7.3.1 Pre-Regulation

The very low input current ripple of the LM2750, which results from internal pre-regulation, adds very little noise to the input line. The core of the LM2750 is very similar to that of a basic switched capacitor doubler: it is composed of four switches and a flying capacitor (external). Regulation is achieved by modulating the on-resistance of the two switches connected to the input pin (one switch in each phase). The regulation is done before the voltage doubling, giving rise to the term pre-regulation. It is pre-regulation that eliminates most of the input current ripple that is a typical and undesirable characteristic of a many switched capacitor converters.

7.3.2 Input, Output, and Ground Connections

Making good input, output, and ground connections is essential to achieve optimal LM2750 performance. The two input pads, pads 8 and 9, must be connected externally. It is strongly recommended that the input capacitor (C_IN) be placed as close to the LM2750 device as possible, so that the traces from the input pads are as short and straight as possible. To minimize the effect of input noise on LM2750 performance, it is best to bring two traces out from the LM2750 all the way to the input capacitor pad, so that they are connected at the capacitor pad. Connecting the two input traces between the input capacitor and the LM2750 input pads could make the LM2750 more susceptible to noise-related performance degradation. TI also recommends that the input capacitor be on the same side of the PCB as the LM2750, and that traces remain on this side of the board as well (vias to traces on other PCB layers are not recommended between the input capacitor and LM2750 input pads).

The two output pads, pads 1 and 2, must also be connected externally. TI recommends that the output capacitor (C_OUT) be placed as close to the LM2750 output pads as possible. It is best if routing of output pad traces follow guidelines similar to those presented for the input pads and capacitor. The flying capacitor (C_FLY) must also be placed as close to the LM2750 device as possible to minimize PCB trace length between the capacitor and the device. Due to the pad-layout of the part, it is likely that the trace from one of the flying capacitor pads (C+ or C–) must be routed to an internal or opposite-side layer using vias. This is acceptable, and it is much more advantageous to route a flying capacitor trace in this fashion than it is to place input traces on other layers.

The GND pads of the LM2750 are ground connections and must be connected externally. These include pads 3 (LM2750-5.0 only), 5, 6, and the die-attach pad (DAP). Large, low-impedance copper fills and via connections to an internal ground plane are the preferred way of connecting together the ground pads of the LM2750, the input capacitor, and the output capacitor, as well as connecting this circuit ground to the system ground of the PCB.

7.3.3 Shutdown

When the voltage on the active-low-logic shutdown pin is low, the LM2750 is in shutdown mode. In shutdown, the LM2750 draws virtually no supply current. There is a 200-kΩ pulldown resistor tied between the SD pin and GND that pulls the SD pin voltage low if the pin is not driven by a voltage source. When pulling the part out of shutdown, the voltage source connected to the SD pin must be able to drive the current required by the 200-kΩ resistor. For voltage management purposes required upon start-up, internal switches connect the output of the LM2750 to an internal pulldown resistor (1 kΩ typical) when the part is shut down. Driving the output of the LM2750 by another supply when the LM2750 is shut down is not recommended, as the pulldown resistor was not sized to sink continuous current.

7.3.4 Soft Start

The LM2750 employs soft-start circuitry to prevent excessive input inrush currents during start-up. The output voltage is programmed to rise from 0 V to the nominal output voltage (5 V) in 500 µs (typical). Soft start is engaged when a part, with input voltage established, is taken out of shutdown mode by pulling the SD pin voltage high. Soft start also engages when voltage is established simultaneously to the input and SD pins.

7.3.5 Output Current Capability

The LM2750-5.0 provides 120 mA of output current when the input voltage is within 2.9 V to 5.6 V. Using the LM2750 to drive loads in excess of 120 mA is possible.

The schematic of Figure 11 is a simplified model of the LM2750 that is useful for evaluating output current capability. The model shows a linear pre-regulation block (Reg), a voltage doubler (2×), and an output resistance (R_OUT). Output resistance models the output voltage droop that is inherent to switched capacitor converters. The output resistance of the LM2750 is 5 Ω (typical), and is approximately equal to twice the resistance of the four LM2750 switches. When the output voltage is in regulation, the regulator in the model controls the voltage V' to keep the output voltage equal to 5 V ± 4%. With increased output current, the voltage drop across R_OUT increases. To prevent droop in output voltage, the voltage drop across the regulator is reduced, V' increases, and V_OUT remains at 5V. When the output current increases to the point that there is zero voltage drop across the regulator, V' equals the input voltage, and the output voltage is "on the edge" of regulation. Additional output current causes the output voltage to fall out of regulation, and the LM2750 operation is similar to a basic open-loop doubler. As in a voltage doubler, increase in output current results in output voltage drop proportional to the output resistance of the doubler. The out-of-regulation LM2750 output voltage can be approximated by:

Equation 1.

Again, Equation 1 only applies at low input voltage and high output current where the LM2750 is not regulating. See Figure 1 and Figure 2 in Typical Characteristics for more details.

Figure 11. LM2750 Output Resistance Model

A more complete calculation of output resistance takes into account the effects of switching frequency, flying capacitance, and capacitor equivalent series resistance (ESR). See Equation 2:

Equation 2.

Switch resistance (5 Ω typical) dominates the output resistance equation of the LM2750. With a 1.7-MHz typical switching frequency, the 1/(F×C) component of the output resistance contributes only 0.6 Ω to the total output resistance. Increasing the flying capacitance only provides minimal improvement to the total output current capability of the LM2750. In some applications it may be desirable to reduce the value of the flying capacitor below 1 µF to reduce solution size and/or cost, but this must be done with care so that output resistance does not increase to the point that undesired output voltage droop results. If ceramic capacitors are used, equivalent series resistance (ESR) is a negligible factor in the total output resistance, as the ESR of quality ceramic capacitors is typically much less than 100 mΩ.

7.3.6 Thermal Shutdown

The LM2750 implements a thermal shutdown mechanism to protect the device from damage due to overheating. When the junction temperature rises to 150°C (typical), the part switches into shutdown mode. The LM2750 releases thermal shutdown when the junction temperature of the part is reduced to 130°C (typical).

Thermal shutdown is most-often triggered by self-heating, which occurs when there is excessive power dissipation in the device and/or insufficient thermal dissipation. LM2750 power dissipation increases with increased output current and input voltage (see Power Efficiency And Power Dissipation). When self-heating brings on thermal shutdown, thermal cycling is the typical result. Thermal cycling is the repeating process where the part self-heats, enters thermal shutdown (where internal power dissipation is practically zero), cools, turns on, and then heats up again to the thermal shutdown threshold. Thermal cycling is recognized by a pulsing output voltage and can be stopped be reducing the internal power dissipation (reduce input voltage and/or output current) or the ambient temperature. If thermal cycling occurs under desired operating conditions, thermal dissipation performance must be improved to accommodate the power dissipation of the LM2750. The WSON package has excellent thermal properties that, when soldered to a PCB designed to aid thermal dissipation, allows the LM2750 to operate under very demanding power dissipation conditions.

7.3.7 Output Current Limiting

The LM2750 contains current limit circuitry that protects the device in the event of excessive output current and/or output shorts to ground. Current is limited to 300 mA (typical) when the output is shorted directly to ground. When the LM2750 is current limiting, power dissipation in the device is likely to be quite high. In this event, thermal cycling must be expected (see Thermal Shutdown).

7.3.8 Programming the Output Voltage of the LM2750-ADJ

As shown in Figure 12, the output voltage of the LM2750-ADJ can be programmed with a simple resistor divider (see resistors R1 and R2). The values of the feedback resistors set the output voltage, as determined by Equation 3:  

In Equation 3, 1.23 V is the nominal voltage of the feedback pin when the feedback loop is correctly established, and the device is operating normally. The sum of the resistance of the two feedback resistors must be from 15 kΩ to 20 kΩ: 15 kΩ < (R1 +  R2) < 20 kΩ.

If larger feedback resistors are desired, a 10-pF capacitor must be placed in parallel with resistor R1.

7.4.1 PWM Brightness/Dimming Control

Brightness of the LEDs can be adjusted in an application by driving the SD pin of the LM2750 with a PWM signal. When the PWM signal is high, the LM2750 is ON, and current flows through the LEDs, as described in the previous section. A low PWM signal turns the part and the LEDs OFF. The perceived brightness of the LEDs is proportional to ON current of the LEDs and the duty cycle (D) of the PWM signal (the percentage of time the LEDs are ON).

To achieve good brightness/dimming control with this circuit, proper selection of the PWM frequency is required. The PWM frequency (ƒ_PWM) must be set higher than 100 Hz to avoid visible flickering of the LED light. An upper bound on this frequency is also needed to accommodate the turn-on time of the LM2750 (T_ON = 0.5 ms typical). This maximum recommended PWM frequency is similarly dependent on the minimum duty cycle (D_MIN) of the application. The next equation puts bounds on the recommended PWM frequency range:

Choosing a PWM frequency within these limits results in fairly linear control of the time-averaged LED current over the full duty-cycle adjustment range. For most applications, a PWM frequency from 100 Hz to 500 Hz is recommended. A PWM frequency up to 1 kHz may be acceptable in some designs.