8

LM2623

SNVS188I – MAY 2004 – REVISED OCTOBER 2017

www.ti.com

Product Folder Links: LM2623

Submit Documentation Feedback

Copyright © 2004–2017, Texas Instruments Incorporated

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

this pin is pulled below 0.15 VDD, the device goes into shutdown mode. The logic input to this pin should be

above 0.7 VDD for the device to work in normal stepup mode.

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

thermal protection circuit disables the MOSFET power switch when the junction temperature (TJ) exceeds about

160°C. The switch is re-enabled when TJ drops below approximately 135°C.