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LM2623-Q1

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SNVSAO5 – OCTOBER 2017

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7.3 Feature Description

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

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.