Data Sheet

LT83201

analog.com

Rev. A 23 of 45

Forced Continuous Mode (FCM)

The LT83201 can operate in FCM for fast transient response and full-frequency operation over a wide load range.

When in FCM, the oscillator operates continuously, and positive SW transitions are aligned to the clock. Negative

inductor current is allowed at light loads or under large transient conditions. In this mode, the LT83201 can sink

current from the output and return this charge to the input, improving load-step transient response (for a

comparison of DCM and FCM, see Figure 52). At light loads, FCM operation is less efficient than DCM operation, but it

may be useful in applications where the output is required to sink current. To enable FCM, connect the pin to INTVCC

or > 1.5V, or float the pin.

Figure 52. Load Step Transient Response with and without FCM

FCM is disabled under VIN overvoltage conditions (VIN pin is held above 18V), if VOUT is too high (PGFB pin is held greater

than 537.5mV) and during start-up until the voltage on VOUT has charged up to ~97.5% of its final value (as indicated

when the PGFB pin rises to above 486.5mV). For the latter two conditions, it is assumed the PGFB pin is connected to

the output voltage through an appropriate resistor divider. When FCM is disabled in these ways, negative inductor

current is not allowed, and the LT83201 operates in DCM.

Discontinuous Conduction Mode (DCM)

When not operating in FCM, the LT83201 operates in Discontinuous Conduction Mode (DCM), which allows

discontinuous operation at light loads. In this mode, the oscillator operates continuously, all switching cycles are

aligned to the clock and negative inductor current is not allowed. This improves efficiency at light loads.

The LT83201 enters discontinuous mode when the inductor valley current drops below 0A and stays in discontinuous

mode until the inductor valley current rises slightly above 0A, at which point the part returns to continuous mode

operation. This allows a smoother transition between discontinuous and continuous mode operation.

In this mode, the LT83201 may also skip switching cycles at very light loads for improved efficiency or at very high

duty cycle to achieve better dropout.

Skipping pulses can result in additional noise at frequencies other than the switching frequency, which could be

undesirable for some applications. The LT83201 operates at full frequency down to very light loads due to the

excellent minimum on-time. Operating at full frequency at light loads helps achieve higher light load efficiency and

predictable noise performance.

FORCED CONTINUOUS MODE

DISCONTINUOUS CONDUCTION MODE

VIN = 12V, VOUT = 3.3V, fSW = 2MHz, RC = 2.32kΩ, CC = 1.5nF

TYPICAL APPLICATION CIRCUIT

50mA

1A

20µs/DIV

ILOAD

1A/DIV

VOUT

20mV/DIV