LTC3613 datasheet(17/44 Pages) AD

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LTC7131-1

Rev. 0

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OPERATION

with the master. The RUN_STBY pins are also connected

together as a handshaking signal between the two so that

both will shutoff together in case of a fault in only one of

the phases, such as overtemperature condition.

See the Applications Information section for further

details regarding 2-phase operation.

Discontinuous/Forced Continuous Operation

The LTC7131-1 can operate in one of two modes select-

able with the MODE/SYNC pin: discontinuous mode or

forced continuous mode. Connecting the MODE/SYNC

pin to INTVCC selects discontinuous mode. Discontinuous

mode is selected when high efficiency at very light loads is

desired. In this mode, when the inductor current reverses,

the bottom MOSFET turns off to minimize the efficiency

loss due to reverse current flow. This reduces the con-

duction loss and slightly improves the efficiency. As the

load reduces, the driver switching frequency drops in

proportion to the load, which further improves efficiency

by minimizing gate charge losses.

Forcing the MODE/SYNC pin low enables forced contin-

uous mode operation. In forced continuous mode, the

bottom MOSFET is always on when the top MOSFET

is off, allowing the inductor current to reverse at low

currents. This mode is less efficient due to conduction

and switching losses, but has the advantage of better

transient response at low currents, constant frequency

operation, and the ability to maintain regulation when

sinking current.

During soft-start, the LTC7131-1 forces the controller to

operate in discontinuous mode until the soft-start volt-

age reaches the internal reference to guarantee smooth

startup into a precharged output capacitor. During margi-

ning transitions and overvoltage conditions, however, the

LTC7131-1 always operates in forced continuous mode

to allow the switcher to sink current.

Fault Response Programming

When an output overvoltage or overcurrent condition

occurs, the LTC7131-1’s response to the fault can be

programmed using the MFR_FAULT_RESPONSE com-

mand. Two fault response modes are available: (1) ignore

or (2) hiccup mode. The response can be independently

programmed for each type of fault.

If ignore option is chosen, the LTC7131-1 will attempt to

correct the fault using the control loop as it would any

time the output goes out of regulation.

If hiccup mode is chosen, a fault timer is started when the

fault is detected. During this time, the control loop tries to

correct the fault. If the fault is still present after the 100µs

timer delay, the drivers are turned off for a period equal to

about 10× the soft-start time (set by the soft-start capac-

itor) before turning the drivers back on and attempting

to regulate again (see timing diagram in Figure 2). The

LTC7131-1 will continue these restart cycles indefinitely

until the fault is removed.

An overvoltage fault response is initiated when the output

voltage exceeds the upper PGOOD threshold. An overcur-

rent fault is initiated when the ITH voltage reached the

clamp set by the ILIM pin.

Undervoltage Lockout

The LTC7131-1 has two functions that help protect the

controller in case of undervoltage conditions. A precision

Figure 2. Fault Response Timing to Overcurrent Event

DRIVERS ON

DRIVERS OFF

7131-1 F02

OVERCURRENT REMOVED

OVERCURRENT EVENT

TIMEOUT DELAY

VOUT

TRACK/SS

TIMEOUT DELAY

FAULT TIMEOUT ENABLED

VREF

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