NCL30002 Просмотр технического описания (PDF) - ON Semiconductor

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NCL30002

Power Factor Corrected Buck LED Driver

ON Semiconductor 

NCL30002

Mode 3 Ipeak = Constant

Mode 2 ton = Constant

Mode 1 Input Current = 0

Figure 24. Theoretical Average Output Current over one half line cycle (conduction angle)

An output capacitor filters the output current in the LED

string. The dynamic LED resistance, line frequency, and the

size of the filter capacitance determine the exact LED ripple.

The NCL30002 operates as a CrM controller. The

controller draws very low currents while the Vcc filter

capacitor charges to the start-up threshold. Since CrM

operation is not clocked at a fixed frequency and depends on

the state of the power circuit to initiate a new switching

cycle, a kick start timer turns on the gate driver to start a new

cycle. The kick start timer will do this anytime the driver is

off for more than about 180 msec as long as none of the

protection circuits are disabling the gate driver output.

The NCL30002 (refer to the block diagram − Figure 1) is

composed of 4 key functional blocks along with protection

circuitry to ensure reliable operation of the controller.

• On−Time Control

• Zero Current Detection Control

• MOSFET Gate Driver

• Startup and VCC Management

On Time Control

The on−time control circuitry (Figure 25) consists of a

precision current source which charges up an external

capacitor (Ct) in a linear ramp. The voltage on Ct (after

removing an internal offset) is compared to an external

control voltage and the output of the comparator is used to

turn off the output driver thus terminating the switching

cycle. A signal from the driver is fed back to the on−time

control block to discharge the Ct capacitor thus preparing the

circuit for the start of the next switching cycle.

The state of Vcontrol is determined by the external

regulation loop. The range of on−time is determined by the

charging slope of the Ct capacitor and is clamped at 4.93 V

nominal. The Ct capacitor is sized to ensure that the required

on−time is reached at maximum output power and the

minimum input line voltage condition.

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