7.3.1 Gated Oscillator Control Scheme

The on/off regulation mode of the LM2623-Q1, along with its ultra-low quiescent current, results in good efficiency over a very wide load range. The internal oscillator frequency can be programmed using an external resistor to be constant or vary with the battery voltage. Adding a capacitor to program the frequency allows the designer to adjust the duty cycle and optimize it for the application. Adding a resistor in addition to the capacitor allows the duty cycle to dynamically compensate for changes to the input/output voltage ratio. This is called a ratio-adaptive gated-oscillator circuit. See the Typical Application for a sample application circuit. Using the correct RC components to adjust the oscillator allows the device to run with low ripple and high efficiency over a wide range of loads and input/output voltages.

Figure 8. Typical Step-Up Regulator Waveforms

7.3.2 Cycle-To-Cycle PFM

When the load does not vary over a wide range, ratio adaptive circuit techniques can be used to achieve cycle to cycle PFM regulation and lower ripple (or smaller output capacitors). The key to success is matching the duty cycle of the circuit closely to what is required by the input-to-output voltage ratio. This ratio then must be dynamically adjusted for input voltage changes (usually caused by batteries running down). The chosen ratio must allow most of the energy in each switching cycle to be delivered to the load and only a small amount to be stored. When the regulation limit is reached, the overshoot is small, and the system settles at an equilibrium point where it adjusts the off time in each switching cycle to meet the current requirements of the load. The off-time adjustment is done by exceeding the regulation limit during each switching cycle and waiting until the voltage drops below the limit again to start the next switching cycle. The current in the coil never goes to zero like it frequently does in the hysteretic operating mode of circuits with wide load variations or duty cycles that are not matched to the input/output voltage ratio. Optimizing the duty cycle for a given set of input/output voltages conditions can be done by referencing the circuit values in AN-1221 LM2623 General Purpose, Boost Converter Circuit.

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7.3.3 Shutdown

The LM2623-Q1 features a shutdown mode that reduces the quiescent current to less than an ensured 2.5 µA over temperature. This extends the life of the battery in battery-powered applications. During shutdown, all feedback and control circuitry is turned off. The output voltage of the regulator drops to one diode drop below the input voltage. Entry into the shutdown mode is controlled by the active-low logic input pin EN (pin 4). When the logic input to this pin is pulled below 0.15 V_DD, the device goes into shutdown mode. The logic input to this pin must be above 0.7 V_DD for the device to work in normal step-up mode. If EN is pulled above 0.7 V_DD the SW pin switches continuously for experimentally tested 5 µs period, regardless of the output voltage. Once this time period is expired the feedback and gated oscillator scheme take over to regulate the output voltage.

7.3.4 Internal Current Limit and Thermal Protection

An internal cycle-by-cycle current limit serves as a protection feature. This is set high enough (2.85 A typical, approximately 4 A maximum) so it does not come into effect during normal operating conditions. An internal thermal protection circuit disables the MOSFET power switch when the junction temperature (T_J) exceeds about 160°C. The switch is re-enabled when T_J drops below approximately 135°C.

7.4.1 Pulse Frequency Modulation (PFM)

Pulse frequency modulation is typically accomplished by switching continuously until the voltage limit is reached and skipping cycles after that to just maintain it. This results in a somewhat hysteretic mode of operation. The coil stores more energy each cycle as the current ramps up to high levels. When the voltage limit is reached, the system usually overshoots to a higher voltage than required, due to the stored energy in the coil (see Figure 8). The system also undershoots somewhat when it starts switching again because it has depleted all the stored energy in the coil and must store more energy to reach equilibrium with the load. Larger output capacitors and smaller inductors reduce the ripple in these situations. The frequency being filtered, however, is not the basic switching frequency. It is a lower frequency determined by the load, the input/output voltage and the circuit parameters. This mode of operation is useful in situations where the load variation is significant. Power-managed computer systems, for instance, may vary from zero to full load while the system is on, and this is usually the preferred regulation mode for such systems.

7.4.2 Low Voltage Start-Up

The LM2623-Q1 can start up from voltages as low as 1.1 V. On start-up, the control circuitry switches the N-channel MOSFET continuously until the output reaches 3 V. After this output voltage is reached, the normal step-up regulator feedback and gated oscillator control scheme take over. Once the device is in regulation, it can operate down to 0.9 V input, because the internal power for the device can be boot-strapped from the output using the V_DD pin.