LM2623

SNVS188H – MAY 2004 – REVISED NOVEMBER 2014

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Feature Description (continued)

Figure 8. Typical Step-Up Regulator Waveforms

7.3.2 Cycle-to-Cycle PFM

When the load doesn't vary over a wide range (like zero to full load), 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

here is matching the duty cycle of the circuit closely to what is required by the input to output voltage ratio. This

ratio then needs to be dynamically adjusted for input voltage changes (usually caused by batteries running

down). The chosen ratio should 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 will be small and the

system will settle 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 aren't matched to the input/output voltage ratio. Optimizing the duty cycle for a given set of

input/output voltages conditions can be done by using the circuit values in the Application Notes.

7.3.3 Shutdown

The LM2623 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 regulator's output voltage 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-2). When the logic input to

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 as not to come into effect during normal operating conditions. An internal

7.4 Device Functional Modes

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 will also undershoot somewhat when it starts switching again because it has depleted all the stored

energy in the coil and needs to store more energy to reach equilibrium with the load. Larger output capacitors

8

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