Overview of inverse buck converter

AN3151

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Doc ID 17051 Rev 1

where:

Fs= 100 kHz is the switching frequency; obtaining an inductance value of 1mH. A smaller

value of 820 uH has been chosen for space saving reasons.

3.1

Average current reading methodology

The current reading methodology is based on an innovative technique suitable for the DC-

DC stage (inverse buck) working in continuous conduction mode (CCM) or even in transition

mode.

The aim is to control the current because the LEDs need a current control in order to

maximize lifetime and brightness, and guarantee the technical specs provided by the LED

manufacturers.

The most important target is to maintain a constant current even though, for thermal issue,

the typical forward voltage can change.

This technique has been patented and is based on microcontroller peripherals allowing the

LEDs current to be sensed, flowing during the MOSFET conduction, on the Ton/2 event.

The Ton/2 current reading and conversion, by the microcontroller, allows the average current

value to be estimated.

The current reading has been implemented on an STM8S208RB but can be easily

implemented on the STM8S2x product family.

The most important feature of this microcontroller is the ADC triggering function by some

timer events. Further details can be seen in the reference manual [1].

The current sense has been obtained avoiding any external circuitry. The sense resistor is

chosen in order to have the best compromise between signal noise ratio and power

dissipation issues. Below are some waveforms describing the proposed technique. The

PWM counter is configured in the center-aligned mode. The counter counts from 0 to the

auto-reload value (content of the TIMx_ARR register) -1, generates a counter overflow

event, then counts from the auto reload value down to 1 and generates a counter underflow

event. Then it restarts counting from 0. The up/down counting defines the center aligned

pattern providing a triangular carrier. The overflow and underflow event can be used for ADC

triggering. In Figure 7 the counting in up/down mode provides a triangular carrier. The OC

register sets the duty cycle. The overflow event, during the Ton/2, triggers the AD conversion

and the result provides the average current value to be used in the control loop.