Table 2. Design Parameters

8.2.2.1 Capacitor Selection

The LM2758 device requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR < 20 mΩ typical). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are not recommended for use with the LM2758 due to their high ESR compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM2758. Ceramic capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over temperature (X7R: ±15% over –55°C to +125°C; X5R: ±15% over –55°C to +85°C). Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2758. Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, –20%) and vary significantly over temperature (Y5V: 22%, –82% over –30°C to +85°C range; Z5U: 22%, –56% over 10°C to 85°C range). Under some conditions, a nominal 1 μF Y5V or Z5U capacitor could have a capacitance of only 0.1 μF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM2758. The voltage rating of the output capacitor must be 6.3 V or more. For example, a 6.3-V, 0603, 2.2-μF output capacitor (TDK C1608X5R0J225) is acceptable for use with the LM2758, as long as the capacitance on the output does not fall below a minimum of 1 μF in the intended application. All other capacitors must have a voltage rating at or above the maximum input voltage of the application and a minimum capacitance of 1 μF.

8.2.2.2 Power Efficiency

Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (P_LED) to the power drawn at the input of the part (P_IN). With a 1×/1.5× charge pump, the input current is equal to the charge pump gain times the output current (total LED current). The efficiency of the LM2758 can be predicted as follows:

For a simple approximation, the current consumed by internal circuitry (I_Q) can be neglected, and the resulting efficiency will become:

Neglecting I_Q results in a slightly higher efficiency prediction, but this impact will be negligible due to the value of I_Q being very low compared to the typical torch and flash current levels (100 mA to 500 mA). It is also worth noting that efficiency as defined here is in part dependent on LED voltage. Variation in LED voltage does not affect power consumed by the circuit and typically does not relate to the brightness of the LED. For an advanced analysis, it is recommended that power consumed by the circuit (V_IN × I_IN) be evaluated rather than power efficiency.