8.2.1.2.1 Capacitors

The LM2750 requires three 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 (≤ 10 mΩ typical). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally are not recommended for use with the LM2750 due to their high ESR, as compared to ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM2750. These capacitors have tight capacitance tolerance (as good as ±10%), hold their value over temperature (X7R: ±15% over –55°C to +125°C; X5R: ±15% over –55°C to

=85°C), and typically have little voltage coefficient. Capacitors with Y5V and/or Z5U temperature characteristic are generally not recommended. These types of capacitors typically have wide capacitance tolerance ( 80%, –20%), vary significantly over temperature (Y5V: 22%, –82% over –30°C to +85°C range; Z5U: 22%, –56% over 10°C to 85°C range), and have poor voltage coefficients. 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 these Y5V and Z5U of capacitors to fail to meet the minimum capacitance requirements of the LM2750.

Table 1 lists some leading ceramic capacitor manufacturers.

8.2.1.2.2 Input Capacitor

The input capacitor (C_IN) is used as a reservoir of charge, helping to quickly transfer charge to the flying capacitor during the charge phase (φ1) of operation. The input capacitor helps to keep the input voltage from drooping at the start of the charge phase, when the flying capacitor is first connected to the input, and helps to filter noise on the input pin that could adversely affect sensitive internal analog circuitry biased off the input line. As mentioned above, an X7R/X5R ceramic capacitor is recommended for use. For applications where the maximum load current required is from 60 mA to 120 mA, a minimum input capacitance of 2 µF is required. For applications where the maximum load current is 60 mA or less, 1 µF of input capacitance is sufficient. Failure to provide enough capacitance on the LM2750 input can result in poor part performance, often consisting of output voltage droop, excessive output voltage ripple and/or excessive input voltage ripple.

A minimum voltage rating of 10 V is recommended for the input capacitor. This is to account for DC bias properties of ceramic capacitors. Capacitance of ceramic capacitors reduces with increased DC bias. This degradation can be quite significant (> 50%) when the DC bias approaches the voltage rating of the capacitor.

8.2.1.2.3 Flying Capacitor

The flying capacitor (C_FLY) transfers charge from the input to the output, providing the voltage boost of the doubler. A polarized capacitor (tantalum, aluminum electrolytic, etc.) must not be used here, as the capacitor is reverse-biased upon start-up of the LM2750. The size of the flying capacitor and its ESR affect output current capability when the input voltage of the LM2750 is low, most notable for input voltages below 3 V. These issues were discussed previously in Output Current Capability. For most applications, a 1-µF X7R/X5R ceramic capacitor is recommended for the flying capacitor.

8.2.1.2.4 Output Capacitor

The output capacitor of the LM2750 plays an important part in determining the characteristics of the output signal of the LM2750, many of which have already been discussed. The ESR of the output capacitor affects charge pump output resistance, which plays a role in determining output current capability. Both output capacitance and ESR affect output voltage ripple. For these reasons, a low-ESR X7R/X5R ceramic capacitor is the capacitor of choice for the LM2750 output.

In addition to these issues previously discussed, the output capacitor of the LM2750 also affects control-loop stability of the part. Instability typically results in the switching frequency effectively reducing by a factor of two, giving excessive output voltage droop and/or increased voltage ripple on the output and the input. With output currents of 60 mA or less, a minimum capacitance of 1 µF is required at the output to ensure stability. For output currents from 60 mA to 120 mA, a minimum output capacitance of 2 µF is required.

A minimum voltage rating of 10 V is recommended for the output capacitor. This is to account for DC bias properties of ceramic capacitors. Capacitance of ceramic capacitors reduces with increased DC bias. This degradation can be quite significant (> 50%) when the DC bias approaches the voltage rating of the capacitor.

8.2.1.2.5 Power Efficiency And Power Dissipation

Efficiency of the LM2750 mirrors that of an unregulated switched capacitor converter followed by a linear regulator. The simplified power model of the LM2750, in Figure 13, is used to discuss power efficiency and power dissipation. In calculating power efficiency, output power (P_OUT) is easily determined as the product of the output current and the 5-V output voltage. Like output current, input voltage is an application-dependent variable. The input current can be calculated using the principles of linear regulation and switched capacitor conversion. In an ideal linear regulator, the current into the circuit is equal to the current out of the circuit. The principles of power conservation mandate the ideal input current of a voltage doubler must be twice the output current. Adding a correction factor for operating quiescent current (I_Q, 5-mA typical) gives an approximation for total input current which, when combined with the other input and output parameter(s), yields Equation 7 for efficiency:

Equation 7.

Comparisons of LM2750 efficiency measurements to calculations using Equation 7 have shown a quite accurate approximation of actual efficiency. Because efficiency is inversely proportional to input voltage, it is highest when the input voltage is low. In fact, for an input voltage of 2.9 V, efficiency of the LM2750 is greater than 80%
(I_OUT ≥ 40 mA) and peak efficiency is 85% (I_OUT = 120 mA). The average efficiency for an input voltage range spanning the Li-Ion range (2.9 V to 4.2 V) is 70% (I_OUT = 120 mA). At higher input voltages, efficiency drops dramatically. In Li-Ion-powered applications, this is typically not a major concern, as the circuit is powered off by a charger in these circumstances. Low efficiency equates to high power dissipation, however, which could become an issue worthy of attention.

The LM2750 power dissipation (P_D) is calculated simply by subtracting output power from input power:

Equation 8.

Power dissipation increases with increased input voltage and output current, up to 772 mW at the ends of the operating ratings (V_IN = 5.6 V, I_OUT = 120 mA). Internal power dissipation self-heats the device. Dissipating this amount power/heat so the LM2750 does not overheat is a demanding thermal requirement for a small surface-mount package. When soldered to a PCB with layout conducive to power dissipation, the excellent thermal properties of the WSON package enable this power to be dissipated from the LM2750 with little or no derating, even when the circuit is placed in elevated ambient temperatures.

Figure 13. LM2750 Model for Power Efficiency and Power Dissipation Calculations

8.2.2.2.1 LED Driver Power Efficiency

Efficiency of an LED driver (E_LED) is typically defined as the power consumed by the LEDs (P_LED) divided by the power consumed at the input of the circuit. Input power consumption of the LM2750 was explained and defined in the previous section titled: Power Efficiency And Power Dissipation. Assuming LED forward voltages and currents match reasonably well, LED power consumption is the product of the number of LEDs in the circuit (N), the LED forward voltage (V_LED), and the LED forward current (I_LED):

Equation 12.

Equation 13.