NCL30161

http://onsemi.com

6

Theory of Operation

This switching power supply is comprised of an inverted

buck regulator controlled by a current mode, hysteretic

control circuit. The buck regulator operates exactly like a

conventional buck regulator except the power device

placement has been inverted to allow for a low side power

FET. Referring to Figure 1, when the FET is conducting,

current flows from the input, through the inductor, the LED

and the FET to ground.

When the FET shuts off, current continues to flow through

the inductor and LED, but is diverted through the diode

(D1). This operation keeps the current in the LED

continuous with a continuous current ramp.

The control circuit controls the current hysteretically.

Figure 2 illustrates the operation of this circuit. The CS

comparator thresholds are set to provide a 10% current

ripple. The peak current comparator threshold of 220 mV

below the average current.

When the FET is conducting, the current in the inductor

ramps up. This current is sensed by the sense resistor that is

connected from CS to ground. When the voltage on the CS

pin reaches 220 mV, the peak current comparator turns off

the power FET. A conventional hysteretic controller would

monitor the load current and turn the switch back on when

the CS pin reaches 180 mV. But in this topology the current

information is not available to the control circuit when the

FET is off. To set the proper FET off time, the CS voltage is

sensed when the FET is turned back on and a correction

signal is sent to the off time circuit to adjust the off time as

necessary. When the FET is turned on, there can be a lot of

ringing on the CS pin that would make the voltage on the CS

pin be an unreliable measure of the current through the FET.

An 85 ns blanking timer is started when the GATE voltage

starts to go high, to allow this ringing to settle down. At the

end of this blanking timer, CS voltage is sensed to determine

the valley current.

Figure 4. Typical Current Waveforms

The current wave shape is triangular, and the peak and

valley currents are controlled. The average value for a

triangular wave shape is halfway between the peak and

valley, so even with changes in duty cycle due to input

voltage variations or load changes, the average current will

remain constant.

Over Current Protection Feature

In the event there is a short−circuit across the LEDs, a

large amount of current could potentially flow through the

circuit during startup. To protect against this, the NCL30161

comes with a short circuit protection feature. If the voltage

on the CS pin is detected to be greater than the over current

protection limit, the NCL30161 will turn off the FET, and

prevent the FET from turning on again until power is

recycled to the NCL30161.

Undervoltage Lockout

When VIN rises above the UVLO threshold voltage,

switching operation of the FET will begin. However, until

the VIN voltage reaches 8 V, the VCC regulator may not

provide the expected gate drive voltage to the FET. This

expected or there not being enough gate drive capability to

operate at the maximum rated switching frequency. For

optimal performance, it is recommended to operate the part

at a VIN voltage of 8 V or greater.

Setting The Output Current

The average output current is determined as being the

middle of the peak and valley of the output current, set by the

CS comparator thresholds. The nominal average output

current will be the current value equivalent to 200 mV at the

output current can be calculated by:

200 mV

PWM Dimming

therefore the brightness of the LED, can be set to a lower

value through the DIM/EN pin. When the DIM/EN pin is

brought low, the internal FET will turn off and switching

will remain off until the DIM/EN pin is brought back into its

high state.

By applying a pulsed signal to DIM/EN, the average

output current can be adjusted to the duty ratio of the pulsed

signal. It is recommended to keep the frequency of the

DIM/EN signal above 100 Hz to avoid any visible flickering

of the LED.