ThinkPad LED Driver #1 (TLD1) is a buck-topology LED driver based on the Texas Instruments TPS92510 controller IC. It provides a single channel of dimmable current-regulated output intended for 3S LED strings operating between 7V to 9.6V and a maximum design output current of 600mA (depending on the components chosen) from an unregulated input of 10-24V.
The primary additional design criteria over existing driver boards already available from Chinese vendors are a wide dimming range (≥25:1), native Thinkpad signaling, and continuous-current operation rather than dimming via PWM.
TLD1 is based on the reference example circuits found in the Texas Instruments TPS92510 data sheet. The design makes three additions to the examples.
A snubber resistor (R6) substantially reduces ground-state ringing through coil L1 during discontinuous-mode operation. Although the circuit meets regulation precision without, R6 reduces regulation error by more than half at low brightness (at the cost of about three-quarters of a percent efficiency).
The PWM dimming input (pin 4 of the TPS92510) is left unused, with dimming implemented by offsetting the current sense voltage at pin 7. C6 provides an AC-coupled current-sense waveform, with the DC component offset by the voltage divider formed by R3 and R5.
U3 charges C5 through RA and RB with each negative-phase of the input PWM dimming control signal. The voltage across C5 also presents across the voltage divider (R3 and R5) to offset the current sense signal at pin 7, dimming the output. The voltage divider does double duty, draining charge from C5 when the PWM waveform is high. In this way, U3, RA, RB, C5, R3 and R5 form a lowpass filter, converting a PWM input signal to an analog voltage offset at pin 7.
The TPS92510 data sheet states that driver current accuracy is +/-3%. In reality, the accuracy error behaves more like a constant current offset. With trimming, regulation error is better than 1/4% across the input and output operating range.
Resistors RA and RB set the ratio of maximum brightness to minimum brightness. The correct values depend on the brightness range of the specific ThinkPad, the desired ratio, and the value of R3. The inherent current offset of the particular driver IC will also affect the minimum brightness by up to 5mA. See the bill of materials for starting values.
Sense resistors RC and RD set the output current at full brightness. As per the data sheet, the effective resistance should be 200mV divided by the desired full-brightness output current. Note that the ratio set by RA and RB is not affected by the values of RC and RD, aside from the hard limitation of the configuration's minimum possible brightness.
The TPS92510 is one of the most static-sensitive ICs I've worked with. It is exceptionally easy to damage the internal sense amplifiers when handling the chip.
Static damage usually manifests subtly as a higher-than expected minimum brightness, or brightness steps on the minimum of the scale proving metastable. The circuit still works, it just 'acts funny'.
Assembled circuits appear to be more normally robust against static, likely due to large external capacitances protecting the relevant sense inputs. As yet, I've not had an assembled board suffer static damage.
TLD1 was originally the first prototype design for a LED backlight conversion board for CCFL-equipped ThinkPads. 50 hand-soldered boards from the prototype run became the first generation of X61 LED kits (Original and Daylight). With the introduction of TLD3, remaining TLD1 boards will be used in 'Original' (335mA) X61 kits until the stock is depleted.