Data Sheet

LT83401/LT83402

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Rev. 0

24 of 44

Forced Continuous Mode (FCM)

The LT83401/LT83402 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

LT83401/LT83402 can sink current from the output and return this charge to the input, improving load-step transient

response (for a comparison of pulse-skip mode and FCM, see Figure 50). At light loads, FCM operation is less efficient

than pulse-skipping operation, but it may be useful in applications where it is necessary to keep switching harmonics

out of the signal band. FCM must be used if the output is required to sink current. To enable FCM, connect the pin to

INTVCC or > 1.5V, or float the pin.

Figure 50. Load Step Transient Response with and without FCM. See Typical Application Circuit (Figure 59)

FCM is disabled under VIN overvoltage conditions (VIN pin is held above 42V), 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 487.5mV). 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 LT83401/LT83402 operate in pulse-skipping mode.

Pulse-Skipping Mode

When not operating in FCM, the LT83401/LT83402 operates in pulse-skipping mode. In this mode, the oscillator

operates continuously, and all switching cycles are aligned to the clock. The negative inductor current is not allowed

in this mode; therefore, at light loads, the LT83401/LT83402 may be operating in discontinuous mode. Additionally,

in pulse-skipping mode, the LT83401/LT83402 may also skip switching cycles at very light loads for improved

efficiency or at very high duty cycles to achieve better dropout. To enable pulse-skipping mode, connect the

SYNC/MODE pin to GND.

Synchronization

To synchronize the LT83401/LT83402 oscillator to an external frequency, connect a square wave to the SYNC/MODE

pin. The square wave amplitude should have valleys below 0.7V and peaks above 1.5V (up to 6V), with a minimum

on-time and off-time of 50ns.

While synchronized to an external clock, the part runs in FCM to maintain regulation. The LT83401/LT83402 can be

synchronized over a 300kHz to 6MHz range. The RT resistor should be chosen to set the LT83401/LT83402 switching

frequency to below the lowest synchronization input. The slope compensation is set by the RT value, while the

minimum slope compensation required to avoid subharmonic oscillations is established by the inductor size, input

voltage, and output voltage. Since the synchronization frequency does not change the inductor current waveform

FORCED CONTINUOUS MODE

PULSE-SKIPPING MODE

0.1A

1.1A

20µs/DIV

ILOAD

1A/DIV

VOUT

20mV/DIV

VIN = 12V, VOUT = 3.3V, fSW = 2MHz, RC = 1.0kΩ, CC = 4.7nF