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LTC1735CS

High Efficiency Synchronous Step-Down Switching Regulator

Linear Technology 

LTC1735

APPLICATIO S I FOR ATIO

tON(MIN) of the LTC1735 (approximately 200ns), the input

voltage and inductor value:

∆IL(SC) = tON(MIN)VIN/L

The resulting short-circuit current is:

ISC

=

30mV

RSENSE

+

1

2

∆IL(SC)

The current foldback function is always active and is not

effected by the current latchoff function.

Fault Conditions: Output Overvoltage Protection

(Crowbar)

The output overvoltage crowbar is designed to blow a

system fuse in the input lead when the output of the

regulator rises much higher than nominal levels. This

condition causes huge currents to flow, much greater than

in normal operation. This feature is designed to protect

against a shorted top MOSFET; it does not protect against

a failure of the controller itself.

The comparator (OV in the Functional Diagram) detects

overvoltage faults greater than 7.5% above the nominal

output voltage. When this condition is sensed the top

MOSFET is turned off and the bottom MOSFET is forced

on. The bottom MOSFET remains on continuously for as

long as the 0V condition persists; if VOUT returns to a safe

level, normal operation automatically resumes.

Note that dynamically changing the output voltage may

cause overvoltage protection to be momentarily activated

during programmed output voltage decreases. This will

not cause permanent latchoff nor will it disrupt the desired

voltage change. With soft-latch overvoltage protection,

dynamically changing the output voltage is allowed and

the overvoltage protection tracks the newly programmed

output voltage, always protecting the load.

the gate charge required to turn on the top MOSFET. Low

duty cycle applications may approach this minimum on-

time limit and care should be taken to ensure that:

tON(MIN)

<

VOUT

VIN(f)

If the duty cycle falls below what can be accommodated by

the minimum on-time, the LTC1735 will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple current and voltage will increase.

The minimum on-time for the LTC1735 in a properly

configured application is generally less than 200ns. How-

ever, as the peak sense voltage decreases, the minimum

on-time gradually increases as shown in Figure 7. This is

of particular concern in forced continuous applications

with low ripple current at light loads. If the duty cycle drops

below the minimum on-time limit in this situation, a

significant amount of cycle skipping can occur with corre-

spondingly larger current and voltage ripple.

If an application can operate close to the minimum on-

time limit, an inductor must be chosen that is low enough

to provide sufficient ripple amplitude to meet the mini-

mum on-time requirement. As a general rule, keep the

inductor ripple current equal or greater than 30% of

IOUT(MAX) at VIN(MAX).

250

200

150

100

50

Minimum On-Time Considerations

Minimum on-time tON(MIN) is the smallest amount of time

that the LTC1735 is capable of turning the top MOSFET on

and off again. It is determined by internal timing delays and

0

0

10

20

30

40

∆IL/IOUT(MAX) (%)

1735 F07

Figure 7. Minimum On-Time vs ∆IL

1735fc

19

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