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LTC3839

3839fa

APPLICATIONS INFORMATION

to INTVCC, it will operate in forced continuous mode at

the RT-programmed frequency. If the MODE/PLLIN pin is

tied to SGND, the LTC3839 will operate in discontinuous

mode at light load and switch into continuous conduction

at the RT programmed frequency as load increases. The TG

on-time during discontinuous conduction is intentionally

slightly extended (approximately 1.2 times the continuous

conduction on-time as calculated from VIN, VOUT and f) to

create hysteresis at the load-current boundary of continu-

ous/discontinuous conduction.

If an application requires very low (approaching minimum)

on-time, the system may not be able to maintain its full

frequency synchronization range. Getting closer to mini-

mum on-time, it may even lose phase/frequency lock at no

load or light load conditions, under which the SW on-time

is effectively longer than TG on-time due to TG/BG dead

times. This is discussed further under Minimum On-Time,

Minimum Off-Time and Dropout Operation.

Minimum On-Time, Minimum Off-Time

and Dropout Operation

The minimum on-time is the smallest duration that

LTC3839’s TG (top gate) pin can be in high or “on” state.

It has dependency on the operating conditions of the

switching regulator, and is a function of voltages on the

VIN and VOUT pins, as well as the value of external resistor

RT. A minimum on-time of 30ns can be achieved when the

VOUT pin is tied to its minimum value of 0.6V while the VIN

is tied to its maximum value of 38V. For larger values of

VOUT and/or smaller values of VIN, the minimum achievable

on-time will be longer. The valley mode control architecture

allows low on-time, making the LTC3839 suitable for high

step-down ratio applications.

The effective on-time, as determined by the SW node

pulse width, can be different from this TG on-time, as it

also depends on external components, as well as loading

Figure 10. Phase and Frequency Locking Behavior During Transient Conditions

PHASE AND

FREQUENCY

LOCK LOST

DUE TO FAST

LOAD STEP

FREQUENCY

RESTORED

QUICKLY

PHASE AND

FREQUENCY

LOCK LOST

DUE TO FAST

LOAD STEP

FREQUENCY

RESTORED

QUICKLY

PHASE LOCK

RESUMED

3839 F10

PHASE AND

FREQUENCY

LOCKED

ILOAD

CLOCK

INPUT

VOUT

LTC3869 Datasheet(PDF) 29 Page - Linear Technology

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