LED 节能灯 电源驱动芯片 OS9910 资料(4)

LED 节能灯 日光灯 电源驱动芯片 OS9910 资料

Current Sense

Linear Dimming

The Linear Dimming pin is used to control the LED current.

The current sense input of the OS9910 goes to the non-inverting inputs of two comparators. The inverting terminal There are two cases when it may be necessary to use the

of one comparator is tied to an internal 250mV reference Linear Dimming pin.whereas the inverting terminal of the other comparator is

In some cases, it may not be possible to nd the exact connected to the LD pin. The outputs of both these compara-RCS value required to obtain the LED current when the tors are fed into an OR gate and the output of the OR gate

internal 250mV is used. In these cases, an external volt-is fed into the reset pin of the ip- op. Thus, the comparator

age divider from the VDD pin can be connected to the LD which has the lowest voltage at the inverting terminal deter-pin to obtain a voltage (less than 250mV) corresponding to mines when the GATE output is turned off.

the desired voltage across RCS.

The outputs of the comparators also include a 150-280ns

Linear dimming may be desired to adjust the current blanking time which prevents spurious turn-offs of the exter-level to reduce the intensity of the LEDs. In these cases, nal FET due to the turn-on spike normally present in peak

an external 0-250mV voltage can be connected to the LD current mode control. In rare cases, this internal blanking

pin to adjust the LED current during operation.might not be enough to lter out the turn-on spike. In these

cases, an external RC lter needs to be added between the

To use the internal 250mV, the LD pin can be connected to external sense resistor (RCS) and the CS pin.

VDD.

Please note that the comparators are fast (with a typical

80ns response time). Hence these comparators are more Note: Although the LD pin can be pulled to GND, the output susceptible to be triggered by noise than the comparators current will not go to zero. This is due to the presence of a of the OS9910. A proper layout minimizing external induc-minimum on-time (which is equal to the sum of the blanking

time and the delay to output time) which is about 450ns. tances will prevent false triggering of these comparators.

This will cause the FET to be on for a minimum of 450ns and thus the LED current when LD = GND will not be zero. This Oscillator

current is also dependent on the input voltage, inductance

The oscillator in the HV9910B is controlled by a single re-value, forward voltage of the LEDs and circuit parasitics. To sistor connected at the RT pin. The equation governing the get zero LED current, the PWMD pin has to be used.oscillator time period tOSC is given by:PWM Dimming

PWM Dimming can be achieved by driving the PWMD pin with a low frequency square wave signal. When the PWM

If the resistor is connected between RT and GND, HV9910B signal is zero, the gate driver is turned off and when the operates in a constant frequency mode and the above equa-PWMD signal if high, the gate driver is enabled. Since the tion determines the time-period. If the resistor is connected PWMD signal does not turn off the other parts of the IC, between RT and GATE, the HV9910B operates in a constant the response of the OS9910 to the PWMD signal is almost off-time mode and the above equation determines the off-instantaneous. The rate of rise and fall of the LED current is

thus determined solely by the rise and fall times of the induc-time.

tor current.

GATE Output

The gate output of the OS9910 is used to drive and external FET. It is recommended that the gate charge of the external FET be less than 25nC for switching frequencies ≤ 100kHz and less than 15nC for switching frequencies > 100kHz.

To disable PWM dimming and enable the OS9910 perma-nently, connect the PWMD pin to VDD.