Saving the designer board space, the TPS9255x from Texas Instruments is a constant current buck LED driver micro-module family whose integrated power switch enables driving a single string of LEDs. The two members of the family are the 14W TPS92550 and 23W TPS92551. By default they deliver a 350mA LED current that is adjustable from 300mA to 450mA using external resistors. Fig. 1 shows the simplified block diagram for both modules and Table 1 compares their features, Table 2 shows LED current settings.
The micro-modules integrate all the power components including the power inductor, Schottky diode and other passive circuits. With up to 96% power efficiency, the module provides a full turn-key, efficient solution for single string LED lighting applications. They require no external current sensing elements and loop compensation network. The only external components are input and output capacitors and resistors to set the LED current.
These modules exhibit EMI that complies with the EN55015 standard. Their fast control loop realizes fine LED current pulse yielding 256–step PWM dimming resolution at 240Hz for general lighting. High speed dimming control input allows precision and high resolution brightness control for applications that require fine brightness adjustment.
Protection features include thermal shutdown, input under-voltage lockout (UVLO), LED open-circuit and short-circuit protection.
The TPS92550 micro-module is available in TO-PMOD 7 pin power package that measures 10.16 x 9.85 x 4.57 mm. Efficient heat dissipation and a back metal plate allow operation up to 125°C junction temperature. The RoHS-compliant package has a θJA (junction-to-ambient) = 20°C/W and a θJC (junction-to-case) = 1.9°C/W
Fig. 2 is an application schematic for the TPS92550 driving an LED string. A resistor from VREF (RVREF) or IADJ (RIADJ) sets LED current regulation. Average LED current is adjustable by varying resistor values according to:
Maximum input voltage determines the number of LEDs (typical forward voltage at 3.2V) that the TPS92550 can accommodate. With 20V VIN (max) it can drive one LED load. If VIN (max.) = 36V, the module can drive from two to 10 LEDs.
Peak Switch Current Limit
The TPS92550 features an integrated switch current limiting mechanism to prevent over-driving the LEDs. If switch current exceeds three times the current level set by the resistors it triggers the current limit, which turns off the internal power switch for 3.6µs to discharge the inductor until the inductor current reduces to its normal level. The current limiting feature avoids permanent damage to the TPS92550 application circuit due to a short circuit of the LED string.
The DIM pin of the TPS92550 is an input with internal pull-up that accepts logic signals for average LED current control. Applying a logic high (above 1.2V) signal to the DIM pin or leaving the DIM pin open enables the module. Applying a logic low signal (below 0.7V) to the DIM pin disables the module’s switching activity but maintains operation of its VCC regulator.
To ensure normal TPS92550 operation it is recommended to limit the dimming frequency to no higher than 1/10 of the switching frequency. The dim pulse on time is tested down to 16µs. In applications that require high dimming contrast ratio, low dimming frequency should be used.
PC Board Layout
Overall performance of the LED driver depends on the PCB layout. Poor board lay out can disrupt the performance of the TPS92550 and surrounding circuitry by contributing to EMI, ground bounce and resistive voltage drop in the traces. These can send erroneous signals to the LED driver, resulting in poor regulation and stability. Good layout can be implemented by following a few simple design rules.
- Place CIN as close as possible to the VIN pin and GND exposed pad (EP).
- Place COUT (optional for reduction of LED current ripple and EMI compliance) as close as possible to the VLED+ pin and VLED- pin.
- The exposed pad (EP) must connect directly to the GND pin.
To minimize EMI, it is important to minimize the di/dt current paths and to connect a 2.2µF capacitor (COUT) across the LED+ pin and LED- pin. This minimizes the ripple current and also reduces radiated EMI.
The TPS9255x is supported by TI's WEBENCH LED Designer and LED Architect. The free online design environment allows the user to automatically create a design customized for the user's input voltage and LED string requirements. The user can run Spice simulations to see the behavior of the circuit under dynamic conditions including PWM dimming. Try it yourself at www.ti.com/webench.