator

The OS9910 can be powered directly from its VIN pin and can work from 8-450VDC at its VIN pin. When a voltage is applied at the VIN pin, the OS9910 maintains a constant