Data Sheet

LT8638-2

analog.com

Rev.0

20 of 35

Forced Continuous Mode

The LT8638-2 can operate in forced continuous mode (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. The LT8638-2 can

sink current from the output and return this charge to the input in this mode, improving load step transient response

(see Figure 47). At light loads, FCM operation is less efficient than Burst Mode operation, but may be desirable 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, float the SYNC/MODE pin. Leakage current on this pin should be <1μA. See

Block Diagram for internal pull-up and pull-down resistance.

Figure 47. LT8638-2 Load Step Transient Response with and without Forced Continuous Mode

FCM is disabled if the VIN pin is held above 37V or if the FB pin is held greater than 7.75% above the feedback reference

voltage. FCM is also disabled during soft-start until the soft-start capacitor is fully charged. When FCM is disabled in

these ways, negative inductor current is not allowed, and the LT8638-2 operates in pulse-skipping mode.

Spread Spectrum Mode

The LT8638-2 features spread spectrum operation to further reduce EMI emissions. To enable spread spectrum

operation, the SYNC/MODE pin should be tied high to INTVCC (or >3V). In this mode, triangular frequency modulation

is used to vary the switching frequency between the value programmed by RT to approximately 24% higher than that

value. The modulation frequency is approximately 3kHz. For example, when the LT8638-2 is programmed to 2MHz,

the frequency varies from 2MHz to approximately 2.5MHz at a 3kHz rate. When spread spectrum operation is

selected, Burst Mode operation is disabled, and the part runs in forced continuous mode.

Synchronization

To synchronize the LT8638-2 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

The LT8638-2 does not enter Burst Mode operation at low output loads while synchronized to an external clock, but

instead runs forced continuous mode to maintain regulation. The LT8638-2 may be synchronized over a 200kHz to

3MHz range. The RT resistor should be chosen to set the LT8638-2 switching frequency equal to or below the lowest

synchronization input. For example, if the synchronization signal is 500kHz and higher, the RT should be selected for

FRONT PAGE APPLICATION

100mA TO 5.1A TRANSIENT

12VIN, 5VOUT, fSW = 1MHz

CC = 220pF, RF = 12.1kΩ

COUT = 47µF, CLEAD = 15pF

50µs/DIV

ILOAD

5A/DIV

VOUT

100mV/DIV

FCM

Burst Mode OPERATION