Table 1. Design Parameters

8.2.2.1 Custom Design With WEBENCH® Tools

Click here to create a custom design using the LM27761 device with the WEBENCH® Power Designer.

1. Start by entering the input voltage (V_IN), output voltage (V_OUT), and output current (I_OUT) requirements.
2. Optimize the design for key parameters such as efficiency, footprint, and cost using the optimizer dial.
3. Compare the generated design with other possible solutions from Texas Instruments.

The WEBENCH Power Designer provides a customized schematic along with a list of materials with real-time pricing and component availability.

In most cases, these actions are available:

• Run electrical simulations to see important waveforms and circuit performance
• Run thermal simulations to understand board thermal performance
• Export customized schematic and layout into popular CAD formats
• Print PDF reports for the design, and share the design with colleagues

Get more information about WEBENCH tools at www.ti.com/WEBENCH.

8.2.2.2 Charge-Pump Voltage Inverter

The main application of the LM27761 is to generate a regulated negative supply voltage. The voltage inverter circuit uses only three external capacitors, and the LDO regulator circuit uses one additional output capacitor.

The voltage inverter portion of the LM27761 contains four large CMOS switches which are switched in sequence to invert the input supply voltage. Energy transfer and storage are provided by external capacitors. Figure 17 shows the voltage switches S2 and S4 are open. In the second time interval, S1 and S3 are open; at the same time, S2 and S4 are closed, and C1 is charging C3. After a number of cycles, the voltage across C3 is pumped into V_IN. Because the anode of C3 is connected to ground, the output at the cathode of C3 equals –(V_IN) when there is no load current. When a load is added the output voltage dropis determined by the parasitic resistance (R_DSON of the MOSFET switches and the equivalent series resistance (ESR) of the capacitors) and the charge transfer loss between the capacitors.

Figure 17. Voltage Inverting Principle

The output characteristic of this circuit can be approximated by an ideal voltage source in series with a resistance. The voltage source equals –(V_IN). The output resistance R_OUT is a function of the ON resistance of the internal MOSFET switches, the oscillator frequency, the capacitance, and the ESR of C1 and C3. Because the switching current charging and discharging C1 is approximately twice as the output current, the effect of the ESR of the pumping capacitor C1 is multiplied by four in the output resistance. The charge-pump output capacitor C3 is charging and discharging at a current approximately equal to the output current; therefore, its ESR only counts once in the output resistance. A good approximation of charge-pump R_OUT is shown in Equation 1:

High capacitance and low-ESR ceramic capacitors reduce the output resistance.

8.2.2.3 Negative Low-Dropout Linear Regulator

At the output of the inverting charge-pump the LM27761 features a low-dropout, linear negative voltage regulator (LDO). The LDO output is rated for a current of 250 mA. This negative LDO allows the device to provide a very low noise output, low output voltage ripple, high PSRR, and low line or load transient response.

8.2.2.4 Power Dissipation

The allowed power dissipation for any package is a measure of the ability of the device to pass heat from the junctions of the device to the heatsink and the ambient environment. Thus, the power dissipation is dependent on the ambient temperature and the thermal resistance across the various interfaces between the die junction and ambient air.

The maximum allowable power dissipation can be calculated by Equation 2:

The actual power being dissipated in the device can be represented by Equation 3:

Equation 2 and Equation 3 establish the relationship between the maximum power dissipation allowed due to thermal consideration, the voltage drop across the device, and the continuous current capability of the device. These equations must be used to determine the optimum operating conditions for the device in a given application.

In lower power dissipation applications the maximum ambient temperature (T_A-MAX) may be increased. In higher power dissipation applications the maximum ambient temperature(T_A-MAX) may have to be derated. T_A-MAX can be calculated using Equation 4:

Alternately, if T_A-MAX cannot be derated, the power dissipation value must be reduced. This can be accomplished by reducing the input voltage as long as the minimum V_IN is not violated, or by reducing the output current, or some combination of the two.

8.2.2.5 Output Voltage Setting

The output voltage of the LM27761 is externally configurable. The value of R1 and R2 determines the output voltage setting. The output voltage can be calculated using Equation 5:

The value for R2 must be no less than 50 kΩ.

8.2.2.6 External Capacitor Selection

The LM27761 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive, and have very low ESR (≤ 15 mΩ typical). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally are not recommended for use with the LM27761 due to their high ESR compared to ceramic capacitors.

For most applications, ceramic capacitors with an X7R or X5R temperature characteristic are preferable for use with the LM27761. These capacitors have tight capacitance tolerances (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).

Using capacitors with a Y5V or Z5U temperature characteristic is generally not recommended for the LM27761. These capacitors 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 1-µF-rated Y5V or Z5U capacitor could have a capacitance as low as 0.1 µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM27761.

Net capacitance of a ceramic capacitor decreases with increased DC bias. This degradation can result in lower-than-expected capacitance on the input and/or output, resulting in higher ripple voltages and currents. Using capacitors at DC bias voltages significantly below the capacitor voltage rating usually minimizes DC bias effects. Consult capacitor manufacturers for information on capacitor DC bias characteristics.

Capacitance characteristics can vary quite dramatically with different application conditions, capacitor types, and capacitor manufacturers. TI strongly recommends that the LM27761 circuit be evaluated thoroughly early in the design-in process with the mass-production capacitor of choice. This helps ensure that any such variability in capacitance does not negatively impact circuit performance.