  The main objectives in driving multiple LED strings include achieving uniform
current control and high performance PWM dimming for all strings. This work
proposes a new multiple string LED driver to achieve not only current balance,
but also flexible and wide range PWM dimming ratio for each string. A compact
single-inductor multiple-output topology is adopted in the driver, accompanied
by synchronous integrators and variable dimming frequency, to achieve both high
efficiency and high performance dimming. By using the proposed variable dimming
frequency scheme, high dimming frequency is applied to a string with high
dimming ratio, which helps to maintain the deviation of LED string current in
an acceptable range, while low dimming frequency is applied to a string with
low dimming ratio, which helps to achieve rectangular LED current waveform.
Meanwhile, the new time multiplexing control scheme automatically optimizes the
LED string's bus voltages, thus minimizes each string's power loss. A three
string LED driver prototype is constructed to validate the effectiveness of the
peoposed control scheme, where the three strings can have different dimming
ratios between 4% and 100%.
