On the author‘s bench lay two ICs, waiting to be tried out: an LM3404 switching regulator (only available in a surface mount package, unfortunately), and a U2352 PWM IC. Together they could be used to make a small dimmer for LEDs. As in the case of the ‘Dimmable LED lamp’ elsewhere in this issue, we use a 6 V lead-acid battery as our source of energy, and a Luxeon 3 W LED as our light source. VCC, therefore, lies between a minimum of around 5.4 V and a maximum of around 7.2 V.
The right-hand part of the circuit shows the switching regulator, which reduces the voltage from the 6 V lead-acid battery to the 4 V required by the high-power LED. As the voltage is reduced the current must increase, and so less current flows through the input power connections than does through the LED. The LM3404 includes all the necessary control electronics along with a MOSFET switch. The voltage across resistor Rsns (CS, pin 5 of IC2) is proportional to the LED current and is compared against an internal reference of 200 mV. If the voltage falls below 200 mV the MOSFET is turned on for a fixed time tON. During this period the current through the inductor and the LED rises in an essentially linear fashion. Time tON is determined by RON and the input voltage VIN:
tON = 0.134 (RON/VIN) μs = 1.83 μs
(where RON is in kΩ and VIN in V).
After this time period has expired the MOSFET is turned off and an approximately linearly-falling current flows through the flyback diode and the LED until Usns, the voltage across Rsns, reaches 200 mV, whereupon a new cycle begins. While the MOSFET is off no current flows in the supply to the regulator. The minimum off time is 0.3 μs.
Ripple current is inversely proportional to the inductance and to the switching frequency. During tON the current rises linearly and the voltage across the coil is UL = UIN–ULED–USNS = 1.8 V.
UL = L(ΔILED/Δt).
With Δt = tON we obtain a ripple current of 66 mA.
When the current reaches its lowest value the voltage across Rsns is 200 mV. The average value of the current is one half of the ripple current greater. With Rsns = 0.3 Ω the average current is given by Iavg = 200 mV/300 mΩ + 66 mA/2 = 700 mA.
This is around the maximum permitted value for a 3 W LED. The LED current can be adjusted by changing Rsns, for which we can use a twisted length of resistance wire. More elegantly, we can use the PWM IC to drive the DIM input of the switching regulator. The U2352 can generate a PWM signal adjustable from 0 % to 100 % using a minimum of external components. The basic frequency of the internal triangle wave oscillator is set by C1 to around 10 kHz: fosc = 55/(Cosc ⋅ Vs) (where fosc is in kHz, Cosc in nF and Vs in V). The triangle wave voltage is compared against a reference voltage set by P1, and at the output of the comparator, we have our PWM signal.
The signal passes through some internal control logic before reaching the output of the device to protect the output from overload. Since we do not require this function it is disabled by taking pin 5 to ground and pin 3 to VCC via R3. It is not clear from the datasheet whether resistor R4 is strictly necessary for internal voltage stabilization. The PWM signal is taken from the output of the U2352 to the DIM input of the LM3404 and imposes a 10 kHz pulse-width modulation on the light produced. The ‘Boost’ switch (or pushbutton) forces the PWM output high and thus the LED to maximum brightness.