8.3.1 Power Conversion

Both converters operate in a fixed-frequency, PWM control scheme. The on-time of the internal switches varies depending on the input-to-output voltage ratio and the load. During the on-time, the inductors connected to the converters charge with current. In the remaining time, the off-time with a time period set by the fixed operating frequency, the inductors discharge into the output capacitors through the rectifier diodes. Usually at higher loads, the inductor currents are continuous. At lighter loads, the boost converter uses an additional internal switch to allow current to flow back to the input. This avoids inductor current becoming discontinuous in the boost converter. At the inverting converter, during light loads, the inductor current can become discontinuous. In this case, the control circuit of the inverting controller output automatically takes care of these changing conditions to operate always with an optimum control setup.

8.3.2 Control

The controller circuits of both converters employ a fixed-frequency, multiple-feedforward controller topology. These circuits monitor input voltage, output voltage, and voltage drop across the switches. Changes in the operating conditions of the converters directly affect the duty cycle and must not take the indirect and slow way through the output voltage-control loops. A self-learning control corrects measurement errors in this feedforward system. An external capacitor damps the output to avoid output-voltage steps due to output changes of this self-learning control system.

The voltage loops, determined by the error amplifiers, must only handle small signal errors. The error amplifiers feature internal compensation. Their inputs are the feedback voltages on the FBP and FBN pins. The device uses a comparison of these voltages with the internal reference voltage to generate an accurate and stable output voltage.

8.3.3 Output Rails Enable or Disable

Both converters can be enabled or disabled individually. Applying a logic HIGH signal at the enable pins (ENP for the boost converter, ENN for the inverting converter) enables the corresponding output. After enabling, internal circuitry, necessary to operate the specific converter, then turns on, followed by the Soft Start.

Applying a low signal at the enable ENP or ENN pin shuts down the corresponding converter. When both enable pins are low, the device enters shutdown mode, where all internal circuitry turns off. The device now consumes shutdown current flowing into the VIN pin. The output loads of the converters can be disconnected from the input, see Load Disconnect.

8.3.4 Load Disconnect

The device supports completely disconnecting the load when the converters are disabled. For the inverting converter, the device turns off the internal PMOS switch. If the inverting converter is turned off, no dc current path remains which could discharge the battery or supply.

This is different for the boost converter. The external rectifying diode, together with the boost inductor, form a dc current path which could discharge the battery or supply if any load connects to the output. The device has no internal switch to prevent current from flowing. For this reason, the device offers a PMOS gate control output (BSW) to enable and disable a PMOS switch in this dc current path, ideally directly between the boost inductor and battery. To be able to fully disconnect the battery, the forward direction of the parasitic backgate diode of this switch must point to the battery or supply. The external PMOS switch, which connects to BSW, turns on when the boost converter is enabled and turns off when the boost converter is disabled.

8.3.5 Soft Start

Both converters have implemented soft-start functions. When each converter is enabled, the implemented switch current limit ramps up slowly to its nominal programmed value in typically 1 ms. The device includes this function to limit the input current during start-up to avoid high peak input currents, which could interfere with other systems connected to the same battery or supply.

If the application includes the Load Disconnect PMOS switch, a current flows from the input to the output of the boost converter at the moment the PMOS switch becomes conducting.

8.3.6 Overvoltage Protection

Both built-in converters (boost and inverter) have implemented individual overvoltage protection. If the feedback voltage under normal operation exceeds the nominal value by typically 5%, the corresponding converter shuts down immediately to protect any connected circuitry from possible damage.

8.3.7 Undervoltage Lockout

An undervoltage lockout prevents the device from starting up and operating if the supply voltage at the VIN pin is lower than the undervoltage lockout threshold. For this case, the device automatically shuts down both converters when the supply voltage at VIN falls below this threshold. Nevertheless, parts of the control circuits remain active, which is different than device shutdown using EN inputs. The device includes the undervoltage lockout function to prevent device malfunction.

8.3.8 Overtemperature Shutdown

The device automatically shuts down both converters if the implemented internal temperature sensor detects a chip temperature above the thermal shutdown temperature. It automatically starts operating again when the chip temperature falls below this threshold plus hysteresis threshold. The built-in hysteresis avoids undefined operation caused by ringing from shutdown and prevents operating at a temperature close to the overtemperature shutdown threshold.

8.4.1 Power-Save Mode

The power-save mode can improve efficiency at light loads. In power-save mode, the converter only operates when the output voltage falls below an device internally set threshold voltage. The converter ramps up the output voltage with one or several operating pulses and goes again into power-save mode once the inductor current becomes discontinuous.

The PSN and PSP logic level selects between power-save mode and continuous-conduction mode. If the specific pins (PSP for the boost converter, PSN for the inverting converter) are HIGH, the power-save mode for the corresponding converter operates at light loads. Similary, a LOW on the PSP pin or PSN pin disables the power-save mode for the corresponding converter.